Editing IPCC:AR6/Glossary

= IPCC AR6 Glossary =

This is a comprehensive glossary of terms from the IPCC Sixth Assessment Report (AR6).

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== 1 ==

=== 1.5°C pathway ===
'''Definition:''' A pathway of emissions of greenhouse gases and other climate forcers that provides an approximately one-in-two to two-in-three chance, given current knowledge of the climate response, of global warming either remaining below 1.5°C or returning to 1.5°C by around 2100 following an overshoot.

=== 1.5°C warmer worlds ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Projected worlds in which global warming has reached and, unless otherwise indicated, been limited to 1.5°C above pre-industrial levels. There is no single 1.5°C warmer world, and projections of 1.5°C warmer worlds look different depending on whether it is considered on a near-term transient trajectory or at climate equilibrium after several millennia, and, in both cases, if it occurs with or without overshoot. Within the 21st century, several aspects play a role for the assessment of risk and potential impacts in 1.5°C warmer worlds: the possible occurrence, magnitude and duration of an overshoot; the way in which emissions reductions are achieved; the ways in which policies might be able to influence the resilience of human and natural systems; and the nature of the regional and sub-regional risks. Beyond the 21st century, several elements of the climate system would continue to change even if the global mean temperatures remain stable, including further increases of sea level.

=== 13C ===
'''Working Groups:''' WGI

'''Definition:''' Stable isotope of carbon having an atomic weight of approximately 13. Measurements of the ratio of 13C/12C in carbon dioxide (CO2) molecules are used to infer the importance of different carbon cycle and climate processes and the size of the terrestrial carbon reservoir.

=== 14C ===
'''Working Groups:''' WGI

'''Definition:''' Unstable isotope of carbon having an atomic weight of approximately 14, and a half-life of about 5700 years. It is often used for dating purposes going back some 40 kyr. Its variation in time is affected by the magnetic fields of the Sun and Earth, which influence its production from cosmic rays (see Cosmogenic radioisotopes).

== 2 ==

=== 2030 Agenda for Sustainable Development ===
'''Working Groups:''' WGII

'''Definition:''' A UN resolution in September 2015 adopting a plan of action for people, planet and prosperity in a new global development framework anchored in 17 Sustainable Development Goals (UN, 2015).

== A ==

=== Ablation ===
'''Working Groups:''' WGI

'''Also known as:''' Ablation (of glaciers, ice sheets, or snow cover)

'''Definition:''' All processes that reduce the mass of a glacier, ice sheet, or snow cover. The main processes are melting, and for glaciers also calving (or, when the glacier nourishes an ice shelf, discharge of ice across the grounding line), but other processes such as sublimation and loss of wind-blown snow can also contribute to ablation. Ablation also refers to the mass lost by any of these processes.

=== Abrupt change ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A change in the system that is substantially faster than the typical rate of the changes in its history.

=== Abrupt climate change ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A large-scale abrupt change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades and causes substantial impacts in human and/or natural systems.

=== Acceptability of policy or system change ===
'''Working Groups:''' WGIII

'''Definition:''' The extent to which a policy or system change is evaluated unfavourably or favourably, or rejected or supported, by members of the general public (public acceptability) or politicians or governments (political acceptability). Acceptability may vary from totally unacceptable/fully rejected to totally acceptable/fully supported; individuals may differ in how acceptable policies or system changes are believed to be.

=== Access ===
'''Working Groups:''' WGII

'''Also known as:''' Access (to food)

'''Definition:''' See Access under Food Security

=== Access to modern energy services ===
'''Working Groups:''' WGIII

'''Definition:''' Access to clean, reliable and affordable energy services for cooking, heating, lighting, communications, and productive uses.

=== Acclimatisation ===
'''Working Groups:''' WGII

'''Definition:''' A change in functional or morphological traits occurring once or repeatedly (e.g., seasonally) during the lifetime of an individual organism in its natural environment. Through acclimatisation, the individual maintains performance across a range of environmental conditions. For a clear differentiation between findings in laboratory and field studies, the term ‘acclimation’ is used in ecophysiology for the respective phenomena when observed in well-defined experimental settings. The term ‘(adaptive) plasticity’ characterises the generally limited scope of changes in phenotype that an individual can reach through the process of acclimatisation.

=== Accumulation ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Accumulation (of glaciers, ice sheets or snow cover)

'''Definition:''' All processes that add to the mass of a glacier, an ice sheet, or snow cover. The main process of accumulation is snowfall. Accumulation also includes deposition of hoar, freezing rain, other types of solid precipitation, gain of wind-blown snow, avalanching, and basal accumulation (often beneath floating ice).

=== Active layer ===
'''Working Groups:''' WGI

'''Definition:''' Layer of ground above permafrost subject to annual thawing and freezing.

=== Acute food insecurity ===
'''Working Groups:''' WGII

'''Definition:''' Acute food insecurity is a situation which can occur at any time with a severity that threatens lives, livelihoods or both, regardless of the causes,context or duration, as a result of shocks risking determinants of food security and nutrition, and used to assess the need for humanitarian action (IPC Global Partners, 2019).

=== Adaptation ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' In human systems, the process of adjustment to actual or expected climate and its effects, in order to moderate harm or exploit beneficial opportunities. In natural systems, the process of adjustment to actual climate and its effects; human intervention may facilitate adjustment to expected climate and its effects.

=== Adaptation Fund ===
'''Working Groups:''' WGII

'''Definition:''' A Fund established under the Kyoto Protocol in 2001 and officially launched in 2007. The Fund finances adaptation projects and programmes in developing countries that are Parties to the Kyoto Protocol. Financing comes mainly from sales of Certified Emissions Reductions (CERs) and a share of proceeds amounting to 2 % of the value of CERs issued each year for Clean Development Mechanism (CDM) projects. The Adaptation Fund can also receive funds from governments, the private sector, and individuals.

=== Adaptation behaviour ===
'''Working Groups:''' WGII

'''Definition:''' Human actions that directly or indirectly affect the risks of climate change impacts.

=== Adaptation deficit ===
'''Working Groups:''' WGII

'''Definition:''' The gap between the current state of a system and a state that minimises adverse impacts from existing climate conditions and variability.

=== Adaptation gap ===
'''Working Groups:''' WGII

'''Definition:''' The difference between actually implemented adaptation and a societally set goal, determined largely by preferences related to tolerated climate change impacts and reflecting resource limitations and competing priorities (UNEP, 2014; UNEP, 2018).

=== Adaptation limits ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The point at which an actor’s objectives (or system needs) cannot be secured from intolerable risks through adaptive actions. • Hard adaptation limit – No adaptive actions are possible to avoid intolerable risks. • Soft adaptation limit – Options may exist but are currently not available to avoid intolerable risks through adaptive action.

=== Adaptation needs ===
'''Working Groups:''' WGII

'''Definition:''' The circumstances requiring action to ensure the safety of populations and the security of assets in response to climate impacts.

=== Adaptation opportunity ===
'''Definition:''' Factors that make it easier to plan and implement adaptation actions, that expand adaptation options, or that provide ancillary co-benefits.

=== Adaptation options ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The array of strategies and measures that are available and appropriate for addressing adaptation. They include a wide range of actions that can be categorised as structural, institutional, ecological or behavioural.

=== Adaptation pathways ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A series of adaptation choices involving trade-offs between short-term and long-term goals and values. These are processes of deliberation to identify solutions that are meaningful to people in the context of their daily lives and to avoid potential maladaptation.

=== Adaptive capacity ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The ability of systems, institutions, humans and other organisms to adjust to potential damage, to take advantage of opportunities or to respond to consequences (MA, 2005).

=== Adaptive governance ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Adjusting to changing conditions, such as climate change, through governance interactions that seek to maintain a desired state in a social-ecological system.

=== Adaptive management ===
'''Working Groups:''' WGII

'''Definition:''' A process of iteratively planning, implementing and modifying strategies for managing resources in the face of uncertainty and change. Adaptive management involves adjusting approaches in response to observations of their effect on, and changes in, the system brought on by resulting feedback effects and other variables.

=== Added value ===
'''Working Groups:''' WGI

'''Definition:''' Improvement of the representation of some climatic aspects by one methodology compared to another methodology. For instance, downscaling a coarse resolution global climate model may improve the representation of regional climate in complex terrain.

=== Additionality ===
'''Working Groups:''' WGIII

'''Definition:''' The property of being additional. Mitigation is additional if the greenhouse gas emission reductions or removals would not have occurred in the absence of the associated policy intervention or activity. [Note: Additionality is one of several key criteria used to ensure the environmental integrity of Offsets (in climate change mitigation) ].

=== Adjustments ===
'''Working Groups:''' WGI

'''Also known as:''' Adjustments (in relation to effective radiative forcing)

'''Definition:''' The response to an agent perturbing the climate system that is driven directly by the agent, independently of any change in global surface temperature. For example, carbon dioxide and aerosols, by altering internal heating and cooling rates within the atmosphere, can each cause changes to cloud cover and other variables thereby producing an effective radiative forcing even in the absence of any surface warming or cooling. Adjustments are usually rapid in the sense that they begin to occur right away, before climate feedbacks which are driven by global surface warming (although some adjustments may still take significant time to proceed to completion, for example those involving vegetation or ice sheets).

=== Advection ===
'''Working Groups:''' WGI

'''Definition:''' Transport of water or air along with its properties (e.g., temperature, chemical tracers) by winds or currents. Regarding the general distinction between advection and convection, the former describes transport by large-scale motions of the atmosphere or ocean, while convection describes the predominantly vertical, locally induced motions.

=== Adverse side-effect ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A negative effect that a policy or measure aimed at one objective has on another objective, thereby potentially reducing the net benefit to society or the environment.

=== Aerosol ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A suspension of airborne solid or liquid particles, with typical particle size in the range of a few nanometres to several tens of micrometres and atmospheric lifetimes of up to several days in the troposphere and up to years in the stratosphere. The term aerosol, which includes both the particles and the suspending gas, is often used in this report in its plural form to mean ‘aerosol particles’. Aerosols may be of either natural or anthropogenic origin in the troposphere; stratospheric aerosols mostly stem from volcanic eruptions. Aerosols can cause an effective radiative forcing directly through scattering and absorbing radiation (aerosol–radiation interaction), and indirectly by acting as cloud condensation nuclei or ice nucleating particles that affect the properties of clouds (aerosol–cloud interaction), and upon deposition on snow- or ice-covered surfaces. Atmospheric aerosols may be either emitted as primary particulate matter or formed within the atmosphere from gaseous precursors (secondary production). Aerosols may be composed of sea salt, organic carbon, black carbon (BC), mineral species (mainly desert dust), sulphate, nitrate and ammonium or their mixtures.

=== Aerosol–cloud interaction ===
'''Working Groups:''' WGI

'''Definition:''' A process by which a perturbation to aerosol affects the microphysical properties and evolution of clouds through the aerosol role as cloud condensation nuclei or ice nuclei, particularly in ways that affect radiation or precipitation; such processes can also include the effect of clouds and precipitation on aerosol. The aerosol perturbation can be anthropogenic or come from some natural source. The radiative forcing from such interactions has traditionally been attributed to numerous indirect aerosol effects, but in this report, only two levels of radiative forcing (or effect) are distinguished:

=== Aerosol effective radiative forcing ===
'''Working Groups:''' WGI

'''Also known as:''' Aerosol effective radiative forcing (ERFari+aci)

'''Definition:''' The total effective radiative forcing due to both aerosol–cloud and aerosol–radiation interactions is denoted aerosol effective radiative forcing (ERFari+aci).

=== Aerosol optical depth ===
'''Working Groups:''' WGI

'''Definition:''' Wavelength-dependent aerosol optical depth is a measure of the aerosol contribution to extinction of top-of-the-atmosphere solar intensity measured at the ground. AOD is unitless.

=== Aerosol–radiation interaction ===
'''Working Groups:''' WGI

'''Definition:''' An interaction of aerosol directly with radiation produces radiative effects. In this report, two levels of radiative forcing (or effect) are distinguished:

=== Afforestation ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Conversion to forest of land that historically has not contained forests. [Note: For a discussion of the term forest and related terms such as afforestation, reforestation and deforestation, see the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and their 2019 Refinement, and information provided by the United Nations Framework Convention on Climate Change (IPCC 2006, 2019; UNFCCC 2021a, b).]

=== Agreement ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' In this report, the degree of agreement within the scientific body of knowledge on a particular finding is assessed based on multiple lines of evidence (e.g., mechanistic understanding, theory, data, models, expert judgement) and expressed qualitatively (Mastrandrea et al., 2010).

=== Agricultural and ecological drought ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Depending on the affected biome: a period with abnormal soil moisture deficit, which results from combined shortage of precipitation and excess evapotranspiration, and during the growing season impinges on crop production or ecosystem function in general.

=== Agriculture, Forestry and Other Land Use ===
'''Working Groups:''' WGIII

'''Also known as:''' Agriculture, Forestry and Other Land Use (AFOLU)

'''Definition:''' In the context of national greenhouse gas (GHG) inventories under the United Nations Framework Convention on Climate Change (UNFCCC), AFOLU is the sum of the GHG inventory sectors Agriculture and Land Use, Land-Use Change and Forestry (LULUCF); see the 2006 IPCC Guidelines for National GHG Inventories for details. Given the difference in estimating the ‘ anthropogenic ’ carbon dioxide (CO 2) removals between countries and the global modelling community, the land-related net GHG emissions from global models included in this report are not necessarily directly comparable with LULUCF estimates in national GHG Inventories.

=== Agroecology ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' ‘The science and practice of applying ecological concepts, principles and knowledge (i.e., the interactions of, and explanations for, the diversity, abundance and activities of organisms) to the study, design and management of sustainable agroecosystems. It includes the roles of human beings as a central organism in agroecology by way of social and economic processes in farming systems. Agroecology examines the roles and interactions among all relevant biophysical, technical and socio-economic components of farming systems and their surrounding landscapes (IPBES, 2019).

=== Agroforestry ===
'''Working Groups:''' WGII

'''Definition:''' Collective name for land-use systems and technologies where woody perennials (trees, shrubs, palms, bamboos, etc.) are deliberately used on the same land-management units as agricultural crops and/or animals, in some form of spatial arrangement or temporal sequence. In agroforestry systems there are both ecological and economical interactions between the different components. Agroforestry can also be defined as a dynamic, ecologically-based, natural resource management system that, through the integration of trees on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic and environmental benefits for land users at all levels (FAO, 2015a).

=== Air mass ===
'''Working Groups:''' WGI

'''Definition:''' A widespread body of air, the approximately homogeneous properties of which (i) have been established while that air was situated over a particular region of the Earth’s surface, and (ii) undergo specific modifications while in transit away from the source region (AMS, 2021).

=== Air pollution ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Degradation of air quality with negative effects on human health or the natural or built environment due to the introduction, by natural processes or human activity, into the atmosphere of substances (gases, aerosols) which have a direct (primary pollutants) or indirect (secondary pollutants) harmful effect.

=== Airborne fraction ===
'''Working Groups:''' WGI

'''Definition:''' The fraction of total carbon dioxide (CO2) emissions (from fossil fuels and land-use change) remaining in the atmosphere.

=== Albedo ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The proportion of sunlight (solar radiation) reflected by a surface or object, often expressed as a percentage. Clouds, snow and ice usually have high albedo; soil surfaces cover the albedo range from high to low; vegetation in the dry season and/or in arid zones can have high albedo, whereas photosynthetically active vegetation and the ocean have low albedo. The Earth’s planetary albedo changes mainly through changes in cloudiness, snow, ice, leaf area and land cover.

=== Alkalinity ===
'''Working Groups:''' WGI

'''Definition:''' Seawater acid–base system.

=== Altimetry ===
'''Working Groups:''' WGI

'''Definition:''' A technique for measuring the height of the Earth’s surface with respect to the geocentre of the Earth within a defined terrestrial reference frame (geocentric sea level).

=== Annular modes ===
'''Working Groups:''' WGI

'''Definition:''' Hemispheric scale patterns of atmospheric variability characterized by opposing and synchronous fluctuations in sea-level pressure between the polar caps and mid-latitudes, with a structure exhibiting a high degree of zonal symmetry, and with no real preferred time scales ranging from days to decades. In each hemisphere, these fluctuations reflect changes in the latitudinal position and strength of the mid-latitude jets and associated storm tracks. Annular modes are defined as the leading mode of variability of extratropical sea-level pressure or geopotential heights and are known as the Northern Annular Mode (NAM) and Southern Annular Mode (SAM) in the two hemispheres, respectively.

=== Anomaly ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The deviation of a variable from its value averaged over a reference period.

=== Antarctic Ice Sheet ===
'''Working Groups:''' WGI

'''Also known as:''' Antarctic Ice Sheet (AIS)

'''Definition:''' There are only two ice sheets in the modern world, one on Greenland and one on Antarctica. The latter is divided into the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet. During glacial periods, there were other ice sheets.

=== Anthropocene ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A proposed new geological epoch resulting from significant human-driven changes to the structure and functioning of the Earth system, including the climate system. Originally proposed in the Earth system science community in 2000, the proposed new epoch is undergoing a formalisation process within the geological community based on the stratigraphic evidence that human activities have changed the Earth system to the extent of forming geological deposits with a signature that is distinct from those of the Holocene, and which will remain in the geological record. Both the stratigraphic and Earth system approaches to defining the Anthropocene consider the mid-20th century to be the most appropriate starting date (Steffen et al., 2016), although others have been proposed and continue to be discussed. The Anthropocene concept has already been informally adopted by diverse disciplines and the public to denote the substantive influence of humans on the Earth system.

=== Anthropogenic ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Resulting from or produced by human activities.

=== Anthropogenic emissions ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Emissions of greenhouse gases (GHGs), precursors of GHGs and aerosols caused by human activities. These activities include the burning of fossil fuels, deforestation, land use and land use changes (LULUC), livestock production, fertilisation, waste management, and industrial processes.

=== Anthropogenic removals ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The withdrawal of greenhouse gases (GHGs) from the atmosphere as a result of deliberate human activities. These include enhancing biological sinks of CO2 and using chemical engineering to achieve long term removal and storage. Carbon capture and storage (CCS), which alone does not remove CO2 from the atmosphere, can help reduce atmospheric CO2 from industrial and energy-related sources if it is combined with bioenergy production (BECCS), or if CO2 is captured from the air directly and stored (DACCS). [Note: In the 2006 IPCC Guidelines for National GHG Inventories (IPCC, 2006), which are used in reporting of emissions to the UNFCCC, ‘anthropogenic’ land-related GHG fluxes are defined as all those occurring on ‘managed land’, i.e. ‘where human interventions and practices have been applied to perform production, ecological or social functions’. However, some removals (e.g. removals associated with CO2 fertilisation and N deposition) are not considered as ‘anthropogenic’, or are referred to as ‘indirect’ anthropogenic effects, in some of the scientific literature assessed in this report. As a consequence, the land-related net GHG emission estimates from global models included in this report are not necessarily directly comparable with Land Use, Land-Use Change and Forestry (LULUCF) estimates in national GHG Inventories.]

=== Anthropogenic subsidence ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Downward motion of the land surface induced by anthropogenic drivers (e.g., loading, extraction of hydrocarbons and/or groundwater, drainage, mining activities) causing sediment compaction or subsidence/deformation of the sedimentary sequence, or oxidation of organic material, thereby leading to relative sea level rise.

=== Apparent hydrological sensitivity ===
'''Working Groups:''' WGI

'''Also known as:''' Apparent hydrological sensitivity (ηa)

'''Definition:''' The change in global mean precipitation per degree Celsius of global mean surface air temperature (GSAT) change with units of % per °C, although it can also be calculated as Wm-2 per °C.

=== Arctic oscillation ===
'''Working Groups:''' WGI

'''Also known as:''' Arctic oscillation (AO)

'''Definition:''' See Northern Annular Mode (NAM) (under Annular modes).

=== Arid zone ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Areas where vegetation growth is severely constrained due to limited water availability. For the most part, the native vegetation of arid zones is sparse. There is high rainfall variability, with annual averages below 300 mm. Crop farming in arid zones requires irrigation.

=== Aridity ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The state of a long-term climatic feature characterised by low average precipitation or available water in a region. Aridity generally arises from widespread persistent atmospheric subsidence or anticyclonic conditions, and from more localised subsidence in the lee side of mountains (adapted from Ogallo and Gbeckor-Kove, 1989; Türkeş, 1999).

=== Artificial ocean upwelling ===
'''Working Groups:''' WGI

'''Also known as:''' Artificial ocean upwelling (AOUpw)

'''Definition:''' A potential carbon dioxide removal (CDR) method that aims to artificially pump up cooler, nutrient-rich waters from deep in the ocean to the surface. The aim is to stimulate phytoplankton activity and thereby increase ocean CO2 uptake.

=== Assets ===
'''Working Groups:''' WGI

'''Definition:''' Natural or human-made resources that provide current or future utility, benefit, economic or intrinsic value to natural or human systems.

=== Atlantic Meridional Mode ===
'''Working Groups:''' WGI

'''Also known as:''' Atlantic Meridional Mode (AMM)

'''Definition:''' The Atlantic Meridional Mode (AMM) refers to the interannual to decadal variability of the cross-equatorial sea surface temperature gradients and surface wind anomalies in the tropical Atlantic. It modulates the strength and latitudinal shifts of the Inter-tropical Convergence Zone (ITCZ), which impacts regional rainfall over Northeast Brazil and Atlantic hurricane activity. See Section AIV.2.5 in Annex IV of the AR6 WGI report.

=== Atlantic Meridional Overturning Circulation ===
'''Working Groups:''' WGI

'''Also known as:''' Atlantic Meridional Overturning Circulation (AMOC)

'''Definition:''' The main current system in the South and North Atlantic Oceans. AMOC transports warm upper-ocean water northwards and cold, deep water southwards, as part of the global ocean circulation system. Changes in the strength of AMOC can affect other components of the climate system.

=== Atlantic Multi-decadal Oscillation ===
'''Working Groups:''' WGI

'''Also known as:''' Atlantic Multi-decadal Oscillation (AMO)

'''Definition:''' A multi-decadal (65- to 75-year) fluctuation in the North Atlantic, in which sea surface temperatures showed warm phases during roughly 1860 to 1880 and 1930 to 1960 and cool phases during 1905 to 1925 and 1970 to 1990 with a range of approximately 0.4°C. See AMO Index in WGI AR5 Box 2.5.

=== Atlantic Multi-decadal Variability ===
'''Working Groups:''' WGI

'''Also known as:''' Atlantic Multi-decadal Variability (AMV)

'''Definition:''' Large-scale fluctuations observed from one decade to the next in a variety of instrumental records and proxy reconstructions over the entire North Atlantic ocean and surrounding continents. Fingerprints of AMV can be found at the surface ocean, which is characterized by swings in basin-scale sea surface temperature anomalies reflecting the interaction with the atmosphere. The positive phase of the AMV is characterized by anomalous warming over the entire North Atlantic, with the strongest amplitude in the subpolar gyre and along sea-ice margin zones in the Labrador Sea and Greenland/Barents Sea and in the subtropical North Atlantic basin to a lower extent. In the AR6 WGI report, the term AMV is preferred to Atlantic Multi-decadal Oscillation (AMO) used in previous IPCC reports because there is no preferred time scale of decadal variability as the term oscillation would indirectly imply. See Section AIV.2.7 in Annex IV of the AR6 WGI report.

=== Atlantic Zonal Mode ===
'''Working Groups:''' WGI

'''Also known as:''' Atlantic Zonal Mode (AZM)

'''Definition:''' An equatorial coupled mode in the Atlantic similar to El Niño–Southern Oscillation (ENSO) in the Pacific, and therefore sometimes referred to as the Atlantic Niño. The AZM is associated with sea surface temperature anomalies near the equatorial Atlantic and rainfall disturbances over the African monsoon domain. Its variations are mostly observed in the interannual scale. It is called also Atlantic equatorial mode. See Section AIV.2.5 in Annex IV of the AR6 WGI report.

=== Atmosphere ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The gaseous envelope surrounding the Earth, divided into five layers – the troposphere which contains half of the Earth’s atmosphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere, which is the outer limit of the atmosphere. The dry atmosphere consists almost entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as argon (0.93 % volume mixing ratio), helium and radiatively active greenhouse gases (GHGs) such as 2) carbon dioxide (CO (0.04% volume mixing ratio), 4) methane (CH, 2 O) nitrous oxide (N and 3) ozone (O. In addition, the atmosphere contains the GHG water vapour (H2O), whose concentrations are highly variable (0–5% volume mixing ratio) as the sources (evapotranspiration) and sinks (precipitation) of water vapour show large spatio-temporal variations, and atmospheric temperature exerts a strong constraint on the amount of water vapour an air parcel can hold. The atmosphere also contains clouds and aerosols.

=== Atmospheric boundary layer ===
'''Working Groups:''' WGI

'''Definition:''' The atmospheric layer adjacent to the Earth’s surface that is affected by friction against that boundary surface, and possibly by transport of heat and other variables across that surface (AMS, 2021). The lowest 100 m of the boundary layer (about 10% of the boundary layer thickness), where mechanical generation of turbulence is dominant, is called the surface boundary layer or surface layer.

=== Atmospheric rivers ===
'''Working Groups:''' WGI

'''Also known as:''' Atmospheric rivers (ARs)

'''Definition:''' Long, narrow (up to a few hundred km wide), shallow (up to a few km deep) and transient corridors of strong horizontal water vapour transport that are typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone (ETC) (Ralph et al., 2018).

=== Attribution ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Attribution is defined as the process of evaluating the relative contributions of multiple causal factors to a change or event with an assessment of confidence.

=== Australian and Maritime Continent monsoon ===
'''Working Groups:''' WGI

'''Definition:''' The Australian–Maritime Continent monsoon (AusMCM) occurs during December-January-February, with the large-scale shift of the Inter-tropical Convergence Zone into the Southern Hemisphere and covering northern Australia and the Maritime Continent up to 10°N. The AusMCM is characterized by the seasonal reversal of prevailing easterly winds to westerly winds and the onset of periods of active convection and heavy rainfall. Over northern Australia, the monsoon season generally lasts from December to March and is associated with west to north-westerly inflow of moist winds, producing convection and heavy precipitation. Over the Maritime Continent, the main rainy season south of the equator is centred on December to February with north-westerly monsoon flow at low levels. Further details on how AusMCM is defined and used throughout the Report are provided in Annex V.

=== Autonomous adaptation ===
'''Working Groups:''' WGII

'''Definition:''' Adaptation in response to experienced climate and its effects, without planning explicitly or consciously focused on addressing climate change. Also referred to as spontaneous adaptation.

=== Autotrophic respiration ===
'''Working Groups:''' WGI

'''Definition:''' Respiration by photosynthetic (see photosynthesis) organisms (e.g., plants and algae).

=== Avalanche ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A mass of snow, ice, earth or rocks, or a mixture of these, falling down a mountainside.

=== Avoid, Shift, Improve ===
'''Working Groups:''' WGIII

'''Also known as:''' Avoid, Shift, Improve (ASI)

'''Definition:''' Reducing greenhouse gas emissions by avoiding the use of an emissions-producing service entirely, shifting to the lowest-emission mode of providing the service, and/or improving the technologies and systems for providing the service in ways that reduce emissions.

== B ==

=== Basal lubrication ===
'''Working Groups:''' WGI

'''Definition:''' Reduction of friction at the base of an ice sheet or glacier due to lubrication by meltwater. This can allow the glacier or ice sheet to slide over its base. Meltwater may be produced by pressure-induced melting, friction or geothermal heat, or surface melt may drain to the base through holes in the ice.

=== Baseline period ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A time period against which differences are calculated (e.g., expressed as anomalies relative to a baseline).

=== Baseline/reference ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The baseline (or reference) isthe state against which change is measured. A baseline period isthe period relative to which anomalies are computed. The baseline concentration of a trace gas is that measured at a location not influenced by local anthropogenic emissions.

=== Baseline scenario ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' See Reference Scenario

=== Behavioural change ===
'''Working Groups:''' WGII

'''Definition:''' In this report, behavioural change refers to alteration of human decisions and actions in ways that mitigate climate change and/or reduce negative consequences of climate change impacts.

=== Benthic ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Occurring at the bottom of a body of water; related to benthos (NOAA, 2018).

=== Benthos ===
'''Working Groups:''' WGII

'''Definition:''' The community of organisms living on the bottom or in sediments of a body of water (such as an ocean, a river or a lake). The ecological zone at the bottom of a body of water, including the sediment surface and some subsurface layers, is known as the benthic zone.

=== Beta diversity ===
'''Working Groups:''' WGII

'''Definition:''' The change in species composition between different areas (spatial turnover) or times (temporal turnover) due to habitat and environmental heterogeneity

=== Biochar ===
'''Working Groups:''' WGIII

'''Definition:''' Relatively stable, carbon-rich material produced by heating biomass in an oxygen-limited environment. Biochar is distinguished from charcoal by its application: biochar is used as a soil amendment with the intention to improve soil functions and to reduce greenhouse gas emissions from biomass that would otherwise decompose rapidly (IBI, 2018).

=== Biochemical oxygen demand ===
'''Also known as:''' Biochemical oxygen demand (BOD)

'''Definition:''' The amount of dissolved oxygen consumed by micro-organisms (bacteria) in the bio-chemical oxidation of organic and inorganic matter in wastewater.

=== Biodiversity ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Biodiversity or biological diversity means the variability among living organisms from all sources including, among other things, terrestrial, marine and other aquatic ecosystems, and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems (UN, 1992).

=== Biodiversity hotspots ===
'''Working Groups:''' WGII

'''Definition:''' Biodiversity hotspots are geographic areas exceptionally rich in species, ecologically distinct, and often contain geographically-rare-endemic species. They are thus priorities for nature conservation action.

=== Bioenergy ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Energy derived from any form of biomass or its metabolic by-products.

=== Bioenergy with carbon dioxide capture and storage ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Bioenergy with carbon dioxide capture and storage (BECCS)

'''Definition:''' Carbon dioxide capture and storage (CCS) technology applied to a bioenergy facility. Note that depending on the total emissions of the BECCS supply chain, carbon dioxide (CO2) can be removed from the atmosphere.

=== Bioethanol ===
'''Definition:''' Ethanol produced from biomass (e.g., sugar cane or corn).

=== Biofuel ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A fuel, generally in liquid form, produced from biomass. Biofuels include bioethanol from sugarcane, sugar beet or maize, and biodiesel from canola or soybeans.

=== Biogenic carbon emissions ===
'''Working Groups:''' WGIII

'''Definition:''' Carbon released as carbon dioxide or methane from combustion or decomposition of biomass or biobased products.

=== Biogenic volatile organic compounds ===
'''Working Groups:''' WGI

'''Also known as:''' Biogenic volatile organic compounds (BVOCs)

'''Definition:''' Organic gas-phase compounds emitted from terrestrial and aquatic ecosystems that are critical in ecology and plant physiology, from abiotic and biotic stress functions to integrated components of metabolism. BVOCs are important in atmospheric chemistry as precursors for 3) ozone (O and secondary organic aerosol formation. Other terms used to represent BVOCs are hydrocarbons (HCs), reactive organic gases (ROGs) and non-methane volatile organic compounds (NMVOCs).

=== Biogeophysical potential ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The mitigation potential constrained by biological, geophysical and geochemical limits and thermodynamics, without taking into account technical, social, economic and/or environmental considerations.

=== Biological pump ===
'''Working Groups:''' WGI

'''Also known as:''' Biological (carbon) pump

'''Definition:''' A series of ocean processes through which inorganic carbon (as carbon dioxide, CO 2) is fixed as organic matter by photosynthesis in sunlit surface water and then transported to the ocean interior, and possibly the sediment, resulting in the storage of carbon.

=== Biomass ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Organic material excluding the material that is fossilised or embedded in geological formations. Biomass may refer to the mass of organic matter in a specific area (ISO, 2014).

=== Biomes ===
'''Working Groups:''' WGII

'''Definition:''' Global-scale zones, generally defined by the type of plant life that they support in response to average rainfall and temperature patterns. For example, tundra, coral reefs or savannas (IPBES, 2019).

=== Biosphere ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Biosphere (terrestrial and marine)

'''Definition:''' The part of the Earth system comprising all ecosystems and living organisms, in the atmosphere, on land (terrestrial biosphere) or in the oceans (marine biosphere), including derived dead organic matter, such as litter, soil organic matter and oceanic detritus.

=== Bipolar seesaw ===
'''Working Groups:''' WGI

'''Also known as:''' Bipolar seesaw (also inter-hemispheric seesaw, inter-hemispheric asymmetry, hemispheric asymmetry)

'''Definition:''' A phenomenon in which temperature changes in the Northern and Southern hemispheres are related but out of phase, generally inferred to represent a change in the magnitude or sign of net heat transport across the equator. Originally called hemispheric asymmetry and linked to changes in thermohaline overturning circulation on multi-millennial scales (Mix et al, 1986), later named bipolar seesaw and applied to millennial scales (Broecker, 1998) with a similar thermohaline mechanism (Stocker and Johnsen, 2003).

=== Black carbon ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Black carbon (BC)

'''Definition:''' A relatively pure form of carbon, also known as soot, arising from the incomplete combustion of fossil fuels, biofuel, and biomass. It only stays in the atmosphere for days or weeks. BC is a climate forcing agent with strong warming effect, both in the atmosphere and when deposited on snow or ice.

=== Blocking ===
'''Working Groups:''' WGI

'''Definition:''' Associated with persistent, slow-moving high-pressure systems that obstruct the prevailing westerly winds in the middle and high latitudes and the normal eastward progress of extratropical transient storm systems. It is an important component of the intra-seasonal climate variability in the extratropics and can cause long-lived weather conditions such as cold spells in winter and summer heatwaves.

=== Blue carbon ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Biologically driven carbon fluxes and storage in marine systems that are amenable to management. Coastal blue carbon focuses on rooted vegetation in the coastal zone, such as tidal marshes, mangroves and seagrasses. These ecosystems have high carbon burial rates on a per unit area basis and accumulate carbon in their soils and sediments. They provide many non-climatic benefits and can contribute to ecosystem-based adaptation. If degraded or lost, coastal blue carbon ecosystems are likely to release most of their carbon back to the atmosphere. There is current debate regarding the application of the blue carbon concept to other coastal and non-coastal processes and ecosystems, including the open ocean.

=== Blue infrastructure ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Blue infrastructure includes bodies of water, watercourses, ponds, lakes and storm drainage, that provide ecological and hydrological functions including evaporation, transpiration, drainage, infiltration and temporary storage of runoff and discharge.

=== Brewer–Dobson circulation ===
'''Working Groups:''' WGI

'''Definition:''' The meridional overturning circulation of the stratosphere transporting air upward in the tropics, poleward to the winter hemisphere, and downward at polar and subpolar latitudes. The Brewer–Dobson circulation is driven by the interaction between upward propagating planetary waves and the mean flow.

=== Burden ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The total mass of a substance of concern in the atmosphere.

=== Business as usual ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Business as usual (BAU)

'''Definition:''' The term business as usual scenario has been used to describe a scenario that assumes no additional policies beyond those currently in place and that patterns of socio-economic development are consistent with recent trends. The term is now used less frequently than in the past.

== C ==

=== CMIP3, CMIP5 and CMIP6 ===
'''Definition:''' Coupled Model Intercomparison Project Phase 3, Phase 5 and Phase 6.

=== CO2 equivalent emission ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The amount of carbon dioxide (CO2) emission that would have an equivalent effect on a specified key measure of climate change, over a specified time horizon, as an emitted amount of another greenhouse gas (GHG) or a mixture of other GHGs. For a mix of GHGs it is obtained by summing the CO2-equivalent emissions of each gas. There are various ways and time horizons to compute such equivalent emissions (see greenhouse gas emission metric). CO2-equivalent emissions are commonly used to compare emissions of different GHGs, but should not be taken to imply that these emissions have an equivalent effect across all key measures of climate change. [Note: Under the Paris Rulebook (Decision 18/CMA.1, annex, paragraph 37), parties have agreed to use GWP-100 values from the IPCC AR5 or GWP-100 values from a subsequent IPCC Assessment Report to report aggregate emissions and removals of GHGs. In addition, parties may use other metrics to report supplemental information on aggregate emissions and removals of GHGs.]

=== Calcification ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The process of biologically precipitating calcium carbonate minerals to create organism shells, skeletons, otoliths, or other body structures. The chemical equation describing calcification is Ca 2+ (aq) + 2HCO 3 − (aq) → CaCO 3 (s) + CO 2 + H 2 O. Aragonite and calcite are two common crystalline forms of biologically precipitated calcium carbonate minerals that have different solubilities.

=== Calving ===
'''Working Groups:''' WGI

'''Also known as:''' Calving (of glaciers or ice sheets)

'''Definition:''' The breaking off of discrete pieces of ice from a glacier, ice sheet or an ice shelf into lake or seawater, producing icebergs. This is a form of mass loss from an ice body.

=== Canopy temperature ===
'''Working Groups:''' WGI

'''Definition:''' The temperature within the canopy of a vegetation structure.

=== Capacity building ===
'''Working Groups:''' WGII

'''Definition:''' The practice of enhancing the strengths and attributes of, and resources available to, an individual, community, society or organisation to respond to change.

=== Carbon–climate feedback ===
'''Working Groups:''' WGI

'''Definition:''' A climate feedback involves changes in the properties of the land and ocean carbon cycle in response to climate change. In the ocean, changes in oceanic temperature and circulation could affect the atmosphere–ocean carbon dioxide (CO2) flux; on the continents, climate change could affect plant photosynthesis and soil microbial respiration and hence the flux of CO2 between the atmosphere and the land biosphere.

=== Carbon cycle ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The flow of carbon (in various forms, e.g., as 2) carbon dioxide (CO, carbon in biomass, and carbon dissolved in the ocean as carbonate and bicarbonate) through the atmosphere, hydrosphere, terrestrial and marine biosphere and lithosphere. In this report, the reference unit for the global carbon cycle is GtCO 2 or GtC (one Gigatonne = 1 Gt = 10 15 grams; 1 GtC corresponds to 3.664 GtCO 2).

=== Carbon dioxide ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Carbon dioxide (CO2)

'''Definition:''' A naturally occurring gas, CO 2 is also a by-product of burning fossil fuels (such as oil, gas and coal), of burning biomass, of land-use changes (LUCs) and of industrial processes (e.g., cement production). It is the principal anthropogenic greenhouse gas (GHG) that affects the Earth’s radiative balance. It is the reference gas against which other GHGs are measured and therefore has a global warming potential (GWP) of 1.

=== Carbon dioxide fertilisation ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Carbon dioxide (CO2) fertilisation

'''Definition:''' The increase of plant photosynthesis and water-use efficiency in response to increased atmospheric carbon dioxide (CO2) concentration. Whether this increased photosynthesis translates into increased plant growth and carbon storage on land depends on the interacting effects of temperature, moisture and nutrient availability.

=== Carbon dioxide capture and storage ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Carbon dioxide capture and storage (CCS)

'''Definition:''' A process in which a relatively pure stream of carbon dioxide (CO2) from industrial and energy-related sources is separated (captured), conditioned, compressed and transported to a storage location for long-term isolation from the atmosphere. Sometimes referred to as carbon capture and storage.

=== Carbon dioxide capture and utilisation ===
'''Working Groups:''' WGIII

'''Also known as:''' Carbon dioxide capture and utilisation (CCU)

'''Definition:''' A process in which carbon dioxide (CO2) is captured and the carbon then used in a product. The climate effect of CCU depends on the product lifetime, the product it displaces, and the CO2 source (fossil, biomass or atmosphere). CCU is sometimes referred to as Carbon Dioxide Capture and Use, or Carbon Capture and Utilisation.

=== Carbon dioxide removal ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Carbon dioxide removal (CDR)

'''Definition:''' Anthropogenic activities removing carbon dioxide (CO 2) from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical CO 2 sinks and direct air carbon dioxide capture and storage (DACCS) but excludes natural CO 2 uptake not directly caused by human activities.

=== Carbon feedback ===
'''Working Groups:''' WGI

'''Definition:''' A climate feedback involves changes in the properties of the land and ocean carbon cycle in response to climate change. In the ocean, changes in oceanic temperature and circulation could affect the atmosphere–ocean carbon dioxide (CO2) flux; on the continents, climate change could affect plant photosynthesis and soil microbial respiration and hence the flux of CO2 between the atmosphere and the land biosphere.

=== Carbon footprint ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Measure of the exclusive total amount of emissions of carbon dioxide (CO2) that is directly and indirectly caused by an activity or is accumulated over the lifecycle stages of a product (Wiedmann and Minx, 2008).

=== Carbon intensity ===
'''Working Groups:''' WGIII

'''Definition:''' The amount of emissions of carbon dioxide (CO2) released per unit of another variable such as gross domestic product (GDP), output energy use or transport.

=== Carbon neutrality ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Condition in which anthropogenic carbon dioxide (CO2) emissions associated with a subject are balanced by anthropogenic CO2 removals. The subject can be an entity such as a country, an organisation, a district or a commodity, or an activity such as a service and an event. Carbon neutrality is often assessed over the lifecycle including indirect (‘scope 3’) emissions, but can also be limited to the emissions and removals, over a specified period, for which the subject has direct control, as determined by the relevant scheme. [Note 1: Carbon neutrality and net zero CO2 emissions are overlapping concepts. The concepts can be applied at global or sub-global scales (e.g., regional, national and sub-national). At a global scale, the terms carbon neutrality and net zero CO2 emissions are equivalent. At sub-global scales, net zero CO2 emissions is generally applied to emissions and removals under direct control or territorial responsibility of the reporting entity, while carbon neutrality generally includes emissions and removals within and beyond the direct control or territorial responsibility of the reporting entity. Accounting rules specified by greenhouse gas (GHG) programmes or schemes can have a significant influence on the quantification of relevant CO2 emissions and removals. Note 2: In some cases achieving carbon neutrality may rely on the supplementary use of offsets to balance emissions that remain after actions by the reporting entity are taken into account.]

=== Carbon price ===
'''Working Groups:''' WGIII

'''Definition:''' The price for avoided or released carbon dioxide (CO2) or CO2-equivalent emissions. This may refer to the rate of a carbon tax, or the price of emission permits. In many models that are used to assess the economic costs of mitigation, carbon prices are used as a proxy to represent the level of effort in mitigation policies.

=== Carbon sequestration ===
'''Working Groups:''' WGI

'''Definition:''' The process of storing carbon in a carbon pool.

=== Carbon stock ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The quantity of carbon in a carbon pool.

=== Carbonaceous aerosol ===
'''Working Groups:''' WGI

'''Definition:''' Aerosol consisting predominantly of organic substances and black carbon.

=== Carbonate pump ===
'''Working Groups:''' WGI

'''Definition:''' Ocean carbon fixation through the biological formation of carbonates, primarily by plankton that generate bio-mineral particles that sink to the ocean interior, and possibly the sediment. It is also called carbonate counter-pump, since the formation of calcium carbonate (CaCO 3) is accompanied by the release of carbon dioxide (CO 2) to surrounding water and subsequently to the atmosphere.

=== Cascading impacts ===
'''Working Groups:''' WGII

'''Definition:''' Cascading impacts from extreme weather/climate events occur when an extreme hazard generates a sequence of secondary events in natural and human systems that result in physical, natural, social or economic disruption, whereby the resulting impact is significantly larger than the initial impact. Cascading impacts are complex and multi-dimensional, and are associated more with the magnitude of vulnerability than with that of the hazard (modified from Pescaroli and Alexander, 2015).

=== Catchment ===
'''Working Groups:''' WGI; WGII

'''Definition:''' An area that collects and drains precipitation.

=== Cenozoic Era ===
'''Working Groups:''' WGI

'''Definition:''' The third and current geological Era, which began 66.0 Ma. It comprises the Paleogene, Neogene and Quaternary Periods.

=== Central Pacific El Niño ===
'''Working Groups:''' WGI

'''Definition:''' An El Niño event in which sea surface temperature anomalies are stronger in the central equatorial Pacific than in the east. Also known as a Modoki El Niño event.

=== Chaotic ===
'''Working Groups:''' WGI

'''Definition:''' A dynamical system such as the climate system, governed by non-linear deterministic equations, may exhibit erratic or chaotic behaviour in the sense that very small changes in the initial state of the system lead to large and apparently unpredictable changes in its temporal evolution. Such chaotic behaviour limits the predictability of the state of a non-linear dynamical system at specific future times, although changes in its statistics may still be predictable given changes in the system parameters or boundary conditions.

=== Charcoal ===
'''Working Groups:''' WGI

'''Definition:''' Material resulting from charring of biomass, usually retaining some of the microscopic texture typical of plant tissues; chemically it consists mainly of carbon with a disturbed graphitic structure, with lesser amounts of oxygen and hydrogen.

=== Chlorofluorocarbons ===
'''Working Groups:''' WGI

'''Also known as:''' Chlorofluorocarbons (CFCs)

'''Definition:''' An organic compound that contains chlorine, carbon, hydrogen, and fluorine and is used for refrigeration, air conditioning, packaging, plastic foam, insulation, solvents, or aerosol propellants. Because they are not destroyed in the lower atmosphere, CFCs drift into the upper atmosphere where, given suitable conditions, they lead to ozone (O3) depletion. They are some of the greenhouse gases (GHGs) covered under the 1987 Montreal Protocol as a result of which manufacturing of these gases has been phased out, and they are being replaced by other compounds, including hydrofluorocarbons (HFCs).

=== Choice architecture ===
'''Working Groups:''' WGIII

'''Definition:''' The presentation of choices to consumers, and the impact that presentation has on consumer decision-making.

=== Chronology ===
'''Working Groups:''' WGI

'''Definition:''' Arrangement of events according to dates or times of occurrence.

=== Circular economy ===
'''Working Groups:''' WGIII

'''Definition:''' A system with minimal input and operational losses of materials and energy through extensive reduce, reuse, recycling, and recovery activities. Ten strategies for circularity include: Refuse, Rethink, Reduce, Reuse, Repair, Refurbish, Remanufacture, Repurpose, Recycle, Recover.

=== Cirrus cloud thinning ===
'''Working Groups:''' WGI

'''Also known as:''' Cirrus cloud thinning (CCT)

'''Definition:''' One of several radiation modification approaches to counter the warming caused by greenhouse gases (GHGs). In this approach, it is proposed to reduce the amount of cirrus clouds by injecting ice nucleating substances in the upper troposphere. The reduction in cirrus clouds is expected to increase the amount of longwave cooling to space resulting in a planetary cooling. Although cirrus cloud thinning primarily affects the longwave radiation budget of our planet, it is often identified as one of the solar radiation modification (SRM) approaches in the literature.

=== Cities ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Cities are open systems, continually exchanging resources, products and services, waste, people, ideas and finances with the hinterlands and broader world. Cities are complex, self-organising, adaptive and constantly evolving. Cities also encompass multiple actors with varying responsibilities, capabilities and priorities, as well as processes that transcend the institutional sector-based approach to city administration. Cities are embedded in broader ecological, economic, technical, institutional, legal and governance structures that enable or often constrain their systemic function, which cannot be separated from wider power relations. Urban processes of a physical, social and economic nature are causally interlinked, with interactions and feedbacks that result in both intended and unintended impacts on emissions.

=== Citizen science ===
'''Working Groups:''' WGIII

'''Definition:''' A voluntary participation of the public in the collection and/or processing of data as part of a scientific study (Silvertown, 2009).

=== City region ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The areal extent of an individual city's material associations and economic or political influence. The city region concept accepts that rural livelihoods and land uses can be incorporated within the functional activities of a city. This will include dormitory settlements, sources for critical inputs of water, some food, and waste disposal.

=== Clathrate ===
'''Working Groups:''' WGI

'''Also known as:''' Clathrate (methane)

'''Definition:''' A partly frozen slushy mix of methane gas and ice, usually found in sediments.

=== Clausius–Clapeyron equation/relationship ===
'''Working Groups:''' WGI

'''Definition:''' The thermodynamic relationship between temperature and the vapour pressure of a substance in which two phases of the substance are in equilibrium (e.g., liquid water and water vapour). For gases such as water vapour, this relation gives the increase in equilibrium (or saturation) vapour pressure per unit change in air temperature.

=== Climate ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' In a narrow sense, climate is usually defined as the average weather, or more rigorously as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization (WMO). The relevant quantities are most often surface variables such as temperature, precipitation and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.

=== Climate–carbon cycle feedback ===
'''Working Groups:''' WGI

'''Definition:''' A climate feedback involves changes in the properties of the land and ocean carbon cycle in response to climate change. In the ocean, changes in oceanic temperature and circulation could affect the atmosphere –ocean carbon dioxide (CO 2) flux; on the continents, climate change could affect plant photosynthesis and soil microbial respiration and hence the flux of CO 2 between the atmosphere and the land biosphere.

=== Climate change ===
'''Definition:''' A change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use. Note that the United Nations Framework Convention on Climate Change (UNFCCC), in its Article 1, defines climate change as: ’a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods’. The UNFCCC thus makes a distinction between climate change attributable to human activities altering the atmospheric composition and climate variability attributable to natural causes.

=== Climate change commitment ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Unavoidable future climate change resulting from inertia in the geophysical and socio-economic systems. Different types of climate change commitment are discussed in the literature (see subterms). Climate change commitment is usually quantified in terms of the further change in temperature, but it includes other future changes, for example in the hydrological cycle, in extreme weather events, in extreme climate events, and in sea level.

=== Climate extreme ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Climate extreme (extreme weather or climate event)

'''Definition:''' The occurrence of a value of a weather or climate variable above (or below) a threshold value near the upper (or lower) ends of the range of observed values of the variable. By definition, the characteristics of what is called extreme weather may vary from place to place in an absolute sense. When a pattern of extreme weather persists for some time, such as a season, it may be classified as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., high temperature, drought, or heavy rainfall over a season). For simplicity, both extreme weather events and extreme climate events are referred to collectively as climate extremes.

=== Climate feedback ===
'''Working Groups:''' WGI; WGII

'''Definition:''' An interaction in which a perturbation in one climate quantity causes a change in a second and the change in the second quantity ultimately leads to an additional change in the first. A negative feedback is one in which the initial perturbation is weakened by the changes it causes; a positive feedback is one in which the initial perturbation is enhanced. The initial perturbation can either be externally forced or arise as part of internal variability.

=== Climate feedback parameter ===
'''Working Groups:''' WGI

'''Definition:''' A way to quantify the radiative response of the climate system to a global surface temperature change induced by a radiative forcing. It is quantified as the change in net energy flux at the top of atmosphere for a given change in annual global surface temperature. It has units of W m -2 °C -1.

=== Climate finance ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' There is no agreed definition of climate finance. The term ‘climate finance‘ is applied to the financial resources devoted to addressing climate change by all public and private actors from global to local scales, including international financial flows to developing countries to assist them in addressing climate change. Climate finance aims to reduce net greenhouse gas emissions and/or to enhance adaptation and increase resilience to the impacts of current and projected climate change. Finance can come from private and public sources, channelled by various intermediaries, and is delivered by a range of instruments, including grants, concessional and non-concessional debt, and internal budget reallocations.

=== Climate forecast ===
'''Working Groups:''' WGI

'''Definition:''' A climate prediction or climate forecast is the result of an attempt to produce (starting from a particular state of the climate system) an estimate of the actual evolution of the climate in the future, for example, at seasonal, interannual or decadal time scales. Because the future evolution of the climate system may be highly sensitive to initial conditions, has chaotic elements and is subject to natural variability, such predictions are usually probabilistic in nature.

=== Climate governance ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The structures, processes and actions through which private and public actors seek to mitigate and adapt to climate change.

=== Climate index ===
'''Working Groups:''' WGI

'''Definition:''' A time series constructed from climate variables that provides an aggregate summary of the state of the climate system. For example, the difference between sea level pressure in Iceland and the Azores provides a simple yet useful historical North Atlantic Oscillation (NAO) index. Because of their optimal properties, climate indices are often defined using principal components — linear combinations of climate variables at different locations that have maximum variance subject to certain normalization constraints (e.g., the Northern Annular Mode (NAM) and Southern Annular Mode (SAM) indices which are principal components of Northern Hemisphere and Southern Hemisphere gridded pressure anomalies, respectively). Definitions of observational indices for Modes of climate variability can be found in Annex VI of the AR6 WGI report.

=== Climate indicator ===
'''Working Groups:''' WGI

'''Definition:''' Measures of the climate system including large-scale variables and climate proxies.

=== Climate information ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Information about the past, current or future state of the climate system that is relevant for mitigation, adaptation and risk management. It may be tailored or “co‑produced“ for specific contexts, taking into account users’ needs and values.

=== Climate justice ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Justice that links development and human rights to achieve a human-centred approach to addressing climate change, safeguarding the rights of the most vulnerable people and sharing the burdens and benefits of climate change and its impacts equitably and fairly (MRFCJ, 2018).

=== Climate literacy ===
'''Working Groups:''' WGII

'''Definition:''' Climate literacy encompasses being aware of climate change, its anthropogenic causes and implications.

=== Climate metrics ===
'''Working Groups:''' WGI

'''Definition:''' Measures of aspects of the overall climate system response to radiative forcing, such as equilibrium climate sensitivity (ECS), transient climate response (TCR), transient climate response to cumulative CO2 emissions (TCRE) and the airborne fraction of anthropogenic carbon dioxide.

=== Climate model ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A qualitative or quantitative representation of the climate system based on the physical, chemical and biological properties of its components, their interactions and feedback processes and accounting for some of its known properties. The climate system can be represented by models of varying complexity; that is, for any one component or combination of components a spectrum or hierarchy of models can be identified, differing in such aspects as the number of spatial dimensions, the extent to which physical, chemical or biological processes are explicitly represented, or the level at which empirical parametrisations are involved. There is an evolution towards more complex models with interactive chemistry and biology. Climate models are applied as a research tool to study and simulate the climate and for operational purposes, including monthly, seasonal and interannual climate predictions.

=== Climate pattern ===
'''Working Groups:''' WGI

'''Definition:''' A set of spatially varying coefficients obtained by ‘projection’ (regression) of climate variables onto a climate index time series. When the climate index is a principal component, the climate pattern is an eigenvector of the covariance matrix, referred to as an empirical orthogonal function (EOF) in climate science.

=== Climate prediction ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A climate prediction or climate forecast is the result of an attempt to produce (starting from a particular state of the climate system) an estimate of the actual evolution of the climate in the future, for example, at seasonal, interannual or decadal time scales. Because the future evolution of the climate system may be highly sensitive to initial conditions, has chaotic elements and is subject to natural variability, such predictions are usually probabilistic in nature.

=== Climate projection ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Simulated response of the climate system to a scenario of future emissions or concentrations of greenhouse gases (GHGs) and aerosols and changes in land use, generally derived using climate models. Climate projections are distinguished from climate predictions by their dependence on the emission/concentration/radiative forcing scenario used, which is in turn based on assumptions concerning, for example, future socio-economic and technological developments that may or may not be realised.

=== Climate refugium ===
'''Working Groups:''' WGII

'''Definition:''' A climate refugium is a geographic area that has had a stable climate on evolutionary time scales, or that is projected to have a stable climate into the future.

=== Climate resilient development ===
'''Working Groups:''' WGII

'''Definition:''' In the WGII report, climate resilient development refers to the process of implementing greenhouse gas mitigation and adaptation measures to support sustainable development for all.

=== Climate resilient development pathways ===
'''Working Groups:''' WGII

'''Also known as:''' Climate resilient development pathways (CRDPs)

'''Definition:''' Trajectories that strengthen sustainable development and efforts to eradicate poverty and reduce inequalities while promoting fair and cross-scalar adaptation to and resilience in a changing climate. They raise the ethics, equity and feasibility aspects of the deep societal transformation needed to drastically reduce emissions to limit global warming (e.g., to well below 2°C) and achieve desirable and liveable futures and well-being for all.

=== Climate-resilient pathways ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Iterative processes for managing change within complex systems in order to reduce disruptions and enhance opportunities associated with climate change.

=== Climate response ===
'''Working Groups:''' WGI

'''Definition:''' A general term for how the climate system responds to a radiative forcing.

=== Climate sensitivity ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The change in the surface temperature in response to a change in the atmospheric carbon dioxide (CO 2) concentration or other radiative forcing.

=== Climate services ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Climate services involve the provision of climate information in such a way as to assist decision-making. The service includes appropriate engagement from users and providers, is based on scientifically credible information and expertise, has an effective access mechanism and responds to user needs (Hewitt et al., 2012).

=== Climate simulation ensemble ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A group of parallel model simulations characterising historical climate conditions, climate predictions, or climate projections. Variation of the results across the ensemble members may give an estimate of modelling-based uncertainty. Ensembles made with the same model but different initial conditions characterise the uncertainty associated with internal climate variability, whereas multi-model ensembles including simulations by several models also include the effect of model differences. Perturbed parameter ensembles, in which model parameters are varied in a systematic manner, aim to assess the uncertainty resulting from internal model specifications within a single model. Remaining sources of uncertainty unaddressed with model ensembles are related to systematic model errors or biases, which may be assessed from systematic comparisons of model simulations with observations wherever available.

=== Climate-smart agriculture ===
'''Working Groups:''' WGII

'''Also known as:''' Climate-smart agriculture (CSA)

'''Definition:''' An approach to agriculture that aims to transform and reorient agricultural systems to effectively support development and ensure food security in a changing climate by sustainably increasing agricultural productivity and incomes, adapting and building resilience to climate change, and reducing and/or removing greenhouse gas emissions, where possible (FAO, 2018).

=== Climate system ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The global system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere and the interactions between them. The climate system changes in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations, orbital forcing, and anthropogenic forcings such as the changing composition of the atmosphere and land-use change.

=== Climate threshold ===
'''Working Groups:''' WGI

'''Definition:''' A limit within the climate system (or its forcing) beyond which the behaviour of the system is qualitatively changed.

=== Climate variability ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Deviations of climate variables from a given mean state (including the occurrence of extremes, etc.) at all spatial and temporal scales beyond that of individual weather events. Variability may be intrinsic, due to fluctuations of processes internal to the climate system (internal variability), or extrinsic, due to variations in natural or anthropogenic external forcing (forced variability).

=== Climate velocity ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The speed at which isolines of a specified climate variable travel across landscapes or seascapes due to changing climate. For example, climate velocity for temperature is the speed at which isotherms move due to changing climate (km yr -1) and is calculated as the temporal change in temperature (°C yr -1) divided by the current spatial gradient in temperature (°C km -1). It can be calculated using additional climate variables such as precipitation or can be based on the climatic niche of organisms.

=== Climatic driver ===
'''Working Groups:''' WGII

'''Also known as:''' Climatic driver (Climate driver)

'''Definition:''' A changing aspect of the climate system that influences a component of a human or natural system.

=== Climatic impact-driver ===
'''Working Groups:''' WGI

'''Also known as:''' Climatic impact-driver (CID)

'''Definition:''' Physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral or a mixture of each across interacting system elements and regions.

=== Cloud condensation nuclei ===
'''Working Groups:''' WGI

'''Also known as:''' Cloud condensation nuclei (CCN)

'''Definition:''' The subset of aerosol particles that serve as an initial site for the condensation of liquid water, which can lead to the formation of cloud droplets, under typical cloud formation conditions. The main factor that determines which aerosol particles are CCN at a given supersaturation is their size.

=== Cloud feedback ===
'''Working Groups:''' WGI

'''Definition:''' A climate feedback involving changes in any of the properties of clouds as a response to a change in the local or global surface temperature. Understanding cloud feedbacks and determining their magnitude and sign requires an understanding of how a change in climate may affect the spectrum of cloud types, the cloud fraction and height, the radiative properties of clouds, and finally the Earth’s radiation budget.

=== Cloud radiative effect ===
'''Working Groups:''' WGI

'''Definition:''' The radiative effect of clouds relative to the identical situation without clouds.

=== Cloud-resolving models ===
'''Working Groups:''' WGI

'''Also known as:''' Cloud-resolving models (CRMs)

'''Definition:''' Numerical models that are that are of high enough resolution and have the necessary physics to represent the dynamical and physical processes of cloud formation.

=== Co-benefits ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A positive effect that a policy or measure aimed at one objective has on another objective, thereby increasing the total benefit to society or the environment. Co-benefits are also referred to as ancillary benefits.

=== Coast ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The land near to the sea. The term ‘coastal’ can refer to that land (e.g., as in ‘coastal communities’), or to that part of the marine environment that is strongly influenced by land-based processes. Thus, coastal seas are generally shallow and near-shore. The landward and seaward limits of the coastal zone are not consistently defined, neither scientifically nor legally. Thus, coastal waters can either be considered as equivalent to territorial waters (extending 12 nautical miles/22.2 km from mean low water), or to the full exclusive economic zone, or to shelf seas, with less than 200 m water depth.

=== Coastal erosion ===
'''Working Groups:''' WGII

'''Definition:''' Coastal erosion, sometimes referred to as shoreline retreat, occurs when a net loss of sediment or bedrock from the shoreline results in landward movement of the high-tide mark.

=== Cold days/cold nights ===
'''Working Groups:''' WGI

'''Definition:''' Days where maximum temperature, or nights where minimum temperature, falls below the 10th percentile, where the respective temperature distributions are generally defined with respect to the 1961-1990 reference period. For the corresponding indices, see Box 2.4.

=== Common era ===
'''Working Groups:''' WGI

'''Also known as:''' Common era (CE)

'''Definition:''' CE (Common Era) and BCE (Before the Common Era) are alternative names for AD (Anno Domini) and BC (Before Christ) in the Gregorian international standard calendar-year system. CE/BCE are preferred in an international context because they are neutral with respect to religion. The numbering of calendar years is the same under both terminologies. The CE began in year AD 1 and extends to the present day.

=== Communicable disease ===
'''Working Groups:''' WGII

'''Definition:''' Illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector or the inanimate environment. Communicable disease pathogens include bacteria, viruses, fungi, parasites and prions.

=== Community-based adaptation ===
'''Working Groups:''' WGII

'''Definition:''' Local, community-driven adaptation. Community-based adaptation focuses attention on empowering and promoting the adaptive capacity of communities. It is an approach that takes context, culture, knowledge, agency, and preferences of communities as strengths.

=== Compatible emissions ===
'''Working Groups:''' WGI

'''Definition:''' Earth system models that simulate the land and ocean carbon cycle can calculate 2) carbon dioxide (CO emissions that are compatible with a given atmospheric CO 2 concentration trajectory. The compatible emissions over a given period of time are equal to the increase of carbon over that same period of time in the sum of the three active reservoirs: the atmosphere, the land and the ocean.

=== Compound risks ===
'''Working Groups:''' WGII

'''Definition:''' arise from the interaction of hazards, which may be characterised by single extreme events or multiple coincident or sequential events that interact with exposed systems or sectors.

=== Compound weather/climate events ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The terms ‘compound events’, ‘compound extremes’ and ‘compound extreme events’ are used interchangeably in the literature and this report and refer to the combination of multiple drivers and/or hazards that contributes to societal and/or environmental risk (Zscheischler et al., 2018).

=== Concentrations scenario ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A plausible representation of the future development of atmospheric concentrations of substances that are radiatively active (e.g., greenhouse gases (GHGs), aerosols, tropospheric ozone), plus human-induced land-cover changes that can be radiatively active via albedo changes, and often used as input to a climate model to compute climate projections.

=== Conference of the Parties ===
'''Working Groups:''' WGIII

'''Also known as:''' Conference of the Parties (COP)

'''Definition:''' The supreme body of UN conventions, such as the United Nations Framework Convention on Climate Change (UNFCCC), comprising parties with a right to vote that have ratified or acceded to the convention. UN Climate Change Conference

=== Confidence ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The robustness of a finding based on the type, amount, quality and consistency of evidence (e.g., mechanistic understanding, theory, data, models, expert judgement) and on the degree of agreement across multiple lines of evidence. In this report, confidence is expressed qualitatively (Mastrandrea et al., 2010).

=== Conservation agriculture ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A farming system that promotes minimum soil disturbance (e.g., by using no till practices), maintenance of a permanent soil cover and diversification of plant species. It aims to prevent land degradation and regenerate degraded lands by enhancing biodiversity and natural biological processes above and below the ground surface, that contribute to increased water and nutrient use efficiency and improved and sustained crop production (FAO, 2016).

=== Constant composition commitment ===
'''Working Groups:''' WGI

'''Definition:''' The constant composition commitment is the remaining climate change that would result if atmospheric composition, and hence radiative forcing, were held fixed at a given value. It results from the thermal inertia of the ocean and slow processes in the cryosphere and land surface.

=== Constant emissions commitment ===
'''Working Groups:''' WGI

'''Definition:''' The constant emissions commitment is the committed climate change that would result from keeping anthropogenic emissions constant.

=== Consumption-based emissions ===
'''Working Groups:''' WGIII

'''Definition:''' Emissions released to the atmosphere in order to generate the goods and services consumed by a certain entity (e.g., a person, firm, country, or region).

=== Convection ===
'''Working Groups:''' WGI

'''Definition:''' Vertical motion driven by buoyancy forces arising from static instability, usually caused by near-surface cooling or increases in salinity in the case of the ocean and near-surface warming or cloud-top radiative cooling in the case of the atmosphere. In the atmosphere, convection gives rise to cumulus clouds and precipitation and is effective at both scavenging and vertically transporting chemical species. In the ocean, convection can carry surface waters to deep within the ocean.

=== Coping ===
'''Working Groups:''' WGII

'''Definition:''' The use of available skills, resources and opportunities to address, manage and overcome adverse conditions, with the aim of achieving basic functioning of people, institutions, organisations and systems in the short to medium term (UNISDR, 2009; IPCC, 2012a).

=== Coping capacity ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The ability of people, institutions, organisations and systems, using available skills, values, beliefs, resources, and opportunities, to address, manage and overcome adverse conditions in the short to medium term (UNISDR, 2009; IPCC, 2012).

=== Coral bleaching ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Loss of coral pigmentation through the loss of intracellular symbiotic algae (known as zooxanthellae) and/or loss of their pigments.

=== Coral reef ===
'''Working Groups:''' WGI; WGII

'''Definition:''' An underwater ecosystem characterised by structure-building stony corals. Warm-water coral reefs occur in shallow seas, mostly in the tropics, with the corals (animals) containing algae (plants) that depend on light and relatively stable temperature conditions. Cold-water coral reefs occur throughout the world, mostly at water depths of 50–500 m. In both kinds of reef, living corals frequently grow on older, dead material, predominantly made of calcium carbonate (CaCO 3). Both warm and cold-water coral reefs support high biodiversity of fish and other groups, and are considered to be especially vulnerable to climate change. From Wikipedia A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

=== Cosmogenic radioisotopes ===
'''Working Groups:''' WGI

'''Definition:''' Rare radioactive isotopes that are created by the interaction of high-energy cosmic ray particles with atomic nuclei. They are often used as indicator of solar activity which modulates the cosmic rays’ intensity or as tracers of atmospheric transport processes, and are also called cosmogenic radionuclides.

=== Cost–benefit analysis ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A type of economic evaluation that compares all monetised all monetised negative and positive impacts associated with a given action. Cost–benefit analysis enables comparison of different interventions, investments or strategies, and reveals how a given investment or policy effort pays off for a particular person, company or country, or at a global scale. Cost–benefit analyses representing society’s point of view are important for climate change decision-making, but there are difficulties in aggregating costs and benefits across different actors and across time scales.

=== Cost-effectiveness analysis ===
'''Working Groups:''' WGIII

'''Also known as:''' Cost-effectiveness analysis (CEA)

'''Definition:''' A type of economic evaluation that compares the costs of different courses of action reaching the same outcome. In this report, CEA focuses on comparing the costs of mitigation strategies designed to meet a prespecified climate change mitigation goal (e.g., an emission-reduction target or a temperature stabilisation target).

=== Coupled Model Intercomparison Project ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Coupled Model Intercomparison Project (CMIP)

'''Definition:''' A climate modelling activity from the World Climate Research Programme (WCRP) which coordinates and archives climate model simulations based on shared model inputs by modelling groups from around the world. The (CMIP3) multi-model data set includes projections using Special Report on Emissions Scenarios (SRES) scenarios. The (CMIP5) data set includes projections using the Representative Concentration Pathways (RCP). The CMIP6 phase involves a suite of common model experiments as well as an ensemble of CMIP-endorsed Model Intercomparison Projects (MIPs).

=== Cryosphere ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The components of the Earth system at and below the land and ocean surface that are frozen, including snow cover, glaciers, ice sheets, ice shelves, icebergs, sea ice, lake ice, river ice, permafrost and seasonally frozen ground.

=== Cultural impacts ===
'''Working Groups:''' WGII

'''Definition:''' Impacts on material and ecological aspects of culture and the lived experience of culture, including dimensions such as identity, community cohesion and belonging, sense of place, worldview, values, perceptions, and tradition. Cultural impacts are closely related to ecological impacts, especially for iconic and representational dimensions of species and landscapes. Culture and cultural practices frame the importance and value of the impacts of change, shape the feasibility and acceptability of adaptation options, and provide the skills and practices that enable adaptation.

=== Cumulative emissions ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The total amount of emissions released over a specified period of time.

== D ==

=== Dansgaard-Oeschger events ===
'''Working Groups:''' WGI

'''Also known as:''' Dansgaard-Oeschger events (D-O events)

'''Definition:''' Millennial-scale events first characterized in Greenland ice cores as abrupt warming from a cold stadial state to a warmer interstadial state, followed by a return to a cold stadial state (Dansgaard et al., 1993), and traced in the ocean via deposits of ice-rafted sand grains (Bond and Lotti, 1995). Named after Willi Dansgaard and Hans Oeschger by Bond and Lotti (1995). An example of a D–O event during the most recent deglacial transition is the Bølling–Allerød interstadial. Warm D–O events in Greenland are associated with cooling events in Antarctica (Blunier and Brook, 2001) through ocean thermohaline circulation (Stocker and Johnsen, 2003).

=== Data assimilation ===
'''Working Groups:''' WGI

'''Definition:''' Mathematical method used to combine different sources of information in order to produce the best possible estimate of the state of a system. This information usually consists of observations of the system and a numerical model of the system evolution. Data assimilation techniques are used to create initial conditions for weather forecast models and to construct reanalyses describing the trajectory of the climate system over the time period covered by the observations.

=== Dead zones ===
'''Working Groups:''' WGI

'''Definition:''' Extremely hypoxic (i.e., low-oxygen) areas in oceans and lakes, caused by excessive nutrient input from human activities coupled with other factors that deplete the oxygen required to support many marine organisms in bottom and near-bottom water.

=== Decadal predictability ===
'''Working Groups:''' WGI

'''Definition:''' Refers to the notion of predictability of the climate system on a decadal time scale.

=== Decadal prediction ===
'''Working Groups:''' WGI

'''Definition:''' A climate prediction on decadal time scales.

=== Decadal variability ===
'''Working Groups:''' WGI

'''Definition:''' Decadal variability refers to climate variability on decadal time scales.

=== Decarbonisation ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Human actions to reduce carbon dioxide emissions from human activities.

=== Decent Living Standard ===
'''Working Groups:''' WGIII

'''Definition:''' A set of minimal material requirements essential for achieving basic human well-being including nutrition, shelter, basic living conditions, clothing, healthcare, education, and mobility (Rao and Baer 2012; Rao and Min 2018; O’Neill et al. 2018).

=== Decoupling ===
'''Working Groups:''' WGIII

'''Definition:''' Decoupling (in relation to climate change) is where economic growth is no longer strongly associated with another relevant indicator such as greenhouse gas emissions. Relative decoupling is where both these indicators grow but the other indicators grow more slowly than the economy. Absolute decoupling is where there is economic growth but there is a decline in the other indicator.

=== Deep uncertainty ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A situation of deep uncertainty exists when experts or stakeholders do not know or cannot agree on: (1) appropriate conceptual models that describe relationships among key driving forces in a system, (2) the probability distributions used to represent uncertainty about key variables and parameters and/or (3) how to weigh and value desirable alternative outcomes (Lempert et al., 2003).

=== Deforestation ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Conversion of forest to non-forest. [Note: For a discussion of the term forest and related terms such as afforestation, reforestation and deforestation, see the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and their 2019 Refinement, and information provided by the United Nations Framework Convention on Climate Change (IPCC 2006, 2019; UNFCCC 2021a, b).]

=== Deglacial or deglaciation or glacial termination ===
'''Working Groups:''' WGI

'''Definition:''' The period of transition from glacial conditions at the end of a glacial period to interglacial conditions characterized by a reduction in land ice volume. Gradual changes can be punctuated by abrupt changes linked to stadial /interstadial events and bipolar seesaw aspect. The last deglacial transition occurred between about 18,000 and 11,000 years ago. It encompasses rapid events such as Meltwater Pulse 1A (MWP-1A) and millennial-scale fluctuations such as the Younger Dryas.

=== Deliberate transformations ===
'''Working Groups:''' WGII

'''Definition:''' A profound shift towards sustainability, envisioned and intended by at least some societal actors, facilitated by changes in individual and collective values and behaviours, and a fairer balance of political, cultural, and institutional power in society.

=== Deliberative governance ===
'''Working Groups:''' WGIII

'''Definition:''' Deliberative governance involves decision making through inclusive public conversation which allows opportunity for developing policy options through public discussion rather than collating individual preferences through voting or referenda (although the latter governance mechanisms can also be proceded and legitimated by public deliberation processes).

=== Demand ===
'''Working Groups:''' WGIII

'''Definition:''' Disciplinary approaches use the term in different ways. In economics, demand by a consumer is willingness and ability to purchase in a marketplace. However, the motivation for purchase may vary and can include economic utility, welfare, Decent standard of living (DSL), or for the good/services.

=== Demand- and supply-side measures ===
'''Working Groups:''' WGIII


=== Demand-side measures ===
'''Working Groups:''' WGIII

'''Definition:''' Policies and programmes for influencing the demand for goods and/or services. In the energy sector, demand-side mitigation measures aim at reducing the amount of greenhouse gas emissions emitted per unit of energy service used.

=== Desertification ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Land degradation in arid, semi-arid, and dry sub-humid areas resulting from many factors, including climatic variations and human activities (UNCCD, 1994). From Wikipedia Desertification is a type of land degradation in drylands in which biological productivity is lost due to natural processes or induced by human activities whereby fertile areas become arid.

=== Detection ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Detection of change is defined as the process of demonstrating that climate or a system affected by climate has changed in some defined statistical sense, without providing a reason for that change. An identified change is detected in observations if its likelihood of occurrence by chance due to internal variability alone is determined to be small, for example, <10%.

=== Detection and attribution ===
'''Working Groups:''' WGI; WGII

'''Definition:''' See Attribution and Detection

=== Developed/developing countries ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Developed/developing countries (Industrialised/developed/developing countries)

'''Definition:''' There is a diversity of approaches for categorising countries on the basis of their level of development, and for defining terms such as ‘industrialised’, ‘developed’ or ‘developing’. Several categorisations are used in this report. (1) In the United Nations (UN) system, there is no established convention for the designation of developed and developing countries or areas. (2) The UN Statistics Division specifies developed and developing regions based on common practice. In addition, specific countries are designated as Least Developed Countries, landlocked developing countries, Small Island Developing States (SIDS) and transition economies. Many countries appear in more than one of these categories. (3) The World Bank uses income as the main criterion for classifying countries as low, lower middle, upper middle and high income. (4) The UN Development Programme (UNDP) aggregates indicators for life expectancy, educational attainment and income into a single composite Human Development Index (HDI) to classify countries as low, medium, high or very high human development.

=== Development pathways ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Development pathways evolve as the result of the countless decisions being made and actions being taken at all levels of societal structure, as well due to the emergent dynamics within and between institutions, cultural norms, technological systems and other drivers of behavioural change.

=== Diatoms ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Microscopic (2–200 μm) unicellular photosynthetic algae that live in surface waters of lakes, rivers and oceans and form shells of opal. In the global ocean, marine diatom species distribution is primarily driven by nutrient availability. On regional scales, their species distribution in ocean sediment cores can be related to past sea surface temperatures (Abrantes et al., 2013).

=== Diet ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The kinds of food that follow a particular pattern that a person or community eats (FAO and Alliance of Biodiversity International and CIAT, 2021).

=== Dimensions of integration ===
'''Working Groups:''' WGI

'''Definition:''' In IPCC AR6, concepts used to synthesize the knowledge of climate change across not just the physical sciences, but also across impacts, adaptation, and mitigation research. The concept of ‘dimensions of integration’ includes (i) emission and c oncentration scenarios underlying the climate change projections assessed in this report, (ii) levels of projected global mean temperature change and (iii) total amounts of cumulative carbon emissions for projections.

=== Direct air capture ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Direct air capture (DAC)

'''Definition:''' Chemical process by which a pure carbon dioxide (CO2) stream is produced by capturing CO2 from the ambient air. From Wikipedia The carbon dioxide (CO2) is captured directly from the ambient air; this is contrast to carbon capture and storage (CCS) which captures CO2 from point sources, such as a cement factory or a bioenergy plant.

=== Direct air carbon dioxide capture and storage ===
'''Working Groups:''' WGIII

'''Also known as:''' Direct air carbon dioxide capture and storage (DACCS)

'''Definition:''' Chemical process by which carbon dioxide (CO 2) is captured directly from the ambient air, with subsequent storage. Also known as direct air capture and storage (DACS).

=== Direct and indirect services ===
'''Working Groups:''' WGIII

'''Definition:''' Direct Services: Services (e.g., passenger mobility) required by end-users (consumers). Indirect services: Services required (e.g., goods transport, manufacturing) for provisioning systems of direct services.

=== Direct emissions ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Emissions that physically arise from activities within well-defined boundaries of, for instance, a region, an economic sector, a company, or a process.

=== Disaster ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A ‘serious disruption of the functioning of a community or a society at any scale due to hazardous events interacting with conditions of exposure, vulnerability and capacity, leading to one or more of the following: human, material, economic and environmental losses and impacts’ (UNGA, 2016). From Wikipedia A disaster is a serious problem occurring over a period of time that causes widespread human, material, economic or environmental loss which exceeds the ability of the affected community or society to cope using its own resources.

=== Disaster management ===
'''Working Groups:''' WGII

'''Definition:''' Social processes for designing, implementing, and evaluating strategies, policies, and measures that promote and improve disaster preparedness, response, and recovery practices at different organisational and societal levels.

=== Disaster risk ===
'''Working Groups:''' WGII

'''Definition:''' The likelihood over a specified time period of severe alterations in the normal functioning of a community or a society due to hazardous physical events interacting with vulnerable social conditions, leading to widespread adverse human, material, economic, or environmental effects that require immediate emergency response to satisfy critical human needs and that may require external support for recovery.

=== Disaster risk management ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Disaster risk management (DRM)

'''Definition:''' Processes for designing, implementing and evaluating strategies, policies and measures to improve the understanding of current and future disaster risk, foster disaster risk reduction and transfer, and promote continuous improvement in disaster preparedness, prevention and protection, response and recovery practices, with the explicit purpose of increasing human security, well-being, quality of life and sustainable development (SD).

=== Disaster risk reduction ===
'''Working Groups:''' WGII

'''Also known as:''' Disaster risk reduction (DRR)

'''Definition:''' Denotes both a policy goal or objective, and the strategic and instrumental measures employed for anticipating future disaster risk; reducing existing exposure, hazard, or vulnerability; and improving resilience. From Wikipedia Disaster risk reduction (DRR) sometimes called disaster risk management (DRM) is a systematic approach to identifying, assessing and reducing the risks of disaster. It aims to reduce socio-economic vulnerabilities to disaster as well as dealing with the environmental and other hazards that trigger them.

=== Discharge ===
'''Working Groups:''' WGI

'''Also known as:''' Discharge (of ice)

'''Definition:''' Rate of the flow of ice through a vertical section of a glacier perpendicular to the direction of the flow of ice. Often used to refer to the loss of mass at marine-terminating glacier fronts (mostly calving of icebergs and submarine melt), or to mass flowing across the grounding line of a floating ice shelf.

=== Discounting ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A mathematical operation that aims to make monetary (or other) amounts received or expended at different times (years) comparable across time. If the discount rate is positive, future values are given less weight than those today. The choice of discount rate(s) is debated as it is a judgement based on hidden and/or explicit values. From Wikipedia In finance, discounting is a mechanism in which a debtor obtains the right to delay payments to a creditor, for a defined period of time, in exchange for a charge or fee.

=== Disruptive innovation ===
'''Working Groups:''' WGIII

'''Definition:''' Demand-led technological change that leads to significant system change and is characterised by strong exponential growth. From Wikipedia

=== Dissolved inorganic carbon ===
'''Working Groups:''' WGI

'''Definition:''' The combined total of different types of non-organic carbon in (seawater) solution, comprising carbonate (CO 3 2–), bicarbonate (HCO 3 –), carbonic acid (H 2 CO 3) and carbon dioxide (CO 2). From Wikipedia Inorganic carbon is found primarily in simple compounds such as carbon dioxide, carbonic acid, bicarbonate, and carbonate (CO2, H2CO3, HCO− 3, CO2− 3 respectively). Dissolved inorganic carbon (DIC) includes three major aqueous species, CO2, HCO− 3,CO2− 3, and to a lesser extent their complexes in solution with metal ions.

=== Distributive equity ===
'''Working Groups:''' WGIII

'''Definition:''' Equity in the consequences, outcomes, costs and benefits of actions or policies. In the case of climate change or climate policies for different people, places and countries, including equity aspects of sharing burdens and benefits for mitigation and adaptation.

=== Diurnal temperature range ===
'''Working Groups:''' WGI

'''Also known as:''' Diurnal temperature range (DTR)

'''Definition:''' The difference between the maximum and minimum temperature during a 24-hour period.

=== Dobson unit ===
'''Working Groups:''' WGI

'''Also known as:''' Dobson unit (DU)

'''Definition:''' A unit to measure the total amount of ozone in a vertical column above the Earth’s surface (total column ozone). The number of Dobson units is the thickness in units of 10 -5 m that the ozone column would occupy if compressed into a layer of uniform density at a pressure of 1013 hPa and a temperature of 0°C. One DU corresponds to a column of ozone containing 2.69 × 10 20 molecules per square metre. A typical value for the amount of ozone in a column of the Earth’s atmosphere, although very variable, is 300 DU. From Wikipedia The Dobson unit (DU) is a unit of measurement of the amount of a trace gas in a vertical column through the Earth's atmosphere. It originated, and continues to be primarily used in respect to, atmospheric ozone, whose total column amount, usually termed "total ozone", and sometimes "column abundance", is dominated by the high concentrations of ozone in the stratospheric ozone layer.

=== Downscaling ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A method that derives local- to regional-scale information from larger-scale models or data analyses. Two main methods exist: dynamical downscaling and empirical/statistical downscaling. The dynamical method uses the output of regional climate models, global models with variable spatial resolution, or high-resolution global models. The empirical/statistical methods are based on observations and develop statistical relationships that link the large-scale atmospheric variables with local/regional climate variables. In all cases, the quality of the driving model remains an important limitation on the quality of the downscaled information. The two methods can be combined, for example, applying empirical/statistical downscaling to the output of a regional climate model, consisting of a dynamical downscaling of a global climate model.

=== Drainage ===
'''Working Groups:''' WGII

'''Definition:''' Artificial lowering of the soil water table (IPCC, 2013).

=== Driver ===
'''Working Groups:''' WGII

'''Definition:''' Any natural or human-induced factor that directly or indirectly causes a change in a system (adapted from MA, 2005).

=== Drought ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' An exceptional period of water shortage for existing ecosystems and the human population (due to low rainfall, high temperature and/or wind). From Wikipedia A drought is a period of drier-than-normal conditions. A drought can last for days, months or years. Drought often has large impacts on the ecosystems and agriculture of affected regions, and causes harm to the local economy.

=== Dynamic global vegetation model ===
'''Working Groups:''' WGI

'''Also known as:''' Dynamic global vegetation model (DGVM)

'''Definition:''' A model that simulates vegetation development and dynamics through space and time, as driven by climate and other environmental changes. From Wikipedia A Dynamic Global Vegetation Model (DGVM) is a computer program that simulates shifts in potential vegetation and its associated biogeochemical and hydrological cycles as a response to shifts in climate. DGVMs use time series of climate data and, given constraints of latitude, topography, and soil characteristics, simulate monthly or daily dynamics of ecosystem processes.

=== Dynamical system ===
'''Working Groups:''' WGI

'''Definition:''' A process or set of processes whose evolution in time is governed by a set of deterministic physical laws. The climate system is a dynamical system.

== E ==

=== Early Eocene Climatic Optimum ===
'''Working Groups:''' WGI

'''Also known as:''' Early Eocene Climatic Optimum (EECO)

'''Definition:''' The EECO is a period of geological time that occurred about 53 to 49 million years ago, during the Eocene Epoch. Continental positions at this time were somewhat different to present due to tectonic plate movements. Geological data indicate that the EECO was a period of relatively high atmospheric CO2 concentrations (about 1150–2500 ppmv) and relative warmth (global mean surface temperature was about 10–18 °C above the 1850–1900 reference), and polar ice sheets were absent.

=== Early warning systems ===
'''Working Groups:''' WGII

'''Also known as:''' Early warning systems (EWS)

'''Definition:''' The set of technical and institutional capacities to forecast, predict, and communicate timely and meaningful warning information to enable individuals, communities, managed ecosystems, and organisations threatened by a hazard to prepare to act promptly and appropriately to reduce the possibility of harm or loss. Depending upon context, EWS may draw upon scientific and/or Indigenous knowledge, and other knowledge types. EWS are also considered for ecological applications, e.g., conservation, where the organisation itself is not threatened by hazard but the ecosystem under conservation is (e.g., coral bleaching alerts), in agriculture (e.g., warnings of heavy rainfall, drought, ground frost, and hailstorms) and in fisheries (e.g., warnings of storm, storm surge, and tsunamis).

=== Earth’s energy budget ===
'''Working Groups:''' WGI

'''Definition:''' encompasses the major energy flows of relevance for the climate system: the top-of-atmosphere energy budget; the surface energy budget; changes in the global energy inventory and internal flows of energy within the climate system that characterize the climate state. From Wikipedia Earth's energy budget (or Earth's energy balance) accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but make a tiny contribution compared to solar energy. The energy budget also accounts for how energy moves through the climate system.

=== Earth's energy flows ===
'''Working Groups:''' WGI

'''Definition:''' The time-mean (or representative) energy exchanges within the climate system (including energy energy exchanges at the surface and top-of-atmosphere). This also includes horizontal ocean and atmospheric heat transports.

=== Earth's energy imbalance ===
'''Working Groups:''' WGI

'''Definition:''' The persistent and positive (downward) net top of atmosphere energy flux associated with greenhouse gas forcing of the climate system.

=== Earth's radiative response ===
'''Working Groups:''' WGI

'''Definition:''' The product of global mean surface air temperature (GSAT) change and the net feedback parameter (i.e. sum of all feedbacks), which determines the net top-of-atmosphere radiative flux that opposes a change in radiative forcing. Units: W m -2.

=== Earth system feedbacks ===
'''Working Groups:''' WGI

'''Definition:''' See Climate feedback.

=== Earth system model ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Earth system model (ESM)

'''Definition:''' A coupled atmosphere –ocean general circulation model (AOGCM) in which a representation of the carbon cycle is included, allowing for interactive calculation of atmospheric carbon dioxide (CO2) or compatible emissions. Additional components (e.g., atmospheric chemistry, ice sheets, dynamic vegetation, nitrogen cycle, but also urban or crop models) may be included.

=== Earth system model of intermediate complexity ===
'''Working Groups:''' WGI

'''Definition:''' EMICs represent climate processes at a lower resolution or in a simpler, more idealized fashion than an Earth system model (ESM).

=== Earth system sensitivity ===
'''Working Groups:''' WGI

'''Definition:''' The equilibrium surface temperature response of the coupled atmosphere – ocean – cryosphere –vegetation– carbon cycle system to a doubling of the atmospheric carbon dioxide (CO 2) concentration is referred to as Earth system sensitivity. Because it allows ice sheets to adjust to the external perturbation, it may differ substantially from the equilibrium climate sensitivity derived from coupled atmosphere–ocean models.

=== East Asian monsoon ===
'''Working Groups:''' WGI

'''Also known as:''' East Asian monsoon (EAsiaM)

'''Definition:''' The East Asian monsoon (EAsiaM) is the seasonal reversal in wind and precipitation occurring over East Asia, including eastern China, Japan and the Korean peninsula. In contrast to the other monsoons it extends quite far north, out of the tropical belt, and it is largely influenced by subtropical systems and by disturbances from the mid-latitudes. The EAsiaM manifests during boreal summer with warm and wet southerly winds, but also during boreal winter with cold and dry northerly winds. In late April/early May, rainfall onsets in the central Indochina Peninsula, and in mid-June the rainy season arrives over East Asia with the formation of the Meiyu front along the Yangtze River valley, Changma in Korea and Baiu in Japan. In July, the monsoon advances up to North China, the Korean peninsula and central Japan. During boreal winter, strong north-westerlies manifest over north and north-east China, Korea and Japan, while strong north-easterlies arrive along the coast of East Asia. Further details on how EAsiaM is defined and used throughout the Report are provided in Annex V. From Wikipedia The East Asian Monsoon is a monsoonal flow that carries moist air from the Indian Ocean and Pacific Ocean to East Asia. It affects approximately one-third of the global population, influencing the climate of Japan, the Korean Peninsula, Taiwan, China, the Philippines and Mainland Southeast Asia but most significantly Vietnam. It is driven by temperature differences between the East Asian continent and the Pacific Ocean.

=== Eastern Pacific El Niño ===
'''Working Groups:''' WGI

'''Definition:''' An El Niño event in which sea surface temperature anomalies are largest in the eastern tropical Pacific.

=== Eastern boundary upwelling systems ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Eastern boundary upwelling systems (EBUS)

'''Definition:''' Eastern boundary upwelling systems (EBUS) are located at the eastern (landward) edges of major ocean basins in both hemispheres, where equatorward winds drive upwelling currents that bring cool, nutrient-rich (and often oxygen-poor) waters from the deep ocean to the surface near the coast.

=== Economic potential ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The portion of the technical potential for which the social benefits exceed the social costs, taking into account a social discount rate and the value of externalities. From Wikipedia

=== Ecosystem ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A functional unit consisting of living organisms, their non-living environment and the interactions within and between them. The components included in a given ecosystem and its spatial boundaries depend on the purpose for which the ecosystem is defined: in some cases they are relatively sharp, while in others they are diffuse. Ecosystem boundaries can change over time. Ecosystems are nested within other ecosystems, and their scale can range from very small to the entire biosphere. In the current era, most ecosystems either contain people as key organisms or are influenced by the effects of human activities in their environment. From Wikipedia An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the system through photosynthesis and is incorporated into plant tissue.

=== Ecosystem-based adaptation ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Ecosystem-based adaptation (EbA)

'''Definition:''' The use of ecosystem management activities to increase the resilience and reduce the vulnerability of people and ecosystems to climate change (Campbell et al., 2009). From Wikipedia Ecosystem-based adaptation (EBA) encompasses a broad set of approaches to adapt to climate change. They all involve the management of ecosystems and their services to reduce the vulnerability of human communities to the impacts of climate change.

=== Ecosystem health ===
'''Working Groups:''' WGII

'''Definition:''' Ecosystem health is a metaphor used to describe the condition of an ecosystem, by analogy with human health. Note that there is no universally accepted benchmark for a healthy ecosystem. Rather, the apparent health status of an ecosystem is judged on the ecosystem’s resilience to change, with details depending upon which metrics are employed in judging it and which societal aspirations are driving the assessment (following IPBES 2019). From Wikipedia Ecosystem health is a metaphor used to describe the condition of an ecosystem. Ecosystem condition can vary as a result of fire, flooding, drought, extinctions, invasive species, climate change, mining, fishing, farming or logging, chemical spills, and a host of other reasons.

=== Ecosystem services ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Ecological processes or functions having monetary or non-monetary value to individuals or society at large. These are frequently classified as (1) supporting services such as productivity or biodiversity maintenance, (2) provisioning services such as food or fibre, (3) regulating services such as climate regulation or carbon sequestration, and (4) cultural services such as tourism or spiritual and aesthetic appreciation.

=== Effective equilibrium climate sensitivity ===
'''Working Groups:''' WGI

'''Definition:''' An estimate of the surface temperature response to a doubling of the atmospheric carbon dioxide (CO2) concentration that is evaluated from model output or observations for evolving non-equilibrium conditions. It is a measure of the strengths of the climate feedbacks at a particular time and may vary with forcing history and climate state, and therefore may differ from equilibrium climate sensitivity.

=== Effective radiative forcing due to aerosol–cloud interactions ===
'''Working Groups:''' WGI

'''Also known as:''' Effective radiative forcing (or effect) due to aerosol–cloud interactions (ERFaci)

'''Definition:''' The final radiative forcing (or effect) from the aerosol perturbation, including the adjustments to the initial change in droplet or crystal formation rate. These adjustments include changes in the strength of convection, precipitation efficiency, cloud fraction, lifetime or water content of clouds, and the formation or suppression of clouds in remote areas due to altered circulations.

=== Effective radiative forcing due to aerosol–radiation interactions ===
'''Working Groups:''' WGI

'''Also known as:''' Effective radiative forcing (or effect) due to aerosol–radiation interactions (ERFari)

'''Definition:''' The final radiative forcing (or effect) from the aerosol perturbation, including adjustments to the initial change in radiation. These adjustments include changes in cloud caused by the impact of the radiative heating on convective or larger-scale atmospheric circulations, traditionally known as semi-direct aerosol forcing (or effect).

=== Ekman transport ===
'''Working Groups:''' WGI

'''Definition:''' The total transport resulting from a balance between the Coriolis force and the frictional stress due to the action of the wind on the ocean surface. From Wikipedia Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind.

=== El Niño–Southern Oscillation ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' El Niño–Southern Oscillation (ENSO)

'''Definition:''' The term El Niño was initially used to describe a warm-water current that periodically flows along the coast of Ecuador and Peru, disrupting the local fishery. It has since become identified with warming of the tropical Pacific Ocean east of the dateline. This oceanic event is associated with a fluctuation of a global-scale tropical and subtropical surface pressure pattern called the Southern Oscillation. This coupled atmosphere–ocean phenomenon, with preferred time scales of two to about seven years, is known as the El Niño–Southern Oscillation (ENSO). The warm and cold phases of ENSO are called El Niño and La Niña, respectively. ENSO is often measured by the surface pressure anomaly difference between Tahiti and Darwin and/or the sea surface temperatures in the central and eastern equatorial Pacific. This phenomenon has a great impact on the wind, sea surface temperature and precipitation patterns in the tropical Pacific. It has climatic effects throughout the Pacific region and in many other parts of the world through global teleconnections. See Section AIV.2.3 in Annex IV of the AR6 WGI report.

=== Electromagnetic spectrum ===
'''Working Groups:''' WGI

'''Definition:''' Wavelength, frequency or energy range of all electromagnetic radiation. In terms of solar radiation, the spectral irradiance is the power arriving at the Earth per unit area, per unit wavelength. From Wikipedia The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

=== Elevation-dependent warming ===
'''Working Groups:''' WGI

'''Also known as:''' Elevation-dependent warming (EDW)

'''Definition:''' Characteristic of many regions where mountains are located, in which past and/or future surface air temperature changes vary neither uniformly nor linearly with elevation. In many cases, warming is enhanced within or above a certain elevation range.

=== Embodied [emissions, water, land] ===
'''Working Groups:''' WGIII

'''Also known as:''' Embodied (embedded) [emissions, water, land]

'''Definition:''' The total emissions [water use, land use ] generated [used] in the production of goods and services regardless of the location and timing of those emissions [water use, land use] in the production process. This includes emissions [water use, land use] within the country used to produce goods or services for the country’s own use, but also includes the emissions [water use, land use] related to the production of such goods or services in other countries that are then consumed in another country through imports. Such emissions [water, land] are termed ‘embodied’ or ‘embedded’ emissions, or in some cases, (particularly with water) as ‘virtual water use’ (Davis and Caldeira, 2010; Allan, 2005; MacDonald et al., 2015).

=== Emergence ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Emergence (of the climate signal)

'''Definition:''' Emergence of a climate change signal or trend refers to when a change in climate (the ‘signal’) becomes larger than the amplitude of natural or internal variations (defining the ‘noise’), This concept is often expressed as a ‘signal-to-noise’ ratio and emergence occurs at a defined threshold of this ratio (e.g., S/N > 1 or 2). Emergence can refer to changes relative to a historical or modern baseline (usually at least 20 years long) and can also be expressed in terms of time (time of emergence) or in terms of a global warming level. Emergence is also used to refer to a time when we can expect to see a response to reducing greenhouse gas (GHG) emissions (emergence with respect to mitigation). Emergence can be estimated using observations and/or model simulations.

=== Emergent constraint ===
'''Working Groups:''' WGI

'''Definition:''' An attempt to reduce the uncertainty in climate projections, using an ensemble of Earth system models (ESMs) to relate a specific feedback or future change to an observation of the past or current climate (typically some trend, variability or change in variability).

=== Emission and Socio-economic Scenario Ensemble ===
'''Working Groups:''' WGIII

'''Definition:''' A set of modelled emission and socio-economic scenarios collected in a database. The scenarios can come from a single multi-model study with systematic variation of harmonised scenario designs (structured ensemble) or from multiple studies in the literature (unstructured ensemble). Depending on the scope of the ensemble, variation of the results across the scenarios in the ensemble give an indication of the spread of results in the literature (unstructured ensemble), or an estimate of uncertainties due to different modelling structures and methodologies (structured ensemble).

=== Emission factor/Emissions intensity ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A coefficient that quantifies the emissions or removals of a gas per unit activity. Emission factors are often based on a sample of measurement data, averaged to develop a representative rate of emission for a given activity level under a given set of operating conditions. From Wikipedia

=== Emission pathways ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Modelled trajectories of global anthropogenic emissions over the 21st century.

=== Emission trajectories ===
'''Working Groups:''' WGIII

'''Definition:''' A projected development in time of the emission of a greenhouse gas (GHG) or group of GHGs, aerosols, and GHG precursors.

=== Emissions scenario ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A plausible representation of the future development of emissions of substances that are radiatively active (e.g., greenhouse gases (GHGs) or aerosols), plus human-induced land-cover changes that can be radiatively active via albedo changes, based on a coherent and internally consistent set of assumptions about driving forces (such as demographic and socio-economic development, technological change, energy and land use) and their key relationships. Concentration scenarios, derived from emission scenarios, are often used as input to a climate model to compute climate projections.

=== Emulation ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Reproducing the behaviour of complex, process-based models (namely, Earth system models, ESMs) via simpler approaches, using either emulators or simple climate models (SCMs). The computational efficiency of emulating approaches opens new analytical possibilities given that ESMs take a lot of computational resources for each simulation.

=== Emulators ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A broad class of heavily parametrized models (’simple climate models’), statistical methods like neural networks, genetic algorithms or other artificial intelligence approaches, designed to reproduce the responses of more complex, process-based Earth system models (ESMs). The main application of emulators is to extrapolate insights from ESMs and observational constraints to a larger set of emission scenarios.

=== Enabling conditions ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Enabling conditions (for adaptation and mitigation options)

'''Definition:''' Conditions that enhance the feasibility of adaptation and mitigation options. Enabling conditions include finance, technological innovation, strengthening policy instruments, institutional capacity, multi-level governance, and changes in human behaviour and lifestyles.

=== Endemic species ===
'''Working Groups:''' WGII

'''Definition:''' Plants and animals that are only found in one geographic region.

=== Energy access ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Access to clean, reliable and affordable energy services for cooking and heating, lighting, communications and productive uses (with special reference to Sustainable Development Goal 7) (AGECC, 2010).

=== Energy balance ===
'''Working Groups:''' WGI

'''Definition:''' The difference between the total incoming and total outgoing energy. If this balance is positive, warming occurs; if it is negative, cooling occurs. Averaged over the globe and over long time periods, this balance must be zero. Because the climate system derives virtually all its energy from the Sun, zero balance implies that, globally, the absorbed solar radiation, that is, incoming solar radiation minus reflected solar radiation at the top of the atmosphere and outgoing longwave radiation emitted by the climate system are equal.

=== Energy balance model ===
'''Working Groups:''' WGI

'''Also known as:''' Energy balance model (EBM)

'''Definition:''' An energy balance model is a simplified climate model that is typically used as an emulator of climate to analyse the energy budget of the Earth to compute changes in the climate. In its simplest form, there is no explicit spatial dimension, and the model then provides an estimate of the changes in globally averaged temperature computed from the changes in radiation. This zero-dimensional energy balance model can be extended to a one-dimensional or two-dimensional model if changes to the energy budget with respect to latitude, or both latitude and longitude, are explicitly considered.

=== Energy budget ===
'''Working Groups:''' WGI

'''Also known as:''' Energy budget (of the Earth)

'''Definition:''' The Earth is a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. The Earth’s energy budget is determined by measuring how much energy comes into the Earth system from the Sun, how much energy is lost to space, and accounting for the remainder on Earth and its atmosphere. Solar radiation is the dominant source of energy into the Earth system. Incoming solar energy may be scattered and reflected by clouds and aerosols or absorbed in the atmosphere. The transmitted radiation is then either absorbed or reflected at the Earth’s surface. The average albedo of the Earth is about 0.3, which means that 30% of the incident solar energy is reflected into space, while 70% is absorbed by the Earth. Radiant solar or shortwave energy is transformed into sensible heat, latent energy (involving different water states), potential energy, and kinetic energy before being emitted as infrared radiation. With the average surface temperature of the Earth of about 15°C (288 K), the main outgoing energy flux is in the infrared part of the spectrum.

=== Energy efficiency ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The ratio of output or useful energy or energy services or other useful physical outputs obtained from a system, conversion process, transmission or storage activity to the input of energy (measured as kWh kWh -1, tonnes kWh -1 or any other physical measure of useful output like tonne-km transported). Energy efficiency is often described by energy intensity.

=== Energy poverty ===
'''Working Groups:''' WGIII

'''Definition:''' The absence of sufficient choice in accessing adequate, affordable, reliable, high quality, safe and environmentally benign energy services to support economic and human development (Reddy, 2000).

=== Energy security ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The goal of a given country, or the global community as a whole, to maintain an adequate, stable and predictable energy supply. Measures encompass safeguarding the sufficiency of energy resources to meet national energy demand at competitive and stable prices and the resilience of the energy supply; enabling development and deployment of technologies; building sufficient infrastructure to generate, store and transmit energy supplies and ensuring enforceable contracts of delivery.

=== Energy services ===
'''Working Groups:''' WGIII

'''Definition:''' A benefit or amenity (e.g., mobility, communication, thermal comfort) received as a result of energy or other resources use.

=== Energy system ===
'''Working Groups:''' WGII

'''Definition:''' The energy system comprises all components related to the production, conversion, delivery and use of energy.

=== Enhanced weathering ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A proposed method to increase the natural rate of removal of carbon dioxide (CO2) from the atmosphere using silicate and carbonate rocks. The active surface area of these minerals is increased by grinding, before they are actively added to soil, beaches or the open ocean.

=== Ensemble ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A collection of comparable datasets that reflect variations within the bounds of one or more sources of uncertainty, and that when averaged can provide a more robust estimate of underlying behaviour. Ensemble techniques are used by the observational, reanalysis and modelling communities.

=== Enteric fermentation ===
'''Working Groups:''' WGIII

'''Definition:''' A natural part of the digestion process in ruminant animal species (domesticated and wild), such as cattle, buffalo, sheep, goats, antelope, etc. Microorganisms (bacteria, archaea, fungi, protozoa and viruses) present in the fore-stomach (reticulorumen or rumen) breakdown plant biomass to produce substrates that can be used by the animal for energy and growth with methane produced as a by-product. Fermentation end-products such as hydrogen, carbon dioxide, formate and methyl-containing compounds are important substrates for the production of methane by the rumen’s methane-forming archaea (known as methanogens).

=== Equality ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A principle that ascribes equal worth to all human beings, including equal opportunities, rights and obligations, irrespective of origins.

=== Equilibrium and transient climate experiment ===
'''Working Groups:''' WGI

'''Definition:''' An equilibrium climate experiment is a climate model experiment in which the model is allowed to fully adjust to a change in radiative forcing. Such experiments provide information on the difference between the initial and final states of the model, but not on the time-dependent response. If the forcing is allowed to evolve gradually according to a prescribed emissions scenario, the time-dependent response of a climate model may be analysed. Such an experiment is called a transient climate experiment.

=== Equilibrium climate sensitivity ===
'''Working Groups:''' WGI

'''Also known as:''' Equilibrium climate sensitivity (ECS)

'''Definition:''' The equilibrium (steady state) change in the surface temperature following a doubling of the atmospheric carbon dioxide (CO2) concentration from pre-industrial conditions.

=== Equilibrium line ===
'''Working Groups:''' WGI

'''Definition:''' The spatially averaged boundary at a given moment, usually chosen as the seasonal mass budget minimum at the end of summer, between the region on a glacier where there is a net annual loss of ice mass (ablation area) and that where there is a net annual gain (accumulation area). The altitude of this boundary is referred to as equilibrium line altitude (ELA).

=== Equity ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The principle of being fair and impartial, and a basis for understanding how the impacts and responses to climate change, including costs and benefits, are distributed in and by society in more or less equal ways. Often aligned with ideas of equality, fairness and justice and applied with respect to equity in the responsibility for, and distribution of, climate impacts and policies across society, generations and gender, and in the sense of who participates and controls the processes of decision-making.

=== Equivalent carbon dioxide emission ===
'''Working Groups:''' WGI

'''Also known as:''' Equivalent carbon dioxide emission (CO2)

'''Definition:''' The amount of carbon dioxide (CO2) emission that would have an equivalent effect on a specified key measure of climate change, over a specified time horizon, as an emitted amount of another greenhouse gas (GHG) or a mixture of other GHGs. For a mix of GHGs it is obtained by summing the CO2-equivalent emissions of each gas. There are various ways and time horizons to compute such equivalent emissions (see greenhouse gas emission metric). CO2-equivalent emissions are commonly used to compare emissions of different GHGs, but should not be taken to imply that these emissions have an equivalent effect across all key measures of climate change. [Note: Under the Paris Rulebook (Decision 18/CMA.1, annex, paragraph 37), parties have agreed to use GWP-100 values from the IPCC AR5 or GWP-100 values from a subsequent IPCC Assessment Report to report aggregate emissions and removals of GHGs. In addition, parties may use other metrics to report supplemental information on aggregate emissions and removals of GHGs.]

=== Ethics ===
'''Working Groups:''' WGII

'''Definition:''' Ethics involves questions of justice and value. Justice is concerned with right and wrong, equity and fairness, and, in general, with the rights to which people and living beings are entitled. Value is a matter of worth, benefit or good.

=== Eudaimonic ===
'''Working Groups:''' WGIII

'''Definition:''' Relational well-being concept based on the premise that experiencing life purpose, challenges and growth leads to flourishing, self-realisation, personal expression, and full functioning (Niemiec 2014; Lamb and Steinberger 2017).

=== Eutrophication ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Over-enrichment of water by nutrients such as nitrogen and phosphorus. It is one of the leading causes of water quality impairment. The two most acute symptoms of eutrophication are hypoxia (or oxygen depletion) and harmful algal blooms.

=== Evaporation ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The physical process by which a liquid (e.g., water) becomes a gas (e.g., water vapour).

=== Evapotranspiration ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The combined processes through which water is transferred to the atmosphere from open water and ice surfaces, bare soil and vegetation that make up the Earth’s surface.

=== Evidence ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Data and information used in the scientific process to establish findings. In this report, the degree of evidence reflects the amount, quality and consistency of scientific/technical information on which the Lead Authors are basing their findings.

=== Evolutionary adaptation ===
'''Working Groups:''' WGII

'''Definition:''' The process whereby a species or population becomes better able to live in a changing environment through the selection of heritable traits. Biologists usually distinguish evolutionary adaptation from acclimatisation, with the latter occurring within an organism’s lifetime.

=== Exergy ===
'''Working Groups:''' WGIII

'''Definition:''' Capacity of energy flows to perform useful work. Exergy is a quality (versatility) indicator of energy flows which ranges from low (e.g., low-temperature heat, biomass) to high (e.g., electricity). Exergy efficiency describes how much useful work can be performed by a particular energy flow in relation to the thermodynamic maximum possible. It can be determined for all energy flows and energy conversion steps, also including alternative service delivery systems. (Grubler et al., 2012).

=== Exposure ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The presence of people; livelihoods; species or ecosystems; environmental functions, services, and resources; infrastructure; or economic, social, or cultural assets in places and settings that could be adversely affected.

=== Extended concentration pathways ===
'''Working Groups:''' WGI

'''Also known as:''' Extended concentration pathways (ECPs)

'''Definition:''' Extended concentration pathways describe extensions of the RCPs from 2100 to 2300 that were calculated using simple rules generated by stakeholder consultations, and do not represent fully consistent scenarios.

=== External forcing ===
'''Working Groups:''' WGI

'''Definition:''' External forcing refers to a forcing agent outside the climate system causing a change in the climate system. Volcanic eruptions, solar variations and changes in Earth’s orbit, as well as anthropogenic changes in the composition of the atmosphere or in land use are external forcings.

=== Externality/external cost/external benefit ===
'''Working Groups:''' WGII

'''Definition:''' Externalities arise from a human activity, when agents responsible for the activity do not take full account of the activity’s impact on others’ production and consumption possibilities, and no compensation exists for such impacts. When the impact is negative, they are external costs. When positive they are referred to as external benefits.

=== Extinction ===
'''Working Groups:''' WGII

'''Definition:''' A population, species or more inclusive taxonomic group has gone extinct when all its individuals have died. A species may go extinct locally (population extinction), regionally (e.g., extinction of all populations in a country, continent or ocean) or globally (IPBES, 2019).

=== Extirpation ===
'''Working Groups:''' WGII

'''Definition:''' The disappearance of a species from an area, sometimes also referred to as local extinction. Its use implies that the species still occurs elsewhere.

=== Extratropical cyclone ===
'''Working Groups:''' WGI

'''Also known as:''' Extratropical cyclone (ETC)

'''Definition:''' Any cyclonic-scale storm that is not a tropical cyclone. Usually refers to a mid- or high-latitude migratory storm system formed in regions of large horizontal temperature variations. Sometimes called extratropical storm or extratropical low.

=== Extratropical jets ===
'''Working Groups:''' WGI

'''Definition:''' Extratropical jets are wind maxima in the upper troposphere marking zones of baroclinic instability. Anomalies in the position of these jets are often associated with storms, blocking, and weather extremes.

=== Extreme climate event ===
'''Working Groups:''' WGI

'''Definition:''' The occurrence of a value of a weather or climate variable above (or below) a threshold value near the upper (or lower) ends of the range of observed values of the variable. By definition, the characteristics of what is called extreme weather may vary from place to place in an absolute sense. When a pattern of extreme weather persists for some time, such as a season, it may be classified as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., high temperature, drought or heavy rainfall over a season). For simplicity, both extreme weather events and extreme climate events are referred to collectively as climate extremes.

=== Extreme/heavy precipitation event ===
'''Working Groups:''' WGI; WGII

'''Definition:''' An extreme/heavy precipitation event is an event that is of very high magnitude with a very rare occurrence at a particular place. Types of extreme precipitation may vary depending on its duration, hourly, daily or multi-days (e.g., 5 days), though all of them qualitatively represent high magnitude. The intensity of such events may be defined with a block maxima approach such as annual maxima or with peaks over threshold approach, such as rainfall above the 95th or 99th percentile at a particular place.

=== Extreme sea level ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Extreme sea level (ESL)

'''Definition:''' The occurrence of an exceptionally low or high local sea-surface height, arising from (a combination of) short term phenomena (e.g., storm surges, tides and waves). Relative sea level changes affect extreme sea levels directly by shifting the mean water levels and indirectly by modulating the propagation of tides, waves and/or surges due to increased water depth. In addition, extreme sea levels can be influenced by changes in the frequency, tracks or strength of weather systems and storms, or due to anthropogenically induced changes such as the modification of coastlines or dredging. In turn, changes in any or all of the contributions to extreme sea levels may lead to long term relative sea-level changes. Alternate expressions for ESL may be used depending on the processes resolved. Extreme still water level (ESWL) refers to the combined contribution of relative sea level change, tides and storm surges. Wind-waves also contribute to coastal sea level via three processes: infragravity waves (lower frequency gravity waves generated by wind waves), wave setup (time-mean sea-level elevation due to wave energy dissipation) and swash (vertical displacement up the shore-face induced by individual waves). Extreme total water level (ETWL) is the ESWL plus wave setup. When considering coastal impacts, swash is also important, and extreme coastal water level (ECWL) is used.

=== Extreme weather event ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' An event that is rare at a particular place and time of year. Definitions of ‘rare’ vary, but an extreme weather event would normally be as rare as, or rarer than, the 10th or 90th percentile of a probability density function estimated from observations. By definition, the characteristics of what is called extreme weather may vary from place to place in an absolute sense.

== F ==

=== Faculae ===
'''Working Groups:''' WGI

'''Definition:''' Bright patches on the Sun. The area covered by faculae is greater during periods of high solar activity.

=== Fairness ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Impartial and just treatment without favouritism or discrimination in which each person is considered of equal worth with equal opportunity.

=== Feasibility ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' In this report, feasibility refers to the potential for a mitigation or adaptation option to be implemented. Factors influencing feasibility are context-dependent, temporally dynamic and may vary between different groups and actors. Feasibility depends on geophysical, environmental-ecological, technological, economic, socio-cultural and institutional factors that enable or constrain the implementation of an option. The feasibility of options may change when different options are combined, and increase when enabling conditions are strengthened.

=== Final energy ===
'''Working Groups:''' WGIII

'''Definition:''' The energy delivered to final users (firms, individuals, institutions), where it becomes usable energy in supplying energy services (e.g., light, heat, mobility).

=== Fine-mode aerosol optical depth ===
'''Working Groups:''' WGI

'''Definition:''' Aerosol optical depth due to aerosol particles smaller than 1 µm in radius.

=== Fingerprint ===
'''Working Groups:''' WGI

'''Definition:''' The climate response pattern in space and/or time to a specific forcing is commonly referred to as a fingerprint. The spatial patterns of sea level response to melting of glaciers or ice sheet s (or other changes in surface loading) are also referred to as fingerprints. Fingerprints are used to detect the presence of this response in observations and are typically estimated using forced climate model simulations.

=== Fire weather ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Weather conditions conducive to triggering and sustaining wildfires, usually based on a set of indicators and combinations of indicators including temperature, soil moisture, humidity, and wind. Fire weather does not include the presence or absence of fuel load.

=== Firn ===
'''Working Groups:''' WGI

'''Definition:''' Snow that has survived at least one ablation season but has not been transformed to glacier ice. Its pore space is at least partially interconnected, allowing air and water to circulate. Firn densities typically are 400–830 kg m –3.

=== Fitness-for-purpose ===
'''Working Groups:''' WGI

'''Definition:''' The suitability of a model (or other resource, such as a dataset or method) for a particular task, such as quantifying the contribution of increased greenhouse gas concentrations to recent changes in global mean surface temperature or projecting changes in drought frequency in a region under a given scenario. Assessment of a model’s fitness-for-purpose can be informed both by how the model represents relevant physical processes and by how it scores on relevant performance metrics.

=== Flaring ===
'''Working Groups:''' WGI

'''Definition:''' Open air burning of waste gases and volatile liquids, through a chimney, at oil wells or rigs, in refineries or chemical plants, and at landfills.

=== Flexibility ===
'''Working Groups:''' WGIII

'''Also known as:''' Flexibility (demand and supply)

'''Definition:''' Adjustment of energy load characteristics by technical and/or non-technical change to balance energy demand and supply.

=== Flexible governance ===
'''Working Groups:''' WGIII

'''Definition:''' Strategies of governance at various levels, which prioritise the use of social learning and rapid feedback mechanisms in planning and policymaking, often through incremental, experimental and iterative management processes.

=== Flood ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The overflowing of the normal confines of a stream or other water body, or the accumulation of water over areas that are not normally submerged. Floods can be caused by unusually heavy rain, for example, during storms and cyclones. Floods include river (fluvial) floods, flash floods, urban floods, rain (pluvial) floods, sewer floods, coastal floods, and glacial lake outburst floods (GLOFs).

=== Flux ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A movement (a flow) of matter (e.g., water vapour, particles), heat or energy from one place to another, or from one medium (e.g., land surface) to another (e.g., atmosphere).

=== Food-borne diseases ===
'''Working Groups:''' WGII

'''Definition:''' Illnesses transmitted through the consumption of unsafe or contaminated food. That contamination can come from a variety of sources, including contaminated water (adapted from UNEP, 2018).

=== Food loss and waste ===
'''Working Groups:''' WGIII

'''Definition:''' ‘The decrease in quantity or quality of food’. Food waste is part of food loss and refers to discarding or alternative (non-food) use of food that is safe and nutritious for human consumption along the entire food supply chain, from primary production to end household consumer level. Food waste is recognised as a distinct part of food loss because the drivers that generate it and the solutions to it are different from those of food losses (FAO, 2015).

=== Food security ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A situation that exists when all people, at all times, have physical, social and economic access to sufficient, safe and nutritious food that meets their dietary needs and food preferences for an active and healthy life. The four pillars of food security are: access; availability; stability; and utilisation. The nutritional dimension is integral to the concept of food security (FAO, 2009,2018).

=== Food system ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' All the elements (environment, people, inputs, processes, infrastructures, institutions, etc.) and activities that relate to the production, processing, distribution, preparation and consumption of food, and the output of these activities, including socio-economic and environmental outcomes (HLPE, 2017). [Note: Whilst there is a global food system (encompassing the totality of global production and consumption), each location’s food system is unique, being defined by that place’s mix of food produced locally, nationally, regionally or globally.]

=== Foraminifera ===
'''Working Groups:''' WGI

'''Definition:''' Single-celled, sand-sized marine organisms (protists) that possess a hard test mainly composed of agglutinated walls (detrital grains glued together with organic cement) or calcium carbonate (predominantly calcite). They are used to reconstruct a range of (paleo)environmental variables such as salinity, temperature, oxygenation, oxygen isotope composition and organic and nutrient flux.

=== Forcing ===
'''Working Groups:''' WGI

'''Definition:''' Radiative forcing is the change in the net, downward minus upward, radiative flux (expressed in W m2) at the tropopause or top of atmosphere due to a change in an driver of climate change, such as a change in the concentration of carbon dioxide or the output of the Sun. The traditional radiative forcing is computed with all tropospheric properties held fixed at their unperturbed values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical equilibrium. Radiative forcing is called instantaneous if no change in stratospheric temperature is accounted for. The radiative forcing once rapid adjustments are accounted for is termed the effective radiative forcing. Radiative forcing is not to be confused with cloud radiative forcing, which describes an unrelated measure of the impact of clouds on the radiative flux at the top of the atmosphere.

=== Forest ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A vegetation type dominated by trees. Many definitions of the term forest are in use throughout the world, reflecting wide differences in biogeophysical conditions, social structure and economics. [Note: For a discussion of the term forest in the context of National GHG inventories, see the 2006 IPCC Guidelines for National GHG Inventories and their 2019 Refinement, and information provided by the United Nations Framework Convention on Climate Change (IPCC 2006, 2019; UNFCCC, 2021a, b).]

=== Forest degradation ===
'''Working Groups:''' WGII

'''Definition:''' A reduction in the capacity of a forest to produce ecosystem services such as carbon storage and wood products as a result of anthropogenic and environmental changes.

=== Forest line ===
'''Working Groups:''' WGII

'''Definition:''' The upper limit of the closed upper montane forest or forest at high latitudes. It is less elevated or less poleward than the tree line.

=== Fossil fuel emissions ===
'''Working Groups:''' WGI

'''Definition:''' Emissions of greenhouse gases (GHGs) (in particular 2) carbon dioxide (CO), other trace gases and aerosols resulting from the combustion of fuels from fossil carbon deposits such as oil, gas and coal.

=== Fossil fuels ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Carbon-based fuels from fossil hydrocarbon deposits, including coal, oil and natural gas.

=== Free atmosphere ===
'''Working Groups:''' WGI

'''Definition:''' The atmospheric layer that is negligibly affected by friction against the Earth’s surface, and which is above the atmospheric boundary layer.

=== Frozen ground ===
'''Working Groups:''' WGI

'''Definition:''' Soil or rock in which part or all of the pore water consists of ice.

=== Fuel poverty ===
'''Working Groups:''' WGIII

'''Definition:''' A condition in which a household is unable to guarantee a certain level of consumption of domestic energy services (especially heating) or suffers disproportionate expenditure burdens to meet these needs.

=== Fugitive emissions ===
'''Working Groups:''' WGIII

'''Also known as:''' Fugitive emissions (oil and natural gas systems)

'''Definition:''' The release of greenhouse gases that occur during the exploration, processing and delivery of fossil fuels to the point of final use. This excludes greenhouse gas emissions from fuel combustion for the production of useful heat or power. It encompasses venting, flaring, and leaks.

== G ==

=== Gender equity ===
'''Working Groups:''' WGIII

'''Definition:''' Equity between women and men with regard to their rights, resources and opportunities. In the case of climate change, gender equity recognises that women are often more vulnerable to the impacts of climate change and may be disadvantaged in the process and outcomes of climate policy.

=== General circulation ===
'''Working Groups:''' WGI

'''Definition:''' The large-scale motions of the atmosphere and the ocean as a consequence of differential heating on a rotating Earth. General circulation contributes to the energy balance of the system through transport of heat and momentum.

=== General circulation model ===
'''Working Groups:''' WGI

'''Also known as:''' General circulation model (GCM)

'''Definition:''' A numerical representation of the atmosphere–ocean–sea ice system based on the physical, chemical and biological properties of its components, their interactions and feedback processes. General circulation models are used for weather forecasts, seasonal to decadal prediction, and climate projections. They are the basis of the more complex Earth system models (ESMs).

=== Geocentric sea level change ===
'''Working Groups:''' WGI

'''Definition:''' The change in local mean sea surface height with respect to the terrestrial reference frame; it is the sea level change observed with instruments from space.

=== Geoid ===
'''Working Groups:''' WGI

'''Definition:''' The equipotential surface having the same geopotential at each latitude and longitude around the world (geodesists denote this potential W0) that best approximates the mean sea level. It is the surface of reference for measurement of altitude. In practice, several variations of definitions of the geoid exist depending on the way the permanent tide (the zero-frequency gravitational tide due to the Sun and Moon) is considered in geodetic studies.

=== Geostrophic winds or currents ===
'''Working Groups:''' WGI

'''Definition:''' A wind or current that is in balance with the horizontal pressure gradient and the Coriolis force, and thus is outside of the influence of friction. Thus, the wind or current is directly parallel to isobars and its speed is proportional to the horizontal pressure gradient.

=== Geothermal energy ===
'''Working Groups:''' WGIII

'''Definition:''' Accessible thermal energy stored in the Earth’s interior, in both rock and trapped steam or liquid water (hydrothermal resources), which may be used to generate electric energy in a thermal power plant, or to supply heat to any process requiring it. The main sources of geothermal energy are the residual energy available from planet formation and the energy continuously generated from radionuclide decay.

=== Gini coefficient ===
'''Working Groups:''' WGIII

'''Definition:''' A statistical measure of dispersion in a distribution and degree of mathematical measure of inequality. For example, it can be used for measuring inequality in income, wealth, carbon emissions, and access to well-being defining services. The dimensionless GINI coefficient ranges between 0 (absolute equality) and 1 (absolute inequality).

=== Glacial-interglacial cycles ===
'''Working Groups:''' WGI

'''Definition:''' Phase of the Earth’s history marked by large changes in continental ice volume and global sea level.

=== Glacial isostatic adjustment ===
'''Working Groups:''' WGI

'''Also known as:''' Glacial isostatic adjustment (GIA)

'''Definition:''' The ongoing changes in gravity, rotation and viscoelastic solid Earth deformation (GRD) in response to past changes in the distribution of ice and water on Earth’s surface. On a time scale of decades to tens of millennia following mass redistribution, Earth’s mantle flows viscously as it evolves toward isostatic equilibrium, causing solid Earth movement and geoid changes, which can result in regional-to-local sea level variations.

=== Glacial lake outburst flood /Glacier lake outburst ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Glacial lake outburst flood (GLOF)/Glacier lake outburst

'''Definition:''' A sudden release of water from a glacier lake, including any of the following types: a glacier-dammed lake, a pro-glacial moraine-dammed lake or water that was stored within, under or on the glacier.

=== Glacial or glaciation ===
'''Working Groups:''' WGI

'''Definition:''' A period characterized by the establishment of expanded ice sheets and glaciers, and associated with global mean sea level (GMSL) substantially lower than present; generally coincides with even-numbered marine isotope stages. Glacial intervals were interrupted by interglacial intervals. The Last Glacial Maximum (LGM) is a specific interval within the most recent glaciation, when ice sheets were near their global maximum volume (Clark et al., 2009; Gowan et al., 2021) and GMSL was nearly at its lowest level (Lambeck et al., 2014; Yokoyama et al., 2018). Local or regional glacial maxima may be diachronous, for example ranging from about 29,000 years ago and 16,000 years ago. For purposes of global synthesis, IPCC AR6 adopts a practical chronostratigraphic definition of LGM of 23,000–19,000 years BP (before 1950; chronozone level 1 of Mix et al., 2001). For modelling purposes, LGM is defined by the model time step nearest to the centre of this interval, 21,000 years ago (Kageyama et al., 2017).

=== Glaciated ===
'''Working Groups:''' WGI

'''Definition:''' State of a surface that was covered by glacier ice in the past, but not at present.

=== Glacier ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A perennial mass of ice, and possibly firn and snow, originating on the land surface by accumulation and compaction of snow and showing evidence of past or present flow. A glacier typically gains mass by accumulation of snow and loses mass by ablation. Land ice masses of continental size (>50,000 km2) are referred to as ice sheets (Cogley et al., 2011).

=== Glacierized ===
'''Working Groups:''' WGI

'''Definition:''' A surface that is currently covered by glacier ice.

=== Global Environment Facility ===
'''Also known as:''' Global Environment Facility (GEF)

'''Definition:''' The Global Environment Facility, established in 1991, helps developing countries fund projects and programmes that protect the global environment. GEF grants support projects related to biodiversity, climate change, international waters, land degradation, the ozone (O3) layer, and persistent organic pollutants.

=== Global carbon budget ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' An assessment of carbon cycle sources and sinks on a global level, through the synthesis of evidence for fossil-fuel and cement emissions, landuse change emissions, ocean and land CO2 sinks, and the resulting atmospheric CO2 growth rate.

=== Global change ===
'''Working Groups:''' WGII

'''Definition:''' A generic term to describe global scale changes in systems, including the climate system, ecosystems and social-ecological systems.

=== Global dimming ===
'''Working Groups:''' WGI

'''Definition:''' Global dimming refers to the observed widespread reduction in the amount of solar radiation received at the Earth’s surface from the 1950s to the 1980s, with an increase in anthropogenic aerosol emissions appearing to have contributed. This was followed by a partial recovery since the 1990s (‘brightening’), particularly in industrialized areas, coincident with a reduction in anthropogenic aerosol emissions.

=== Global energy budget ===
'''Working Groups:''' WGI

'''Definition:''' For a given time period, the global energy budget expresses the balance between change in the global energy inventory, the time-integrated effective radiative forcing and time-integrated radiative response of the climate system. Typical units: Joules.

=== Global energy inventory ===
'''Working Groups:''' WGI

'''Definition:''' quantifies the excess energy absorbed or lost by the Earth system (ocean, land, atmosphere and cryosphere), mostly in the form of heat, associated with radiative forcing of the climate. Typical units: Joules.

=== Global mean sea level change ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Global mean sea level (GMSL) change

'''Definition:''' The increase or decrease in the volume of the ocean divided by the ocean surface area. It is the sum of changes in ocean density through temperature changes (global mean thermosteric sea level change) and changes in the ocean mass as a result of changes in the cryosphere or land water storage (barystatic sea level change).

=== Global mean surface air temperature ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Global mean surface air temperature (GSAT)

'''Definition:''' Global average of near-surface air temperatures over land, oceans and sea ice. Changes in GSAT are often used as a measure of global temperature change in climate models.

=== Global mean surface temperature ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Global mean surface temperature (GMST)

'''Definition:''' Estimated global average of near-surface air temperatures over land and sea ice, and sea surface temperature (SST) over ice-free ocean regions, with changes normally expressed as departures from a value over a specified reference period.

=== Global monsoon ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The global monsoon (GM) is a global-scale solstitial mode that dominates the annual variation of tropical and sub-tropical precipitation and circulation. The GM domain is defined as the area where the annual range of precipitation (local summer minus winter mean precipitation rate) is greater than 2.5 mm day -1, following on from the definition as in Kitoh et al. (2013). Further details on how the GM is defined, used and related to regional monsoons throughout the Report are provided by WGI AR6 Annex V (IPCC 2021b).

=== Global warming ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Global warming refers to the increase in global surface temperature relative to a baseline reference period, averaging over a period sufficient to remove interannual variations (e.g., 20 or 30 years). A common choice for the baseline is 1850–1900 (the earliest period of reliable observations with sufficient geographic coverage), with more modern baselines used depending upon the application.

=== Global warming potential ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Global warming potential (GWP)

'''Definition:''' An index measuring the radiative forcing following an emission of a unit mass of a given substance, accumulated over a chosen time horizon, relative to that of the reference substance, carbon dioxide (CO2). The GWP thus represents the combined effect of the differing times these substances remain in the atmosphere and their effectiveness in causing radiative forcing.

=== Governance ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The structures, processes and actions through which private and public actors interact to address societal goals. This includes formal and informal institutions and the associated norms, rules, laws and procedures for deciding, managing, implementing and monitoring policies and measures at any geographic or political scale, from global to local.

=== Governance capacity ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The ability of governance institutions, leaders, and non-state and civil society to plan, coordinate, fund, implement, evaluate and adjust policies and measures over the short, medium and long term, adjusting for uncertainty, rapid change and wide-ranging impacts and multiple actors and demands.

=== Gravitational, rotational and deformational effects ===
'''Working Groups:''' WGI

'''Also known as:''' Gravitational, rotational and deformational (GRD) effects

'''Definition:''' Changes in Earth gravity, Earth rotation and viscoelastic solid Earth deformation (GRD) result from the redistribution of mass between terrestrial ice and water reservoirs and the ocean. Contemporary terrestrial mass loss leads to elastic solid Earth uplift and a nearby relative sea level fall (for a single source of terrestrial mass loss this is within ~2000 km, for multiple sources the distance depends on the interaction of the different relative sea level patterns). Farther away (more than ~7000 km for a single source of terrestrial mass loss), relative sea level rises more than the global average, due (to first order) to gravitational effects. Earth deformation associated with adding water to the oceans and a shift of the Earth’s rotation axis towards the source of terrestrial mass loss leads to second-order effects that increase spatial variability of the pattern globally. GRD effects due to the redistribution of ocean water within the ocean itself are referred to as self-attraction and loading effects.

=== Gravity Recovery and Climate Experiment ===
'''Working Groups:''' WGI

'''Also known as:''' Gravity Recovery and Climate Experiment (GRACE)

'''Definition:''' A pair of satellites that measured the Earth’s gravity field anomalies from 2002 to 2017. These fields have been used, among other things, to study mass changes of the polar ice sheets and glaciers.

=== Grazing land ===
'''Working Groups:''' WGIII

'''Definition:''' The sum of rangelands and pastures not considered as cropland, and subject to livestock grazing or hay production. It includes a wide range of ecosystems, for example, systems with vegetation that fall below the threshold used in the forest land category, silvo-pastoral systems, as well as natural, managed grasslands and semi-deserts.

=== Green Climate Fund ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Green Climate Fund (GCF)

'''Definition:''' The Green Climate Fund was established by the 16th Session of the Conference of the Parties (COP) in 2010 as an operating entity of the financial mechanism of the United Nations Framework Convention on Climate Change (UNFCCC), in accordance with Article 11 of the Convention, to support projects, programmes and policies and other activities in developing country Parties. The Fund is governed by a board and will receive guidance from the COP.

=== Green infrastructure ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The strategically planned interconnected set of natural and constructed ecological systems, green spaces and other landscape features that can provide functions and services including air and water purification, temperature management, floodwater management and coastal defence often with co-benefits for human and ecological well-being. Green infrastructure includes planted and remnant native vegetation, soils, wetlands, parks and green open spaces, as well as building and street-level design interventions that incorporate vegetation (Culwick and Bobbins, 2016).

=== Greenhouse effect ===
'''Working Groups:''' WGI

'''Definition:''' The infrared radiative effect of all infrared-absorbing constituents in the atmosphere. Greenhouse gases (GHGs), clouds, and some aerosols absorb terrestrial radiation emitted by the Earth’s surface and elsewhere in the atmosphere. These substances emit infrared radiation in all directions, but, everything else being equal, the net amount emitted to space is normally less than would have been emitted in the absence of these absorbers because of the decline of temperature with altitude in the troposphere and the consequent weakening of emission. An increase in the concentration of GHGs increases the magnitude of this effect; the difference is sometimes called the enhanced greenhouse effect. The change in a GHG concentration because of anthropogenic emissions contributes to an instantaneous radiative forcing. Earth’s surface temperature and troposphere warm in response to this forcing, gradually restoring the radiative balance at the top of the atmosphere.

=== Greenhouse gas emission metric ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A simplified relationship used to quantify the effect of emitting a unit mass of a given greenhouse gas (GHG) on a specified key measure of climate change. A relative GHG emission metric expresses the effect from one gas relative to the effect of emitting a unit mass of a reference GHG on the same measure of climate change. There are multiple emission metrics, and the most appropriate metric depends on the application. GHG emission metrics may differ with respect to: (i) the key measure of climate change they consider; (ii) whether they consider climate outcomes for a specified point in time or integrated over a specified time horizon; (iii) the time horizon over which the metric is applied; (iv) whether they apply to a single emission pulse, emissions sustained over a period of time, or a combination of both; and (v) whether they consider the climate effect from an emission compared to the absence of that emission or compared to a reference emissions level or climate state. [Note:Most relative GHG emission metrics (such as the g lobal warming potential (GWP), global temperature change potential (GTP), global damage potential, and GWP*), use carbon dioxide (CO 2) as the reference gas. Emissions of non-CO2 gases, when expressed using such metrics, are often referred to as ‘carbon dioxide equivalent’ emissions. A metric that establishes equivalence regarding one key measure of the climate system response to emissions does not imply equivalence regarding other key measures. The choice of a metric, including its time horizon, should reflect the policy objectives for which the metric is applied.]

=== Greenhouse gas neutrality ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Condition in which metric-weighted anthropogenic greenhouse gas (GHG) emissions associated with a subject are balanced by metric-weighted anthropogenic GHG removals. The subject can be an entity such as a country, an organisation, a district or a commodity, or an activity such as a service or an event. GHG neutrality is often assessed over the lifecycle, including indirect (‘scope 3’) emissions, but can also be limited to the emissions and removals, over a specified period, for which the subject has direct control, as determined by the relevant scheme. The quantification of GHG emissions and removals depends on the GHG emission metric chosen to compare emissions and removals of different gases, as well as the time horizon chosen for that metric [Note 1: Greenhouse gas neutrality and net zero greenhouse gas emissions are overlapping concepts. The concepts can be applied at global or sub-global scales (e.g., regional, national and sub-national). At a global scale, the terms greenhouse gas neutrality and net zero greenhouse gas emissions are equivalent. At sub-global scales, net zero GHG emissions is generally applied to emissions and removals under direct control or territorial responsibility of the reporting entity, while GHG neutrality generally includes emissions and removals within and beyond the direct control or territorial responsibility of the reporting entity. Accounting rules specified by GHG programmes or schemes can have a significant influence on the quantification of relevant emissions and removals. Note 2: Under the Paris Rulebook (Decision 18/CMA.1, annex, paragraph 37), parties have agreed to use GWP100 values from the IPCC AR5 or GWP100 values from a subsequent IPCC Assessment Report to report aggregate emissions and removals of GHGs. In addition, parties may use other metrics to report supplemental information on aggregate emissions and removals of GHGs. Note 3: In some cases, achieving greenhouse gas neutrality may rely on the supplementary use of offsets to balance emissions that remain after actions by the reporting entity are taken into account.]

=== Greenhouse gases ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Greenhouse gases (GHGs)

'''Definition:''' Gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapour (H 2 O), 2) carbon dioxide (CO, 2 O) nitrous oxide (N, 4) methane (CH and 3) ozone (O are the primary GHGs in the Earth’s atmosphere. Human-made GHGs include 6), sulphur hexafluoride (SF hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs) and perfluorocarbons (PFCs); several of these are also O 3 -depleting (and are regulated under the Montreal Protocol).

=== Greenland Ice Sheet ===
'''Working Groups:''' WGI

'''Also known as:''' Greenland Ice Sheet (GrIS)

'''Definition:''' There are only two ice sheets in the modern world, one on Greenland and one on Antarctica. The latter is divided into the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet. During glacial periods, there were other ice sheets.

=== Grey infrastructure ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Engineered physical components and networks of pipes, wires, tracks and roads that underpin energy, transport, communications (including digital), built form, water and sanitation and solid waste management systems.

=== Gross domestic product ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Gross domestic product (GDP)

'''Definition:''' The sum of gross value added, at purchasers’ prices, by all resident and non-resident producers in the economy, plus any taxes and minus any subsidies not included in the value of the products in a country or a geographic region for a given period, normally one year. GDP is calculated without deducting for depreciation of fabricated assets or depletion and degradation of natural resources.

=== Gross primary production ===
'''Working Groups:''' WGI

'''Also known as:''' Gross primary production (GPP)

'''Definition:''' The total amount of carbon fixed by photosynthesis over a specified time period.

=== Ground-level ozone ===
'''Working Groups:''' WGI

'''Definition:''' Atmospheric ozone (O3) is formed naturally or from human-emitted precursors near Earth’s surface, thus affecting human health, agriculture and ecosystems. Ozone is a greenhouse gas (GHG), but ground-level ozone, unlike stratospheric ozone, also directly affects organisms at the surface. Ground-level ozone is sometimes referred to as tropospheric ozone, although much of the troposphere is well above the surface and thus does not directly expose organisms at the surface.

=== Grounding line ===
'''Working Groups:''' WGI

'''Definition:''' The junction between a glacier or ice sheet and an ice shelf; the place where ice starts to float. This junction normally occurs over a zone, rather than at a line.

=== Groundwater recharge ===
'''Working Groups:''' WGII

'''Definition:''' The process by which external water is added to the zone of saturation of an aquifer, either directly into a geologic formation that traps the water or indirectly by way of another formation.

=== Gyre ===
'''Working Groups:''' WGI

'''Definition:''' Basin-scale ocean horizontal circulation pattern with slow flow circulating around the ocean basin, closed by a strong and narrow (100 to 200 km wide) boundary current on the western side. The subtropical gyres in each ocean are associated with high pressure in the centre of the gyres; the subpolar gyres are associated with low pressure.

== H ==

=== Habitability ===
'''Working Groups:''' WGII

'''Also known as:''' Habitability (human)

'''Definition:''' The ability of a place to support human life by providing protection from hazards which challenge human survival, and by assuring adequate space, food and freshwater.

=== Hadley circulation ===
'''Working Groups:''' WGI

'''Definition:''' A direct, thermally driven overturning cell in the atmosphere consisting of poleward flow in the upper troposphere, subsiding air into the subtropical anticyclones, return flow as part of the trade winds near the surface, and with rising air near the equator in the so-called Inter-tropical Convergence Zone.

=== Halocarbons ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A collective term for the group of partially halogenated organic species, which includes the chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), halons, methyl chloride and methyl bromide. Many of the halocarbons have large global warming potentials. The chlorine and bromine-containing halocarbons are also involved in the depletion of the ozone layer.

=== Halocline ===
'''Working Groups:''' WGI

'''Definition:''' A layer in the oceanic water column in which salinity changes rapidly with depth. Generally, saltier water is denser and lies below less salty water. In some high-latitude oceans the surface waters may be colder than the deep waters, and the halocline is responsible for maintaining water column stability and isolating the surface waters from the deep waters.

=== Halosteric ===
'''Working Groups:''' WGI

'''Definition:''' Density changes induced by temperature changes only are called thermosteric, while density changes induced by salinity changes are called halosteric.

=== Halosteric sea level change ===
'''Working Groups:''' WGI

'''Definition:''' Halosteric sea level change occurs as a result of salinity variations: higher salinity leads to higher density and decreases the volume per unit of mass. Although both processes can be relevant on regional to local scales, only thermosteric changes impact the global mean sea level (GMSL) change, whereas the global mean halosteric change is negligible (Gregory et al., 2019).

=== Hazard ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The potential occurrence of a natural or human-induced physical event or trend that may cause loss of life, injury, or other health impacts, as well as damage and loss to property, infrastructure, livelihoods, service provision, ecosystems and environmental resources.

=== Health ===
'''Working Groups:''' WGII

'''Definition:''' Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity (WHO).

=== Heat index ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A measure of how hot the air feels to the human body. The index is mainly based on surface air temperature and relative humidity and thus reflects the combined effect of high temperature and humidity on human physiology and provides a relative indication of potential health risks.

=== Heat stress ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A range of conditions in, for example, terrestrial or aquatic organisms when the body absorbs excess heat during overexposure to high air or water temperatures or thermal radiation. In aquatic water-breathing animals, hypoxia and acidification can exacerbate vulnerability to heat. Heat stress in mammals (including humans) and birds, both in air, is exacerbated by a detrimental combination of ambient heat, high humidity and low wind speeds, causing regulation of body temperature to fail.

=== Heatwave ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A period of abnormally hot weather, often defined with reference to a relative temperature threshold, lasting from two days to months. Heatwaves and warm spells have various and, in some cases, overlapping definitions.

=== Heavy precipitation event ===
'''Working Groups:''' WGI; WGII

'''Definition:''' An extreme/heavy precipitation event is an event that is of very high magnitude with a very rare occurrence at a particular place. Types of extreme precipitation may vary depending on its duration, hourly, daily or multi-days (e.g., 5 days), though all of them qualitatively represent high magnitude. The intensity of such events may be defined with block maxima approach such as annual maxima or with peak over threshold approach, such as rainfall above 95th or 99th percentile at a particular space.

=== Hedonic ===
'''Working Groups:''' WGIII

'''Definition:''' Subjective well-being concept based on the idea that attaining pleasure and avoiding pain leads to happiness (Ryan and Deci, 2001).

=== Heinrich event ===
'''Working Groups:''' WGI

'''Definition:''' Distinct layers of coarse-grained sediments comprised of ice-rafted debris identified across marine sediment cores in the North Atlantic. These sedimentary layers are closely associated with millennial-scale cooling events in the North Atlantic and a distinct pattern of global temperature and hydrological changes that are largely consistent with evidence for a slowdown, or even near-collapse, of the Atlantic Meridional Overturning Circulation (AMOC) during these times.

=== Heterotrophic respiration ===
'''Working Groups:''' WGI

'''Definition:''' The conversion of organic matter to 2) carbon dioxide (CO by organisms other than autotrophs.

=== Hindcast or retrospective forecast ===
'''Working Groups:''' WGI

'''Definition:''' A forecast made for a period in the past using only information available before the beginning of the forecast. A sequence of hindcasts can be used to calibrate the forecast system and/or provide a measure of the average skill that the forecast system has exhibited in the past as a guide to the skill that might be expected in the future.

=== Holocene ===
'''Working Groups:''' WGI

'''Definition:''' The current interglacial geological epoch, the second of two epochs within the Quaternary Period, the preceding being the Pleistocene. The International Commission on Stratigraphy (ICS) defines the start of the Holocene Epoch at 11,700 years before 2000 (Walker et al., 2019). It encompasses the mid-Holocene (MH), the 1000-year-long interval centred at 6000 years before 1950; a period of long-standing focus for climate modelling, with enhanced seasonality in the Northern Hemisphere and decreased seasonality in the Southern Hemisphere. The early part of the Holocene is marked by the late stages of deglaciation of Pleistocene land ice, sea level rise, and the occurrence of warm phases that affected different regions at different times, often referred to as the ‘Holocene Thermal Maximum’. In addition, the epoch includes the post-glacial interval, which began approximately 7000 years ago when the fundamental features of the modern climate system were essentially in place, as the influence of remnant Pleistocene ice sheets waned.

=== Household carbon footprint ===
'''Working Groups:''' WGIII

'''Definition:''' The carbon footprint of an individual household, inclusive of the direct and indirect carbon dioxide (CO 2) emissions associated with home energy use, transportation, food provision, and consumption of other goods and services associated with household expenditures.

=== Human behaviour ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The responses of persons or groups to a particular situation, here likely to relate to climate change. Human behaviour covers the range of actions by individuals, communities, organisations, governments and at the international level.

=== Human influence on the climate system ===
'''Working Groups:''' WGI

'''Definition:''' Human-driven activities that lead to changes in the climate system due to perturbations of the Earth’s energy budget (also called anthropogenic forcing). Human influence results from emissions of greenhouse gases, aerosols, ozone-depleting substances (ODSs), and land-use change.

=== Human mobility ===
'''Working Groups:''' WGII

'''Definition:''' The permanent or semi-permanent move by a person for at least 1 year and involving crossing an administrative, but not necessarily a national, border.

=== Human rights ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Rights that are inherent to all human beings, universal, inalienable, and indivisible, typically expressed and guaranteed by law. They include the right to life, economic, social, and cultural rights, and the right to development and self-determination (UNOHCHR, 2018).

=== Human security ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A condition that is met when the vital core of human lives is protected, and when people have the freedom and capacity to live with dignity. In the context of climate change, the vital core of human lives includes the universal and culturally specific, material and non-material elements necessary for people to act on behalf of their interests and to live with dignity.

=== Human system ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Any system in which human organisations and institutions play a major role. Often, but not always, the term is synonymous with society or social system. Systems such as agricultural systems, urban systems, political systems, technological systems and economic systems are all human systems in the sense applied in this report.

=== Hydroclimate ===
'''Working Groups:''' WGI

'''Definition:''' Part of the climate pertaining to the hydrology of a region.

=== Hydrofluorocarbons ===
'''Working Groups:''' WGI

'''Also known as:''' Hydrofluorocarbons (HFCs)

'''Definition:''' A type of greenhouse gas (GHG), HFCs are organic compounds that contain fluorine, carbon and hydrogen atoms and they are produced commercially as a substitute for chlorofluorocarbons (CFCs). They are mainly used in refrigeration and semiconductor manufacturing.

=== Hydrological cycle ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The cycle in which water evaporates from the ocean and the land surface, is carried over the Earth in atmospheric circulation as water vapour, condenses to form clouds, precipitates over the ocean and land as rain or snow, which on land can be intercepted by trees and vegetation, potentially accumulating as snow or ice, provides runoff on the land surface, infiltrates into soils, recharges groundwater, discharges into streams, and ultimately, flows into the oceans as rivers, polar glaciers and ice sheets, from which it will eventually evaporate again. The various systems involved in the hydrological cycle are usually referred to as hydrological systems.

=== Hydrological drought ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A period with large runoff and water deficits in rivers, lakes and reservoirs.

=== Hydrological sensitivity ===
'''Working Groups:''' WGI

'''Also known as:''' Hydrological sensitivity (η)

'''Definition:''' The linear change in global mean precipitation per degree Celsius of global mean surface air temperature (GSAT) change once precipitation changes related to fast atmospheric and land surface adjustments to radiative forcings have occurred. Units are % per °C although it can also be calculated as W m –2 per °C.

=== Hydropower ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Power harnessed from the flow of water.

=== Hydrosphere ===
'''Working Groups:''' WGI

'''Definition:''' The component of the climate system comprising liquid surface and subterranean water, such as in oceans, seas, rivers, freshwater lakes, underground water, wetlands, etc.

=== Hyperthermal events ===
'''Working Groups:''' WGII

'''Definition:''' Geologically abrupt global warming events of the past associated with disturbances of the carbon cycle and impacts on the biosphere.

=== Hypoxic ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Conditions of low dissolved oxygen in shallow water ocean and freshwater environments. There is no universal threshold for hypoxia. A value around 60 μmol kg –1 has commonly been used for some estuarine systems, although this does not necessarily directly translate into biological impacts. Anoxic conditions occur where there is no oxygen present at all.

=== Hypoxic events ===
'''Working Groups:''' WGII

'''Definition:''' Events that lead to deficiencies of oxygen in water bodies.

=== Hypsometry ===
'''Working Groups:''' WGI

'''Definition:''' The distribution of land or ice surface as a function of altitude.

== I ==

=== Ice age ===
'''Working Groups:''' WGI

'''Definition:''' An informal term for a geological period characterized by a long-term reduction in the temperature of the Earth’s climate, resulting in the presence or expansion of ice sheets and glaciers. Among the Earth’s ice ages is the current Quaternary Period, characterized by alternating glacial and interglacial intervals.

=== Ice–albedo feedback ===
'''Working Groups:''' WGI

'''Definition:''' A climate feedback involving changes in the Earth’s surface albedo. Snow and ice have an albedo much higher (up to ~0.8) than the average planetary albedo (~0.3). With increasing temperatures, it is anticipated that snow and ice extent will decrease, the Earth’s overall albedo will decrease and more solar radiation will be absorbed, warming the Earth further.

=== Ice core ===
'''Working Groups:''' WGI

'''Definition:''' A cylinder of ice drilled out of a glacier or ice sheet to determine the physical properties of the ice body and to gain information on past changes in climate and composition of the atmosphere that are preserved in the ice or in air trapped in the ice.

=== Ice sheet ===
'''Working Groups:''' WGI; WGII

'''Definition:''' An ice body originating on land that covers an area of continental size, generally defined as covering >50,000 km 2, and that has formed over thousands of years through accumulation and compaction of snow. An ice sheet flows outward from a high central ice plateau with a small average surface slope. The margins usually slope more steeply, and most ice is discharged through fast-flowing ice streams or outlet glaciers, often into the sea or into ice shelves floating on the sea. There are only two ice sheets in the modern world, one on Greenland and one on Antarctica. The latter is divided into the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet. During glacial periods, there were other ice sheets.

=== Ice shelf ===
'''Working Groups:''' WGI

'''Definition:''' A floating slab of ice originating from land of considerable thickness extending from the coast (usually of great horizontal extent with a very gently sloping surface), resulting from the flow of ice sheets, initially formed by the accumulation of snow, and often filling embayments in the coastline of an ice sheet. Nearly all ice shelves are in Antarctica, where most of the ice discharged into the ocean flows via ice shelves.

=== Ice stream ===
'''Working Groups:''' WGI

'''Definition:''' A stream of ice with strongly enhanced flow that is part of an ice sheet. It is often separated from surrounding ice by strongly sheared, crevassed margins.

=== Iceberg ===
'''Working Groups:''' WGI

'''Definition:''' Large piece of freshwater ice broken off from a glacier or an ice shelf during calving and floating in open water (at least 5 m height above sea level). Smaller pieces of floating ice known as ‘bergy bits’ (less than 5 m above sea level) or ‘growlers’ (less than 2 m above sea level) can originate from glaciers or ice shelves, or from the breaking up of a large iceberg. Icebergs can also be classified by shape, most commonly being either tabular (steep sides and a flat top) or non-tabular (varying shapes, with domes and spires) (NOAA, 2021). In lakes, icebergs can originate by breaking off shelf ice, which forms through freezing of a lake surface.

=== Impacts ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The consequences of realised risks on natural and human systems, where risks result from the interactions of climate-related hazards (including extreme weather/climate events), exposure, and vulnerability. Impacts generally refer to effects on lives, livelihoods, health and well-being, ecosystems and species, economic, social and cultural assets, services (including ecosystem services), and infrastructure. Impacts may be referred to as consequences or outcomes, and can be adverse or beneficial.

=== Income ===
'''Working Groups:''' WGII

'''Definition:''' The maximum amount that a household, or other unit, can consume without reducing its real net worth. Total income is the broadest measure of income and refers to regular receipts such as wages and salaries, income from self-employment, interest and dividends from invested funds, pensions or other benefits from social insurance, and other current transfers receivable. OECD (2003).

=== Incremental adaptation ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Adaptation that maintains the essence and integrity of a system or process at a given scale (Park et al., 2012). In some cases, incremental adaptation can accrue to result in transformational adaptation (Tàbara et al., 2019; Termeer et al., 2017). Incremental adaptations to change in climate are understood as extensions of actions and behaviours that already reduce the losses or enhance the benefits of natural variations in extreme weather/climate events.

=== Indian Ocean Dipole ===
'''Working Groups:''' WGI

'''Also known as:''' Indian Ocean Dipole (IOD)

'''Definition:''' A mode of interannual variability that features an east–west dipole of sea surface temperature anomalies in the tropical Indian Ocean. Its positive phase shows concurrent sea surface cooling off Sumatra and Java and warming off Somalia in the west, combined with anomalous surface easterlies along the equator, while the opposite anomalies are seen in the negative phase. The IOD typically develops in boreal summer and matures in boreal autumn and controls part of the rainfall interannual variability in Australia, South Eastern Asia and Eastern Africa. See Section AIV.2.4 in Annex IV of the AR6 WGI report. Wikipedia Page

=== Indian Ocean basin mode ===
'''Working Groups:''' WGI

'''Also known as:''' Indian Ocean basin (IOB) mode

'''Definition:''' A mode of interannual variability characterized by a temporal alternation of basin-wide warming and cooling of the Indian Ocean sea surface. It mostly develops in response to El Niño–Southern Oscillation (ENSO), but often persists after ENSO’s equatorial eastern Pacific signal has dissipated. The IOB affects atmospheric circulation, temperature, and precipitation in South, South East, and East Asia as well as Africa, and modulates tropical cyclone activity in the north western Pacific. See Section AIV.2.4 in Annex IV of the AR6 WGI report.

=== Indigenous Peoples ===
'''Working Groups:''' WGII

'''Definition:''' Indigenous Peoples and Nations are those that, having a historical continuity with pre-invasion and pre-colonial societies that developed on their territories, consider themselves distinct from other sectors of the societies now prevailing on those territories, or parts of them. They form at present principally non-dominant sectors of society and are often determined to preserve, develop, and transmit to future generations their ancestral territories, and their ethnic identity, as the basis of their continued existence as peoples, in accordance with their own cultural patterns, social institutions and common law system. Cobo (1987).

=== Indigenous knowledge ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Indigenous knowledge (IK)

'''Definition:''' The understandings, skills and philosophies developed by societies with long histories of interaction with their natural surroundings. For many indigenous peoples, IK informs decision-making about fundamental aspects of life, from day-to-day activities to longer term actions. This knowledge is integral to cultural complexes, which also encompass language, systems of classification, resource use practices, social interactions, values, ritual and spirituality. These distinctive ways of knowing are important facets of the world’s cultural diversity (UNESCO, 2018).

=== Indirect emissions ===
'''Working Groups:''' WGIII

'''Definition:''' Emissions that are a consequence of the activities within well-defined boundaries of, for instance, a region, an economic sector, a company or process, but which occur outside the specified boundaries. For example, emissions are described as indirect if they relate to the use of heat but physically arise outside the boundaries of the heat user, or to electricity production but physically arise outside of the boundaries of the power supply sector.

=== Indirect land-use change ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Indirect land-use change (iLUC)

'''Definition:''' Land-use change outside the area of focus that occurs as a consequence of change in use or management of land within the area of focus, such as through market or policy drivers. For example, if agricultural land is diverted to biofuel production, forest clearance may occur elsewhere to replace the former agricultural production.

=== Industrial revolution ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A period of rapid industrial growth with far-reaching social and economic consequences, beginning in Britain during the second half of the 18th century and spreading to Europe and later to other countries including the United States. The invention of the steam engine was an important trigger of this development. The industrial revolution marks the beginning of a strong increase in the use of fossil fuels, initially coal, and hence emission of 2) carbon dioxide (CO.

=== Inequality ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Uneven opportunities and social positions, and processes of discrimination within a group or society, based on gender, class, ethnicity, age and (dis)ability, often produced by uneven development. Income inequality refers to gaps between the highest and lowest income earners within a country and between countries.

=== Informal settlement ===
'''Working Groups:''' WGII

'''Definition:''' A term given to settlements or residential areas that by at least one criterion fall outside official rules and regulations. Most informal settlements have poor housing (with widespread use of temporary materials) and are developed on land that is occupied illegally with high levels of overcrowding. In most such settlements, provision for safe water, sanitation, drainage, paved roads and basic services is inadequate or lacking. The term ‘slum’ is often used for informal settlements, although it is misleading as many informal settlements develop into good quality residential areas, especially where governments support such development.

=== Infrastructure ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The designed and built set of physical systems and corresponding institutional arrangements that mediate between people, their communities and the broader environment to provide services that support economic growth, health, quality of life and safety (Chester, 2019; Dawson et al., 2018).

=== Insolation ===
'''Working Groups:''' WGI

'''Definition:''' The amount of solar radiation reaching the Earth by latitude and by season measured in W m –2. Usually, insolation refers to the radiation arriving at the top of the atmosphere. Sometimes it is specified as referring to the radiation arriving at the Earth’s surface.

=== Instantaneous radiative forcing due to aerosol–cloud interactions ===
'''Working Groups:''' WGI

'''Also known as:''' Instantaneous radiative forcing (or effect) due to aerosol–cloud interactions (IRFaci)

'''Definition:''' The radiative forcing (or radiative effect, if the perturbation is internally generated) due to the change in number or size distribution of cloud droplets or ice crystals that is the proximate result of an aerosol perturbation, with other variables (in particular total cloud water content) remaining equal. In liquid clouds, an increase in cloud droplet concentration and surface area would increase the cloud albedo. This effect is also known as the cloud albedo effect, first indirect effect, or Twomey effect. It is a largely theoretical concept that cannot readily be isolated in observations or comprehensive process models due to the ubiquity of adjustments.

=== Instantaneous radiative forcing due to aerosol–radiation interactions ===
'''Working Groups:''' WGI

'''Also known as:''' Instantaneous radiative forcing (or effect) due to aerosol–radiation interactions (IRFari)

'''Definition:''' The radiative forcing (or radiative effect, if the perturbation is internally generated) of an aerosol perturbation due directly to aerosol–radiation interactions, with all environmental variables remaining unaffected. Traditionally known in the literature as the direct aerosol forcing (or effect).

=== Institutional capacity ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Building and strengthening individual organisations and providing technical and management training to support integrated planning and decision-making processes between organisations and people, as well as empowerment, social capital, and an enabling environment, including culture, values and power relations (Willems and Baumert, 2003).

=== Institutions ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Rules, norms and conventions that guide, constrain or enable human behaviours and practices. Institutions can be formally established, for instance through laws and regulations, or informally established, for instance by traditions or customs. Institutions may spur, hinder, strengthen, weaken or distort the emergence, adoption and implementation of climate action and climate governance. [Note: Institutions can also refer to a large organisation.]

=== Insurance/reinsurance ===
'''Working Groups:''' WGII

'''Definition:''' A family of financial instruments for sharing and transferring risk among a pool of at-risk households, businesses and/or governments.

=== Integrated assessment ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A method of analysis that combines results and models from the physical, biological, economic and social sciences and the interactions among these components in a consistent framework to evaluate the status and consequences of environmental change and the policy responses to it.

=== Integrated assessment model ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Integrated assessment model (IAM)

'''Definition:''' Models that integrate knowledge from two or more domains into a single framework. They are one of the main tools for undertaking integrated assessments. One class of IAM used with respect to climate change mitigation may include representations of: multiple sectors of the economy, such as energy, land use and land-use change; interactions between sectors; the economy as a whole; associated greenhouse gas (GHG) emissions and sinks; and reduced representations of the climate system. This class of model is used to assess linkages between economic, social and technological development and the evolution of the climate system. Another class of IAM additionally includes representations of the costs associated with climate change impacts, but includes less detailed representations of economic systems. These can be used to assess impacts and mitigation in a cost–benefit framework and have been used to estimate the social cost of carbon.

=== Integrated assessment scenario ensemble ===
'''Working Groups:''' WGIII

'''Definition:''' A set of modelled scenarios from an intercomparison of integrated assessment models (IAMs) based on a systematic variation of harmonised scenario designs.

=== Inter-decadal Pacific Oscillation ===
'''Working Groups:''' WGI

'''Also known as:''' Inter-decadal Pacific Oscillation (IPO)

'''Definition:''' An equatorially symmetric pattern of sea surface temperature variability at decadal-to-inter-decadal time scales. While the Pacific Decadal Oscillation (PDO) and its South Pacific counterpart, the South Pacific Decadal Oscillation (SPDO), are considered as physically distinct modes, the tropical Pacific decadal–inter-decadal variability can drive both the PDO and SPDO, forming the IPO as a synchronized pan-Pacific variability. Its spatial pattern of sea surface temperature anomalies is similar to that of the El Niño–Southern Oscillation (ENSO), but with a broader meridional extent in the tropical signal and more weights in the extratropics compared to the tropics. In the AR6 WGI report, it is encapsulated within the definition and description of Pacific Decadal Variability (PDV). See also Section AIV.2.6 in Annex IV of the AR6 WGI report.

=== Inter-tropical Convergence Zone ===
'''Working Groups:''' WGI

'''Also known as:''' Inter-tropical Convergence Zone (ITCZ)

'''Definition:''' The Inter-tropical Convergence Zone is an equatorial zonal belt of low pressure, strong convection and heavy precipitation near the equator where the north-east trade winds meet the south-east trade winds. This band moves seasonally.

=== Interglacial or interglaciation ===
'''Working Groups:''' WGI

'''Definition:''' A globally warm period lasting thousands of years between glacial periods within an ice age. Generally coincides with odd-numbered marine isotope stages (MIS) when mean sea level was close to present. The Last Interglacial (LIG) occurred between about 129 and 116 ka (thousand years) before present (defined as 1950) although the warm period started in some areas a few thousand years earlier. In terms of MIS, interglaciations are defined as the interval between the midpoint of the preceding termination and the onset of the next glaciation. The LIG coincides with MIS 5e. The present interglaciation, the Holocene, started at 11,700 years before 2000 CE, although global mean sea level did not approach its present position until roughly 7000 years ago.

=== Internal climate variability ===
'''Working Groups:''' WGI

'''Definition:''' Deviations of climate variables from a given mean state (including the occurrence of extremes, etc.) at all spatial and temporal scales beyond that of individual weather events. Variability may be intrinsic, due to fluctuations of processes internal to the climate system.

=== Internal variability ===
'''Working Groups:''' WGI

'''Definition:''' Fluctuations of the climate dynamical system when subject to a constant or periodic external forcing (such as the annual cycle).

=== Internet of Things ===
'''Working Groups:''' WGIII

'''Also known as:''' Internet of Things (IoT)

'''Definition:''' The network of computing devices embedded in everyday objects such as cars, phones and computers, connected via the internet, enabling them to send and receive data.

=== Interpolation uncertainty ===
'''Working Groups:''' WGI

'''Definition:''' Uncertainty arising from a statistical or physical model-based interpolation of a field between available estimates to create a more spatio-temporally complete estimate.

=== Interstadial or interstade ===
'''Working Groups:''' WGI

'''Definition:''' A brief period of regional climatic warming during a glacial or interglacial interval, often characterized by transient glacial retreats. Interstadials are generally of short duration (hundreds to a few thousand years) compared to glacial or interglacial intervals (lasting many thousands to tens of thousands of years). One example of a regional interstadial event is based on millennial scale warming recorded by oxygen isotope ratios in Greenland ice cores, the so called “Greenland Interstadials” (Johnsen et al., 1992).

=== Invasive species ===
'''Working Groups:''' WGII

'''Definition:''' A species that is not native to a specific location or nearby, lacking natural controls, and that has a tendency to rapidly increase in abundance, displacing native species. Invasive species may also damage the human economy or human health.

=== Irreversibility ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A perturbed state of a dynamical system is defined as irreversible on a given time scale if the recovery from this state due to natural processes takes substantially longer than the time scale of interest.

=== Isostatic or Isostasy ===
'''Working Groups:''' WGI

'''Definition:''' Isostasy refers to the response of the Earth to changes in surface load. It includes the deformational and gravitational response. This response is elastic on short time scales, as in the Earth– ocean response to recent changes in mountain glaciation, or viscoelastic on longer time scales, as in the response to the last deglaciation following the Last Glacial Maximum.

=== Isotopes ===
'''Working Groups:''' WGI

'''Definition:''' Atoms of the same chemical element that have the same the number of protons but differ in the number of neutrons. Some proton–neutron configurations are stable (stable isotopes), others are unstable undergoing spontaneous radioactive decay (radioisotopes). Most elements have more than one stable isotope. Isotopes can be used to trace transport processes or to study processes that change the isotopic ratio. Radioisotopes provide, in addition, time information that can be used for radiometric dating.

== J ==

=== Just transitions ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A set of principles, processes and practices that aim to ensure that no people, workers, places, sectors, countries or regions are left behind in the transition from a high-carbon to a low-carbon economy. It stresses the need for targeted and proactive measures from governments, agencies and authorities to ensure that any negative social, environmental or economic impacts of economy-wide transitions are minimised, while benefits are maximised for those disproportionally affected. Key principles of just transitions include: respect and dignity for vulnerable groups; fairness in energy access and use, social dialogue and democratic consultation with relevant stakeholders; the creation of decent jobs; social protection; and rights at work. Just transitions could include fairness in energy, land use and climate planning and decision-making processes; economic diversification based on low-carbon investments; realistic training/retraining programs that lead to decent work; gender-specific policies that promote equitable outcomes; the fostering of international cooperation and coordinated multilateral actions; and the eradication of poverty. Lastly, just transitions may embody the redressing of past harms and perceived injustices (ILO 2015; UNFCCC 2016).

=== Justice ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Justice is concerned with setting out the moral or legal principles of fairness and equity in the way people are treated, often based on the ethics and values of society.

== K ==

=== Kaya identity ===
'''Working Groups:''' WGIII

'''Definition:''' In this identity, global emissions are equal to the population size, multiplied by per capita output (gross world product), multiplied by the energy intensity of production, multiplied by the carbon intensity of energy.

=== Key climate indicators ===
'''Working Groups:''' WGI

'''Definition:''' Key indicators constitute a finite set of distinct variables that may collectively point to important overall changes in the climate system of broad societal relevance across the atmospheric, oceanic, cryospheric and biospheric domains, with land as an implicit cross-cutting theme. Taken together, these indicators would be expected to both have changed and continue to change in the future in a coherent and consistent manner. See Cross-Chapter Box 2.2, Table 1 in the AR6 WGI report.

=== Key risk ===
'''Working Groups:''' WGII

'''Definition:''' Key risks have potentially severe adverse consequences for humans and social-ecological systems resulting from the interaction of climate related hazards with vulnerabilities of societies and systems exposed.

=== Kriging ===
'''Working Groups:''' WGI

'''Definition:''' Kriging is a method of interpolation (normally spatial interpolation when used with atmospheric or oceanographic data) in which the interpolated values are estimated using a Gaussian process governed by prior covariances.

== L ==

=== Land ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The terrestrial portion of the biosphere that comprises the natural resources (soil, near-surface air, vegetation and other biota, and water), the ecological processes, topography, and human settlements and infrastructure that operate within that system (FAO, 2007; UNCCD, 1994).

=== Land cover ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The biophysical coverage of land (e.g., bare soil, rocks, forests, buildings and roads or lakes). Land cover is often categorised in broad land-cover classes (e.g., deciduous forest, coniferous forest, mixed forest, grassland bare ground). [Note: In some literature, land cover and land use are used interchangeably, but the two represent distinct classification systems. For example, the land cover class woodland can be under various land uses such as livestock grazing, recreation, conservation, or wood harvest.]

=== Land-cover change ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Change from one land cover class to another, due to change in land use or change in natural conditions (Pongratz et al., 2018).

=== Land degradation ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A negative trend in land condition, caused by direct or indirect human-induced processes including anthropogenic climate change, expressed as a long-term reduction or loss of at least one of the following: biological productivity, ecological integrity or value to humans. [Note: This definition applies to forest and non-forest land. Changes in land condition resulting solely from natural processes (such as volcanic eruptions) are not considered to be land degradation. Reduction of biological productivity or ecological integrity or value to humans can constitute degradation, but any one of these changes need not necessarily be considered degradation.]

=== Land degradation neutrality ===
'''Working Groups:''' WGIII

'''Definition:''' A state whereby the amount and quality of land resources necessary to support ecosystem functions and services and enhance food security remain stable or increase within specified temporal and spatial scales and ecosystems (UNCCD, 2020).

=== Land management ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The sum of land-use practices (e.g., sowing, fertilising, weeding, harvesting, thinning and clear-cutting) that take place within broader land-use categories (Pongratz et al., 2018).

=== Land management change ===
'''Working Groups:''' WGIII

'''Definition:''' A change in land management that occurs within a land-use category.

=== Land potential ===
'''Working Groups:''' WGIII

'''Definition:''' The inherent, long-term potential of the land to sustainably generate ecosystem services, which reflects the capacity and resilience of the land-based natural capital, in the face of ongoing environmental change (UNEP, 2016).

=== Land rehabilitation ===
'''Working Groups:''' WGIII

'''Definition:''' Direct or indirect actions undertaken with the aim of reinstating a level of ecosystem functionality, where the goal is provision of goods and services rather than ecological restoration (McDonald et al., 2016).

=== Land restoration ===
'''Working Groups:''' WGIII

'''Definition:''' The process of assisting the recovery of land from a degraded state (IPBES, 2018; McDonald et al. 2016).

=== Land surface air temperature ===
'''Working Groups:''' WGI

'''Also known as:''' Land surface air temperature (LSAT)

'''Definition:''' The near-surface air temperature over land, typically measured at 1.25–2 m above the ground using standard meteorological equipment.

=== Land use ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The total of arrangements, activities and inputs applied to a parcel of land. The term land use is also used in the sense of the social and economic purposes for which land is managed (e.g., grazing, timber extraction, conservation and city dwelling). In national greenhouse gas (GHG) inventories, land use is classified according to the IPCC land-use categories of forest land, cropland, grassland, wetlands, settlements and other lands (see the 2006 IPCC Guidelines for National GHG Inventories and their 2019 Refinement for details (IPCC, 2006, 2019)).

=== Land use, land-use change and forestry ===
'''Working Groups:''' WGIII

'''Also known as:''' Land use, land-use change and forestry (LULUCF)

'''Definition:''' In the context of national greenhouse gas (GHG) inventories under the United Nations Framework Convention on Climate Change (UNFCCC, 2019), LULUCF is a GHG inventory sector that covers anthropogenic emissions and removals of GHG in managed lands, excluding non-CO2 agricultural emissions. Following the 2006 IPCC Guidelines for National GHG Inventories and their 2019 Refinement, ‘anthropogenic’ land-related GHG fluxes are defined as all those occurring on ‘managed land’, that is, ‘where human interventions and practices have been applied to perform production, ecological or social functions’. Since managed land may include carbon dioxide (CO2) removals not considered as ‘anthropogenic’ in some of the scientific literature assessed in this report (e.g., removals associated with CO2 fertilisation and N deposition), the land-related net GHG emission estimates from global models included in this report are not necessarily directly comparable with LULUCF estimates in National GHG Inventories. (IPCC 2006, 2019).

=== Land-use change ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Land-use change (LUC)

'''Definition:''' The change from one land use category to another. Note that in some scientific literature, land-use change encompasses changes in land-use categories as well as changes in land management.

=== Land water storage ===
'''Working Groups:''' WGI

'''Also known as:''' Land water storage (LWS)

'''Definition:''' Land water storage (LWS) includes all surface water, soil moisture, groundwater storage and snow, but excludes water stored in glaciers and ice sheets. Changes in LWS can be caused either by direct human intervention in the water cycle (e.g., storage of water in reservoirs by building dams in rivers, groundwater extraction from groundwater reservoirs for consumption and irrigation, or deforestation) or by climate variations (e.g., changes in the amount of water in endorheic lakes and wetlands, the canopy, the soil, the permafrost and the snowpack). Land water storage changes caused by climate variations may also be indirectly affected by anthropogenic influences.

=== Lapse rate ===
'''Working Groups:''' WGI

'''Definition:''' The rate of change of an atmospheric variable, usually temperature, with height. The lapse rate is considered positive when the variable decreases with height.

=== Large-scale ===
'''Working Groups:''' WGI

'''Definition:''' The climate system involves process interactions from the micro- to the global-scale. Any threshold for defining ‘large-scale’ is arbitrary. Understanding of large-scale climate variability and change requires knowledge of both the response to external forcings and the role of internal variability. Many external forcings have substantial hemispheric or continental scale variations. Modes of climate variability are driven by ocean- basin-scale processes. Thus we define large-scale to include ocean-basin and continental scales as well as hemispheric and global scales.

=== Last millennium ===
'''Working Groups:''' WGI

'''Definition:''' The interval of the Common Era (CE) between 1001 and 2000 CE. Encompasses the Little Ice Age, a roughly defined period characterized by multiple expansions of mountain glaciers worldwide, the timing of which differs among regions but generally occurred between 1400 CE and 1900 CE. The last millennium also mostly encompasses the Medieval Warm Period (also called the Medieval Climate Anomaly), a roughly defined period of relatively warm conditions or other climate excursions such as extensive drought, the timing and magnitude of which differ among regions, but generally occurred between 900 and 1400 CE. Transient climate model experiments by the Paleoclimate Modelling Intercomparison Project (PMIP) for the last millennium extend from 850–1849 CE.

=== Latent heat flux ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The turbulent flux of heat from the Earth’s surface to the atmosphere that is associated with evaporation or condensation of water vapour at the surface; a component of the surface energy budget.

=== Leakage ===
'''Working Groups:''' WGIII

'''Definition:''' The effects of policies that result in a displacement of the environmental impact, thereby counteracting the intended effects of the initial policies.

=== Leapfrogging ===
'''Working Groups:''' WGIII

'''Definition:''' The ability of developing countries to bypass intermediate technologies and jump straight to advanced clean technologies.

=== Least Developed Countries ===
'''Working Groups:''' WGII

'''Also known as:''' Least Developed Countries (LDCs)

'''Definition:''' A list of countries designated by the Economic and Social Council of the United Nations (ECOSOC) as meeting three criteria: (1) a low income criterion below a certain threshold of gross national income per capita of between USD 750 and USD 900, (2) a human resource weakness based on indicators of health, education and adult literacy, and (3) an economic vulnerability weakness based on indicators on instability of agricultural production, instability of export of goods and services, economic importance of non-traditional activities, merchandise export concentration and the handicap of economic smallness. Countries in this category are eligible for a number of programmes focused on assisting countries most in need. These privileges include certain benefits under the articles of the United Nations Framework Convention on Climate Change (UNFCCC).

=== Lifecycle assessment ===
'''Working Groups:''' WGIII

'''Also known as:''' Lifecycle assessment (LCA)

'''Definition:''' Compilation and evaluation of the inputs, outputs and the potential environmental impacts of a product or service throughout its lifecycle (ISO, 2018).

=== Lifetime ===
'''Working Groups:''' WGI

'''Definition:''' Lifetime is a general term used for various time scales characterizing the rate of processes affecting the concentration of trace gases. The following lifetimes may be distinguished:

=== Light-absorbing particles ===
'''Working Groups:''' WGI

'''Definition:''' Light-absorbing particles (LAP), for example, black carbon (BC), brown carbon and dust, are particles that absorb solar radiation and convert it into internal energy, thus raising the particle’s temperature and emitting thermal-infrared radiation that is selectively absorbed by the surrounding medium. LAP affect the energy balance of the atmosphere and clouds, and when deposited on snow and ice, they reduce snow/ice albedo, increasing heating and accelerating melting. These particles have a warming effect on climate.

=== Likelihood ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The chance of a specific outcome occurring, where this might be estimated probabilistically. Likelihood is expressed in this report using a standard terminology (Mastrandrea et al., 2010).

=== Lithosphere ===
'''Working Groups:''' WGI

'''Definition:''' The upper layer of the solid Earth, both continental and oceanic, which comprises all crustal rocks and the cold, mainly elastic part of the uppermost mantle. Volcanic activity, although part of the lithosphere, is not considered as part of the climate system, but acts as an external forcing factor.

=== Livelihood ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The resources used and the activities undertaken in order for people to live. Livelihoods are usually determined by the entitlements and assets to which people have access. Such assets can be categorised as human, social, natural, physical or financial.

=== Local extinction ===
'''Working Groups:''' WGII

'''Definition:''' See extirpation

=== Local knowledge ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Local knowledge (LK)

'''Definition:''' The understandings and skills developed by individuals and populations, specific to the places where they live. Local knowledge informs decision-making about fundamental aspects of life, from day-to-day activities to longer-term actions. This knowledge is a key element of the social and cultural systems which influence observations of and responses to climate change; it also informs governance decisions (UNESCO, 2018).

=== Local sea level change ===
'''Working Groups:''' WGI

'''Definition:''' Change in sea level relative to a datum (such as present-day mean sea level) at spatial scales smaller than 10 km.

=== Lock-in ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A situation in which the future development of a system, including infrastructure, technologies, investments, institutions and behavioural norms, is determined or constrained (‘locked in’) by historical developments.

=== Long-lived climate forcers ===
'''Working Groups:''' WGIII

'''Also known as:''' Long-lived climate forcers (LLCFs)

'''Definition:''' [TERM NOT USED - Term name change to ''Long-lived greenhouse gases (LLGHGs) in WGI report] A set of well-mixed greenhouse gases with long atmospheric lifetimes. This set of compounds includes carbon dioxide (CO2) and nitrous oxide (N2O), together with some fluorinated gases. They have a warming effect on climate. These compounds accumulate in the atmosphere at decadal to centennial timescales, and their effect on climate hence persists for decades to centuries after their emission. On timescales of decades to a century already emitted emissions of long-lived climate forcers can only be abated by greenhouse gas removal (GGR).

=== Long-lived greenhouse gases ===
'''Working Groups:''' WGI

'''Also known as:''' Long-lived greenhouse gases (LLGHGs)

'''Definition:''' A set of well-mixed greenhouse gases with long atmospheric lifetimes. This set of compounds includes 2) carbon dioxide (CO and 2 O) nitrous oxide (N, together with some halogenated compounds. They have a warming effect on climate. These compounds accumulate in the atmosphere at decadal to centennial time scales, and their effect on climate hence persists for decades to centuries after their emission. On time scales of decades to a century, already emitted emissions of long-lived climate forcers can only be abated by greenhouse gas removal.

=== Loss and Damage, and losses and damages ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Research has taken Loss and Damage (capitalised letters) to refer to political debate under the United Nations Framework Convention on Climate Change (UNFCCC) following the establishment of the Warsaw International Mechanism for Loss and Damage in 2013, which is to ‘address loss and damage associated with impacts of climate change, including extreme events and slow onset events, in developing countries that are particularly vulnerable to the adverse effects of climate change.’ Lowercase letters (losses and damages) have been taken to refer broadly to harm from (observed) impacts and (projected) risks and can be economic or non-economic (Mechler et al., 2018).

=== Low Elevation Coastal Zones ===
'''Working Groups:''' WGII

'''Also known as:''' Low Elevation Coastal Zones (LECZ)

'''Definition:''' Coastal areas below 10 m of elevation above sea level that are hydrologically connected to the sea.

=== Low-likelihood, high impact outcomes ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Outcomes/events whose probability of occurrence is low or not well known (as in the context of deep uncertainty) but whose potential impacts on society and ecosystems could be high. To better inform risk assessment and decision-making, such low-likelihood outcomes are considered if they are associated with very large consequences and may therefore constitute material risks, even though those consequences do not necessarily represent the most likely outcome.

== M ==

=== Madden–Julian Oscillation ===
'''Working Groups:''' WGI

'''Also known as:''' Madden–Julian Oscillation (MJO)

'''Definition:''' The largest mode of tropical atmospheric intra-seasonal variability with typical periods ranging from 20 to 90 days. The MJO corresponds to planetary-scale disturbances of pressure, wind and deep convection moving predominantly eastward along the equator. As it progresses, the MJO is associated with the temporal alternation of large-scale enhanced and suppressed rainfall, with maximum loading over the Indian and western Pacific oceans, although influences of the MJO can be tracked over the Atlantic/Africa in dynamical fields. See Section AIV.2.8 in Annex IV of the AR6 WGI report.

=== Maladaptive actions ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Maladaptive actions (Maladaptation)

'''Definition:''' Actions that may lead to increased risk of adverse climate-related outcomes, including via increased greenhouse gas (GHG) emissions, increased or shifted vulnerability to climate change, more inequitable outcomes, or diminished welfare, now or in the future. Most often, maladaptation is an unintended consequence.

=== Malnutrition ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Deficiencies, excesses, or imbalances in a person’s intake of energy and/or nutrients. The term malnutrition addresses three broad groups of conditions: undernutrition, which includes wasting (low weight-for-height), stunting (low height-for-age) and underweight (low weight-for-age); micronutrient-related malnutrition, which includes micronutrient deficiencies (a lack of important vitamins and minerals) or micronutrient excess; and overweight, obesity and diet-related non-communicable diseases (such as heart disease, stroke, diabetes and some cancers) (WHO, 2018). Micronutrient deficiencies are sometimes termed ‘hidden hunger’ to emphasise that people can be malnourished in the sense of deficient without being deficient in calories. Hidden hunger can apply even where people are obese.

=== Managed forest ===
'''Working Groups:''' WGIII

'''Definition:''' Forests subject to human interventions (notably silvicultural management such as planting, pruning, thinning), timber and fuelwood harvest, protection (fire suppression, insect suppression) and management for amenity values or conservation, with defined geographical boundaries (Ogle et al., 2018). [Note: For a discussion of the term ‘forest’ in the context of National GHG inventories, see the 2006 IPCC Guidelines for National GHG Inventories (IPCC 2006).]

=== Managed grassland ===
'''Working Groups:''' WGIII

'''Definition:''' Grasslands on which human interventions are carried out, such as grazing domestic livestock or hay removal.

=== Managed land ===
'''Working Groups:''' WGIII

'''Definition:''' In the context of national greenhouse gas (GHG) inventories under the United Nations Framework Convention on Climate Change (UNFCCC), the 2006 IPCC Guidelines for National GHG Inventories (IPCC, 2006) defines managed land ‘where human interventions and practices have been applied to perform production, ecological or social functions’. IPCC (2006) defines anthropogenic GHG emissions and removals in the LULUCF sector as all those occurring on ‘managed land’. The key rationale for this approach is that the preponderance of anthropogenic effects occurs on managed lands. [Note: More details can be found in IPCC 2006 Guidelines for National GHG Inventories, Volume 4, Chapter 1.]

=== Marine-based ice sheet ===
'''Working Groups:''' WGI

'''Definition:''' An ice sheet containing a substantial region that rests on a bed lying below sea level and whose perimeter is in contact with the ocean. The best known example is the West Antarctic Ice Sheet.

=== Marine cloud brightening ===
'''Working Groups:''' WGI

'''Also known as:''' Marine cloud brightening (MCB)

'''Definition:''' One of several solar radiation modification (SRM) approaches to increase the planetary albedo. In this approach, it is proposed to inject sea salt aerosols into persistent marine low clouds. This is expected to increase the cloud droplet concentration of these clouds and their reflectivity.

=== Marine heatwave ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A period during which water temperature is abnormally warm for the time of the year relative to historical temperatures, with that extreme warmth persisting for days to months. The phenomenon can manifest in any place in the ocean and at scales of up to thousands of kilometres.

=== Marine ice cliff instability ===
'''Working Groups:''' WGI

'''Also known as:''' Marine ice cliff instability (MICI)

'''Definition:''' A hypothetical mechanism of an ice cliff failure. In case a marine-terminated ice sheet loses its buttressing ice shelf, an ice cliff can be exposed. If the exposed ice cliff is tall enough (about 800 m of the total height, or about 100 m of the above-water part), the stresses at the cliff face exceed the strength of the ice, and the cliff fails structurally in repeated calving events.

=== Marine ice sheet instability ===
'''Working Groups:''' WGI

'''Also known as:''' Marine ice sheet instability (MISI)

'''Definition:''' A mechanism of irreversible (on the decadal to centennial time scale) retreat of a grounding line for the marine-terminating glaciers, in case the glacier bed slopes towards the ice sheet interior.

=== Marine isotope stage ===
'''Working Groups:''' WGI

'''Also known as:''' Marine isotope stage (MIS)

'''Definition:''' Geological periods of alternating glacial and interglacial conditions, each typically lasting tens of thousands of years as inferred from the oxygen isotope composition of microfossils from deep sea sediment cores. MIS numbers increase back in time from the present, which is MIS 1. Even-number MISs coincide with glacial periods, and odd-numbered MISs are interglacials.

=== Market failure ===
'''Working Groups:''' WGIII

'''Definition:''' When private decisions are based on market prices that do not reflect the real scarcity of goods and services but rather reflect market distortions, they do not generate an efficient allocation of resources but cause welfare losses. A market distortion is any event in which a market reaches a market clearing price that is substantially different from the price that a market would achieve while operating under conditions of perfect competition and state enforcement of legal contracts and the ownership of private property. Examples of factors causing market prices to deviate from real economic scarcity are environmental externalities, public goods, monopoly power, information asymmetry, transaction costs, and non-rational behaviour.

=== Mass balance/budget ===
'''Working Groups:''' WGI

'''Also known as:''' Mass balance/budget (of glaciers or ice sheets)

'''Definition:''' Difference between the mass input (accumulation) and the mass loss (ablation) of an ice body (e.g., a glacier or ice sheet) over a stated time period, which is often a year or a season. Surface mass balance refers to the difference between surface accumulation and surface ablation.

=== Material substitution ===
'''Working Groups:''' WGIII

'''Definition:''' Replacement of one material (including an energy carrier used as a feedstock) by another, due to scarcity, price, technological change, or because of lower environmental impacts or greenhouse gas emissions.

=== Mean sea level ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The surface level of the ocean at a particular point averaged over an extended period of time such as a month or year. Mean sea level is often used as a national datum to which heights on land are referred.

=== Measurement ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' ‘Processes of data collection over time, providing basic datasets, including associated accuracy and precision, for the range of relevant variables. Possible data sources are field measurements, field observations, detection through remote sensing and interviews’ (UN-REDD, 2009).

=== Measurement, Reporting and Verification ===
'''Working Groups:''' WGIII

'''Also known as:''' Measurement, Reporting and Verification (MRV)

'''Definition:''' Measurement ‘Processes of data collection over time, providing basic datasets, including associated accuracy and precision, for the range of relevant variables. Possible data sources are field measurements, field observations, detection through remote sensing and interviews’ (UN-REDD, 2009). Reporting ‘The process of formal reporting of assessment results to the UNFCCC, according to predetermined formats and according to established standards, especially the Intergovernmental Panel on Climate Change (IPCC) Guidelines and GPG (Good Practice Guidance)’ (UN-REDD, 2009). Verification ‘The process of formal verification of reports, for example, the established approach to verify national communications and national inventory reports to the UNFCCC’ (UN-REDD, 2009).

=== Megacity ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' An urban agglomeration with 10 million inhabitants or more (United Nations, Department of Economic and Social Affairs, Population Division (2019).

=== Megadrought ===
'''Working Groups:''' WGII

'''Definition:''' A very lengthy and pervasive drought, lasting much longer than normal, usually a decade or more.

=== Meltwater Pulse 1A ===
'''Working Groups:''' WGI

'''Also known as:''' Meltwater Pulse 1A (MWP-1A)

'''Definition:''' A particular interval of rapid global sea level rise between about 14,700 and 14,300 years ago, associated with the end of the last ice age and attributed to freshwater flux to the ocean from accelerated melting of ice sheets and glaciers. First defined based on oxygen isotope data (Duplessy et al., 1981), and later shown to be reflected by high rates of sea level rise (Fairbanks, 1989).

=== Mental health ===
'''Working Groups:''' WGII

'''Definition:''' The state of well-being in which an individual realises his or her own abilities, can cope with the normal stresses of life, can work productively and is able to contribute to his or her community.

=== Meridional overturning circulation ===
'''Working Groups:''' WGI

'''Also known as:''' Meridional overturning circulation (MOC)

'''Definition:''' Meridional (north–south) overturning circulation in the ocean quantified by zonal (east–west) sums of mass transports in depth or density layers. In the North Atlantic, away from the subpolar regions, the MOC (which is in principle an observable quantity) is often identified with the thermohaline circulation (THC), which is a conceptual and incomplete interpretation. The MOC is also driven by wind, and can also include shallower overturning cells such as occur in the upper ocean in the tropics and subtropics, in which warm (light) waters moving poleward are transformed to slightly denser waters and subducted equatorward at deeper levels.

=== Meteorological drought ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A period with an abnormal precipitation deficit.

=== Methane ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Methane (CH4)

'''Definition:''' The greenhouse gas methane is the major component of natural gas and associated with all hydrocarbon fuels. Significant anthropogenic emissions also occur as a result of animal husbandry and paddy rice production. Methane is also produced naturally where organic matter decays under anaerobic conditions, such as in wetlands. Under future global warming, there is potential for increased methane emissions from thawing permafrost, wetlands and sub-sea gas hydrates.

=== Metric ===
'''Working Groups:''' WGII

'''Definition:''' A consistent measurement of a characteristic of an object or activity that is otherwise difficult to quantify. Within the context of the evaluation of climate models, this is a quantitative measure of agreement between a simulated and an observed quantity which can be used to assess the performance of individual models.

=== Microclimate ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Local climate at or near the Earth’s surface.

=== Microwave sounding unit ===
'''Working Groups:''' WGI

'''Also known as:''' Microwave sounding unit (MSU)

'''Definition:''' A microwave sounder on U.S. National Oceanic and Atmospheric Administration (NOAA) polar orbiter satellites that estimates the temperature of thick layers of the atmosphere by measuring the thermal emission of oxygen molecules from a complex of emission lines near 60 GHz. A series of nine MSUs began making this kind of measurement in late 1978. Beginning in mid-1998, a follow-on series of instruments, the Advanced Microwave Sounding Units (AMSUs), began operation.

=== Migrant ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Any person who is moving or has moved across an international border or within a State away from his/her habitual place of residence, regardless of (1) the person’s legal status; (2) whether the movement is voluntary or involuntary; (3) what the causes for the movement are; or (4) what the length of the stay is (IOM, 2018).

=== Migration ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Migration (of humans)

'''Definition:''' Movement of a person or a group of persons, either across an international border, or within a State. It is a population movement, encompassing any kind of movement of people, whatever its length, composition and causes; it includes migration of refugees, displaced persons, economic migrants, and persons moving for other purposes, including family reunification (IOM, 2018).

=== Mineralization/Remineralization ===
'''Working Groups:''' WGI

'''Definition:''' The conversion of an element from its organic form to an inorganic form as a result of microbial decomposition. In nitrogen mineralization, organic nitrogen from decaying plant and animal residues (proteins, nucleic acids, amino sugars and urea) is converted to ammonia (NH 3) and ammonium (NH 4 +) by biological activity.

=== Mitigation ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Mitigation (of climate change)

'''Definition:''' A human intervention to reduce emissions or enhance the sinks of greenhouse gases.

=== Mitigation measures ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' In climate policy, mitigation measures are technologies, processes or practices that contribute to mitigation, for example, renewable energy technologies, waste minimisation processes, and public transport commuting practices.

=== Mitigation option ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A technology or practice that reduces greenhouse gas (GHG) emissions or enhances sinks.

=== Mitigation pathways ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A temporal evolution of a set of mitigation scenario features, such as greenhouse gas (GHG) emissions and socio-economic development.

=== Mitigation potential ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The quantity of net greenhouse gas emission reductions that can be achieved by a given mitigation option relative to specified emission baselines. [Note: Net greenhouse gas emission reduction is the sum of reduced emissions and/or enhanced sinks.]

=== Mitigation scenario ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A plausible description of the future that describes how the (studied) system responds to the implementation of mitigation policies and measures.

=== Model initialization ===
'''Working Groups:''' WGI

'''Definition:''' A climate prediction typically proceeds by integrating a climate model forward in time from an initial state that is intended to reflect the actual state of the climate system. Available observations of the climate system are assimilated into the model. Initialization is a complex process that is limited by available observations, observational errors and, depending on the procedure used, may be affected by uncertainty in the history of climate forcing. The initial conditions will contain errors that grow as the forecast progresses, thereby limiting the time period over which the forecast will be useful.

=== Model spread ===
'''Working Groups:''' WGI

'''Definition:''' The range or spread in results from climate models, such as those assembled for Coupled Model Intercomparison Project Phase 6 (CMIP6). Does not necessarily provide an exhaustive and formal estimate of the uncertainty in feedbacks, forcing or projections even when expressed numerically, for example, by computing a standard deviation of the models’ responses. In order to quantify uncertainty, information from observations, physical constraints and expert judgement must be combined, using a statistical framework.

=== Models ===
'''Working Groups:''' WGII

'''Definition:''' Structured imitations of a system’s attributes and mechanisms to mimic the appearance or functioning of systems, for example, the climate, the economy of a country, or a crop. Mathematical models assemble (many) variables and relations (often in a computer code) to simulate system functioning and performance for variations in parameters and inputs.

=== Modes of climate variability ===
'''Working Groups:''' WGI

'''Definition:''' Recurrent space-time structures of natural variability of the climate system with intrinsic spatial patterns, seasonality and time scales. Modes can arise through the dynamical characteristics of the atmospheric circulation but also through coupling between the ocean and the atmosphere, with some interactions with land surfaces and sea ice. Many modes of variability are driven by internal climate processes and are a critical potential source of climate predictability on sub-seasonal to decadal time scales. See Annex IV of the AR6 WGI report.

=== Mole fraction or mixing ratio ===
'''Working Groups:''' WGI

'''Definition:''' Mole fraction, or mixing ratio, is the ratio of the number of moles of a constituent in a given volume to the total number of moles of all constituents in that volume. It is usually reported for dry air. Typical values for well-mixed greenhouse gases are in the order of μmol mol –1 (parts per million: ppm), nmol mol –1 (parts per billion: ppb), and fmol mol –1 (parts per trillion: ppt). Mole fraction differs from volume mixing ratio, often expressed in ppmv, etc., by the corrections for non-ideality of gases. This correction is significant relative to measurement precision for many greenhouse gases (Schwartz and Warneck, 1995).

=== Monitoring and evaluation ===
'''Working Groups:''' WGII

'''Also known as:''' Monitoring and evaluation (M&E)

'''Definition:''' Mechanisms put in place to respectively monitor and evaluate efforts to reduce greenhouse gas emissions and/or adapt to the impacts of climate change with the aim of systematically identifying, characterising and assessing progress over time.

=== Montreal Protocol ===
'''Working Groups:''' WGI

'''Definition:''' The Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in Montreal in 1987, and subsequently adjusted and amended (including London (1990), Copenhagen (1992), Vienna (1995), Montreal (1997), Beijing (1999) and Kigali(2016)). It controls the consumption and production of chlorine- and bromine-containing chemicals that destroy 3) stratospheric ozone (O, such as chlorofluorocarbons (CFCs), methyl chloroform, carbon tetrachloride and many others. Since the Kigali Amendment in 2016, hydrofluorocarbons (HFCs), which were used as alternatives to ozone-depleting substances (ODSs), have been targeted for a phase-down due to their climate effect as greenhouse gases (GHGs).

=== Mountains ===
'''Working Groups:''' WGII

'''Definition:''' A mountain is a landform formed through plate tectonics that rises above its surrounding area, characterised by verticality and ruggedness such as gentle or steep sloping sides, sharp or rounded ridges and a high point called a peak or a summit. Mountain regions consist of mountains and mountain ranges as defined by ruggedness, intermontane valleys, plateaus and tablelands, and hills and hilly forelands, together forming a complex terrain. To delineate mountain regions, a combination of terrain characteristics is used, such as elevation above sea level, steepness of slope and relative relief or local elevational range. Three mountain characterisations using different combinations of the above criteria applied to digital elevation models have been developed to arrive at mountain area statistics, described and analysed in detail by Sayre et al. (2018), namely K1 (Kapos et al., 2000), K2 (Körner et al., 2011) and K3 (Karagulle et al., 2017).

=== Multi-level governance ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The dispersion of governance across multiple levels of jurisdiction and decision-making, including, global, regional, national and local, as well as trans-regional and trans-national levels.

== N ==

=== Narrative ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Qualitative descriptions of plausible future world evolutions, describing the characteristics, general logic and developments underlying a particular quantitative set of scenarios. Narratives are also referred to in the literature as “storylines”.

=== Native species ===
'''Working Groups:''' WGII

'''Definition:''' Indigenous species of animals or plants that naturally occur in a given region or ecosystem. Under climate change, many species colonise new areas where they may become native over time (following IPBES 2019).

=== Natural systems ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The dynamic physical, physicochemical and biological components of the Earth system that would operate independently of human activities.

=== Natural variability ===
'''Working Groups:''' WGI

'''Definition:''' Natural variability refers to climatic fluctuations that occur without any human influence, that is internal variability combined with the response to external natural factors such as volcanic eruptions, changes in solar activity and, on longer time-scales, orbital effects and plate tectonics.

=== Nature-based solutions ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Nature-based solutions (NbS)

'''Definition:''' Actions to protect, sustainably manage and restore natural or modified ecosystems that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits. (IUCN, 2016)

=== Nature's contributions to people ===
'''Working Groups:''' WGIII

'''Also known as:''' Nature's contributions to people (NCP)

'''Definition:''' All the contributions, both positive and negative, of living nature (i.e., diversity of organisms, ecosystems, and their associated ecological and evolutionary processes) to the quality of life for people. Beneficial contributions from nature include such things as food provision, water purification, flood control, and artistic inspiration, whereas detrimental contributions include disease transmission and predation that damages people or their assets. Many NCP may be perceived as benefits or detriments depending on the cultural, temporal or spatial context (Díaz et al, 2018).

=== Near-surface permafrost ===
'''Working Groups:''' WGI

'''Definition:''' Permafrost within about 3–4 m of the ground surface. The depth is not precise, but describes what commonly is highly relevant for people and ecosystems. Deeper permafrost is often progressively less ice-rich and responds more slowly to warming than near-surface permafrost. The presence or absence of near-surface permafrost is not the only significant metric of permafrost change, and deeper permafrost may persist when near-surface permafrost is absent.

=== Negative greenhouse gas emissions ===
'''Working Groups:''' WGI

'''Definition:''' Removal of greenhouse gases (GHGs) from the atmosphere by deliberate human activities, that is, in addition to the removal that would occur via natural carbon cycle or atmospheric chemistry processes.

=== Net negative greenhouse gas emissions ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A situation of net negative greenhouse gas emissions is achieved when metric-weighted anthropogenic greenhouse gas (GHG) removals exceed metric-weighted anthropogenic GHG emissions. Where multiple GHG are involved, the quantification of net emissions depends on the metric chosen to compare emissions of different gases (such as global warming potential, global temperature change potential, and others, as well as the chosen time horizon).

=== Net primary production ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Net primary production (NPP)

'''Definition:''' The amount of carbon fixed by photosynthesis minus the amount lost by respiration over a specified time period.

=== Net zero CO2 emissions ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Condition in which anthropogenic carbon dioxide (CO2) emissions are balanced by anthropogenic CO2 removals over a specified period. Note: Carbon neutrality and net zero CO2 emissions are overlapping concepts. The concepts can be applied at global or sub-global scales (e.g., regional, national and sub-national). At a global scale, the terms carbon neutrality and net zero CO2 emissions are equivalent. At sub-global scales, net zero CO2 emissions is generally applied to emissions and removals under direct control or territorial responsibility of the reporting entity, while carbon neutrality generally includes emissions and removals within and beyond the direct control or territorial responsibility of the reporting entity. Accounting rules specified by GHG programmes or schemes can have a significant influence on the quantification of relevant CO2 emissions and removals.

=== Net zero greenhouse gas emissions ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Condition in which metric-weighted anthropogenic greenhouse gas (GHG) emissions are balanced by metric-weighted anthropogenic GHG removals over a specified period. The quantification of net zero GHG emissions depends on the GHG emission metric chosen to compare emissions and removals of different gases, as well as the time horizon chosen for that metric. [Note 1: Greenhouse gas neutrality and net zero GHG emissions are overlapping concepts. The concept of net zero GHG emissions can be applied at global or sub-global scales (e.g., regional, national and sub-national). At a global scale, the terms GHG neutrality and net zero GHG emissions are equivalent. At sub-global scales, net zero GHG emissions is generally applied to emissions and removals under direct control or territorial responsibility of the reporting entity, while GHG neutrality generally includes anthropogenic emissions and anthropogenic removals within and beyond the direct control or territorial responsibility of the reporting entity. Accounting rules specified by GHG programmes or schemes can have a significant influence on the quantification of relevant emissions and removals. Note 2: Under the Paris Rulebook (Decision 18/CMA.1, annex, paragraph 37), parties have agreed to use GWP100 values from the IPCC AR5 or GWP100 values from a subsequent IPCC Assessment Report to report aggregate emissions and removals of GHGs. In addition, parties may use other metrics to report supplemental information on aggregate emissions and removals of GHGs.]

=== New Urban Agenda ===
'''Working Groups:''' WGII

'''Definition:''' The New Urban Agenda was adopted at the United Nations Conference on Housing and Sustainable Urban Development (Habitat III) in Quito, Ecuador, on 20 October 2016. It was endorsed by the United Nations General Assembly at its 68 th plenary meeting of the 71 st session on 23 December 2016.

=== Nitrogen deposition ===
'''Working Groups:''' WGI

'''Definition:''' Nitrogen deposition is defined as the nitrogen transferred from the atmosphere to the Earth’s surface by the processes of wet deposition and dry deposition.

=== Nitrous oxide ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Nitrous oxide (N2O)

'''Definition:''' The main anthropogenic source of N2O, a greenhouse gas (GHG), is agriculture (soil and animal manure management), but important contributions also come from sewage treatment, fossil fuel combustion, and chemical industrial processes. N2O is also produced naturally from a wide variety of biological sources in soil and water, particularly microbial action in wet tropical forests.

=== Non-CO2 emissions and radiative forcing ===
'''Working Groups:''' WGI

'''Definition:''' Non-CO 2 emissions included in this report are all anthropogenic emissions other than 2) carbon dioxide (CO that result in radiative forcing. These include short-lived climate forcers, such as methane (CH 4), some fluorinated gases, ozone (O 3) precursors, aerosols or aerosol precursors, such as black carbon and sulphur dioxide, respectively, as well as long-lived greenhouse gases, such as nitrous oxide (N 2 O) or other fluorinated gases. The radiative forcing associated with non-CO 2 emissions and changes in surface albedo (e.g., resulting from land-use change) is referred to as non-CO 2 radiative forcing.

=== Non-climatic driver ===
'''Working Groups:''' WGII

'''Also known as:''' Non-climatic driver (Non-climate driver)

'''Definition:''' An agent or process outside the climate system that influences a human or natural system.

=== Non-communicable diseases ===
'''Working Groups:''' WGII

'''Definition:''' Non-communicable diseases (NCDs), also known as chronic diseases, tend to be of long duration and are the result of a combination of genetic, physiological, environmental and behavioural factors. The main types of NCDs are cardiovascular diseases (such as heart attacks and stroke), cancers, chronic respiratory diseases (such as chronic obstructive pulmonary disease and asthma) and diabetes (WHO).

=== Non-linearity ===
'''Working Groups:''' WGI

'''Definition:''' A process is called non-linear when there is no simple proportional relation between cause and effect. The climate system contains many such non-linear processes, resulting in a system with potentially very complex behaviour. Such complexity may lead to abrupt climate change and tipping points.

=== Non-methane volatile organic compounds ===
'''Working Groups:''' WGI

'''Also known as:''' Non-methane volatile organic compounds (NMVOCs)

'''Definition:''' NMVOCs are major contributors (together with NOX and CO) to the formation of photochemical oxidants such as ozone.

=== Non-overshoot pathways ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Pathways that stay below a specified concentration, forcing, or global warming level during a specified period of time (e.g., until 2100).

=== North American monsoon ===
'''Working Groups:''' WGI

'''Also known as:''' North American monsoon (NAmerM)

'''Definition:''' The North American monsoon (NAmerM) is a regional-scale atmospheric circulation system with increases in summer precipitation over northwestern Mexico and southwest United States. The monsoonal characteristics of the region include a pronounced annual maximum of precipitation in boreal summer (June–July–August) accompanied by a surface low pressure system and an upper-level anticyclone, although seasonal reversal of the surface winds is primarily limited to the northern Gulf of California. Further details on how NAmerM is defined and used throughout the Report are provided in Annex V.

=== North Atlantic Oscillation ===
'''Working Groups:''' WGI

'''Also known as:''' North Atlantic Oscillation (NAO)

'''Definition:''' The leading mode of large-scale atmospheric variability in the North Atlantic basin characterized by alternating (see-saw) variations in sea level pressure or geopotential height between the Azores High in the subtropics and the Icelandic Low in the mid- to high latitudes, with some northward extension deep into the Arctic. It is associated with fluctuations in the strength and latitudinal position of the main westerly winds across a vast North Atlantic–Europe domain, and thus with fluctuations in the embedded extratropical cyclones and associated frontal systems leading to strong teleconnection over the entire North Atlantic adjacent continents. The positive and negative phases of the NAO show similar characteristics described for the Northern Annular Mode (NAM). See Section AIV.2.1 in Annex IV of the AR6 WGI report.

=== Northern Annular Mode ===
'''Working Groups:''' WGI

'''Also known as:''' Northern Annular Mode (NAM)

'''Definition:''' A see-saw latitudinal fluctuation in Northern Hemisphere sea-level pressure or geopotential height between the Arctic and the mid-latitudes. The NAM has some links with the stratospheric polar vortex and is related to the fluctuation in strength and latitude of the mean westerlies. Its variance is maximum in winter and its pattern has a strong regional expression in the North Atlantic being strongly correlated with the North Atlantic Oscillation index. The NAM is also known as the Arctic Oscillation (AO). In its positive phase, the NAM is characterized by anomalously low pressure over the Arctic and high pressure over the mid-latitudes/subtropics, with a strengthening of the zonally averaged westerly winds on their polar flank that confines colder air across the Arctic. The negative NAM phase is characterized by a more distorted wind pattern and jet meanders that increase storminess in the mid-latitude regions. See Section AIV.2.1 in Annex IV of the AR6 WGI report.

== O ==

=== Ocean ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The interconnected body of saline water that covers 71% of the Earth’s surface, contains 97% of the Earth’s water and provides 99% of the Earth’s biologically habitable space. It includes the Arctic, Atlantic, Indian, Pacific and Southern Oceans, as well as their marginal seas and coastal waters.

=== Ocean acidification ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Ocean acidification (OA)

'''Definition:''' A reduction in the pH of the ocean, accompanied by other chemical changes (primarily in the levels of carbonate and bicarbonate ions), over an extended period, typically decades or longer, which is caused primarily by uptake of carbon dioxide (CO2) from the atmosphere, but can also be caused by other chemical additions or subtractions from the ocean. Anthropogenic OA refers to the component of pH reduction that is caused by human activity (IPCC, 2011, p. 37).

=== Ocean alkalinization/Ocean alkalinity enhancement ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A proposed carbon dioxide removal (CDR) method that involves deposition of alkaline minerals or their dissociation products at the ocean surface. This increases surface total alkalinity, and may thus increase ocean 2) carbon dioxide (CO uptake and ameliorate surface ocean acidification.

=== Ocean carbon cycle ===
'''Working Groups:''' WGI

'''Definition:''' The ocean carbon cycle is the set of processes that exchange carbon between various pools within the ocean, as well as between the atmosphere, Earth’s interior, cryosphere, and the sea-floor.

=== Ocean deoxygenation ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The loss of oxygen in the ocean. It results from ocean warming, which reduces oxygen solubility and increases oxygen consumption and stratification, thereby reducing the mixing of oxygen into the ocean interior. Deoxygenation can also be exacerbated by the addition of excess nutrients in the coastal zone.

=== Ocean dynamic sea level change ===
'''Working Groups:''' WGI

'''Definition:''' Change in mean sea level relative to the geoid associated with circulation and density-driven changes in the ocean. Ocean dynamic sea level change is regionally varying but by definition has a zero global mean and conventionally is inverse-barometer corrected (i.e., the effect of the hydrostatic depression of the sea surface by atmospheric pressure changes is removed). Changes in ocean currents occur due to variations in heating and cooling, variability in winds and changes in seasonally to annually averaged air temperature and humidity.

=== Ocean fertilisation ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A proposed carbon dioxide removal (CDR) method that relies on the deliberate increase of nutrient supply to the near-surface ocean with the aim of sequestering additional CO2 from the atmosphere through biological production. Methods include direct addition of micro-nutrients or macro-nutrients. To be successful, the additional carbon needs to reach the deep ocean where it has the potential to be sequestered on climatically relevant time scales.

=== Ocean heat uptake efficiency ===
'''Working Groups:''' WGI

'''Definition:''' This is a measure (W m –2 °C –1) of the rate at which heat storage by the global ocean increases as global surface temperature rises. It is a useful parameter for climate change simulations in which the radiative forcing is changing monotonically, when it can be compared with the c limate feedback parameter to gauge the relative importance of radiative response and ocean heat uptake in determining the rate of climate change. It can be estimated from such an experiment as the ratio of the rate of increase of ocean heat content to the surface temperature change.

=== Ocean stratification ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Process of forming of layers of ocean water with different properties such as salinity, density and temperature that act as barriers to water mixing. The strengthening of near-surface stratification generally results in warmer surface waters, decreased oxygen levels in deeper water and intensification of ocean acidification (OA) in the upper ocean.

=== Offset ===
'''Working Groups:''' WGIII

'''Also known as:''' Offset (in climate policy)

'''Definition:''' The reduction, avoidance or removal of a unit of greenhouse gas (GHG) emissions by one entity, purchased by another entity to counterbalance a unit of GHG emissions by that other entity. Offsets are commonly subject to rules and environmental integrity criteria intended to ensure that offsets achieve their stated mitigation outcome. Relevant criteria include, but are not limited to, the avoidance of double counting and leakage, use of appropriate baselines, additionality, and permanence or measures to address impermanence.

=== Orbital forcing ===
'''Working Groups:''' WGI

'''Definition:''' Orbital forcing is the influence of slow, systematic and predictable changes in orbital parameters (eccentricity, obliquity and precession of the equinox) on incoming solar radiation (insolation), especially its latitudinal and seasonal distribution. It is an external forcing and a key driver of glacial–interglacial cycles.

=== Organic aerosol ===
'''Working Groups:''' WGI

'''Definition:''' Component of the aerosol that consists of organic compounds, mainly carbon, hydrogen, oxygen and lesser amounts of other elements.

=== Organic farming ===
'''Working Groups:''' WGIII

'''Definition:''' An agricultural production system that aims to utilise natural processes and cycles to limit off-farm and notably synthetic inputs, while also aiming to enhance agroecosystems and society. Organic farming is often legally defined and governed by standards, typically guided by principles outlined by the International Federation of Organic Agriculture Movements (IFOAM – Organics International) (IFOAM – Organics International, 2014).

=== Outbreak ===
'''Working Groups:''' WGII

'''Definition:''' Often used synonymously with ‘epidemic’, usually to indicate localised as opposed to generalised epidemics (WHO, 2020).

=== Outgoing longwave radiation ===
'''Working Groups:''' WGI

'''Definition:''' Net outgoing radiation in the infrared part of the spectrum at the top of the atmosphere.

=== Outlet glacier ===
'''Working Groups:''' WGI

'''Definition:''' A glacier, usually between rock walls, that is part of, and drains, an ice sheet.

=== Overshoot pathways ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Pathways that first exceed a specified concentration, forcing, or global warming level, and then return to or below that level again before the end of a specified period of time (e.g., before 2100). Sometimes the magnitude and likelihood of the overshoot are also characterised. The overshoot duration can vary from one pathway to the next, but in most overshoot pathways in the literature and referred to as overshoot pathways in the AR6, the overshoot occurs over a period of at least one decade and up to several decades.

=== Oxygen minimum zone ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Oxygen minimum zone (OMZ)

'''Definition:''' The midwater layer (200–1000 m) in the open ocean in which oxygen saturation is the lowest in the ocean. The degree of oxygen depletion depends on the largely bacterial consumption of organic matter, and the distribution of the OMZs is influenced by large-scale ocean circulation. In coastal oceans, OMZs extend to the shelves and may also affect benthic ecosystems.

=== Ozone ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Ozone (O3)

'''Definition:''' The triatomic form of oxygen, and a gaseous atmospheric constituent. In the troposphere, O 3 is created both naturally and by photochemical reactions involving gases resulting from human activities (e.g., smog). Tropospheric O 3 acts as a greenhouse gas (GHG). In the stratosphere, O 3 is created by the interaction between solar ultraviolet radiation and molecular oxygen (O 2). Stratospheric O 3 plays a dominant role in the stratospheric radiative balance. Its concentration is highest in the ozone layer.

=== Ozone-depleting substances ===
'''Working Groups:''' WGI

'''Also known as:''' Ozone-depleting substances (ODSs)

'''Definition:''' Man-made gases that destroy 3) ozone (O once they reach the ozone layer in the stratosphere. Ozone-depleting substances include: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrobromofluorocarbons (HBFCs), halons, methyl bromide, carbon tetrachloride and methyl chloroform. They are used as refrigerants in commercial, home and vehicle air conditioners and refrigerators, foam blowing agents, components in electrical equipment, industrial solvents, solvents for cleaning (including dry cleaning), aerosol spray propellants and fumigants.

=== Ozone layer ===
'''Working Groups:''' WGI

'''Definition:''' A layer of Earth’s stratosphere that absorbs most of the Sun’s ultraviolet radiation. It contains high concentrations of 3) ozone (O in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth’s atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 kilometres (9.3 to 21.7 miles) above Earth, although its thickness varies seasonally and geographically.

=== Ozonesonde ===
'''Working Groups:''' WGI

'''Definition:''' An ozonesonde is a radiosonde measuring 3) ozone (O concentrations. The radiosonde is usually carried on a weather balloon and transmits measured quantities by radio to a ground-based receiver.

== P ==

=== Pacific Decadal Oscillation ===
'''Working Groups:''' WGI

'''Also known as:''' Pacific Decadal Oscillation (PDO)

'''Definition:''' The leading mode of variability obtained from decomposition in empirical orthogonal function of sea surface temperature over the North Pacific north of 20°N, and characterized by a strong decadal component. The positive phase of the PDO features a dipole of sea surface temperature anomalies in the North Pacific, with a cold lobe near the centre of the basin and extending westward along the Kuroshio, encircled by warmer conditions along the coast of North America and in the subtropics. A positive PDO is accompanied by an intensified Aleutian Low and an associated cyclonic circulation enhancement leading to teleconnections over the continents adjacent to the North Pacific. In the AR6 WGI report, the PDO is encapsulated within the definition and description of Pacific Decadal Variability (PDV). See also Section AIV.2.6 in Annex IV of the AR6 WGI report. From Wikipedia During a "warm", or "positive", phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a "cool", or "negative", phase, the opposite pattern occurs.

=== Pacific Decadal Variability ===
'''Working Groups:''' WGI

'''Also known as:''' Pacific Decadal Variability (PDV)

'''Definition:''' Coupled decadal-to-inter-decadal variability of the atmospheric circulation and underlying ocean that is typically observed over the entire Pacific Basin beyond the El Niño–Southern Oscillation (ENSO) time scale. In the AR6 WGI report, PDV encapsulates the Pacific Decadal Oscillation (PDO), the South Pacific Decadal Oscillation (SPDO), tropical Pacific decadal variability (also called decadal ENSO), and the Inter-decadal Pacific Oscillation (IPO). Typically, the positive phase of the PDV is characterized by anomalously high sea surface temperatures in the central-eastern tropical Pacific that extend to the extratropical North and South Pacific along the American coasts, encircled to the west by cold sea surface anomalies in the mid-latitude North and South Pacific. The negative phase is accompanied by sea surface temperature anomalies of the opposite sign. Those sea surface temperature anomalies are linked to anomalies in atmospheric and oceanic circulation throughout the whole Pacific Basin. The PDV is associated with decadal modulations in the relative occurrence of El Niño and La Niña. See Section AIV.2.6 in Annex IV of the AR6 WGI report.

=== Pacific-North American pattern ===
'''Working Groups:''' WGI

'''Also known as:''' Pacific-North American (PNA) pattern

'''Definition:''' An atmospheric large-scale wave pattern featuring a sequence of tropospheric high and low pressure anomalies stretching from the subtropical west Pacific to the east coast of North America.

=== Palaeocene–Eocene Thermal Maximum ===
'''Working Groups:''' WGI

'''Also known as:''' Palaeocene–Eocene Thermal Maximum (PETM)

'''Definition:''' The PETM is a transient event that occurred between 55.9 and 55.7 million years ago. Continental positions at this time were somewhat different to present due to tectonic plate movements. Geological data indicate that the PETM was characterised by a warming (global mean surface temperature rose to about 4°C–7 °C warmer than the preceding mean state), and an increase in atmospheric CO2 (from about 900 to about 2000 ppmv). In addition, ocean pH and oxygen content decreased; many deep-sea species went extinct and tropical coral reefs diminished.

=== Paleoclimate ===
'''Working Groups:''' WGI

'''Definition:''' Climate during periods prior to the development of measuring instruments, including historic and geologic time, for which only proxy climate records are available.

=== Pandemic ===
'''Working Groups:''' WGII

'''Definition:''' A worldwide outbreak of a disease in humans in numbers clearly in excess of normal (WHO, 2020).

=== Pareto optimum ===
'''Working Groups:''' WGIII

'''Definition:''' A state in which no one’s welfare can be increased without reducing someone one’s welfare. Wikipedia

=== Participatory governance ===
'''Working Groups:''' WGIII

'''Definition:''' A governance system that enables direct public engagement in decision-making using a variety of techniques, for example, referenda, community deliberation, citizen juries or participatory budgeting. The approach can be applied in formal and informal institutional contexts from national to local, but is usually associated with devolved decision making (Fung and Wright, 2003; Sarmiento and Tilly, 2018).

=== Particulate matter ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Particulate matter (PM)

'''Definition:''' Atmospheric aerosols involved in air pollution issues. Of greatest concern for health are particles of aerodynamic diameter less than or equal to 10 micrometers, usually designated as PM 10 and particles of diameter less than or equal to 2.5 micrometers, usually designated as PM 2.5.

=== Pasture ===
'''Working Groups:''' WGII

'''Definition:''' Area covered with grass or other plants used or suitable for grazing of livestock; grassland.

=== Path dependence ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The generic situation where decisions, events, or outcomes at one point in time constrain adaptation, mitigation or other actions or options at a later point in time.

=== Pathways ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The temporal evolution of natural and/or human systems towards a future state. Pathway concepts range from sets of quantitative and qualitative scenarios or narratives of potential futures to solution-oriented decision-making processes to achieve desirable societal goals. Pathway approaches typically focus on biophysical, techno-economic, and/or socio-behavioural trajectories and involve various dynamics, goals, and actors across different scales.

=== Pattern scaling ===
'''Working Groups:''' WGI

'''Definition:''' Techniques used to represent the spatial variations in climate at a given increase in global mean surface air temperature (GSAT) are referred to as ‘pattern scaling’.

=== Peat ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Soft, porous or compressed, sedentary deposit of which a substantial portion is partly decomposed plant material with high water content in the natural state (up to about 90%).

=== Peatlands ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Peatlands are wetland ecosystems where soils are dominated by peat. In peatlands, net primary production exceeds organic matter decomposition as a result of waterlogged conditions, which leads to the accumulation of peat.

=== Pelagic ===
'''Working Groups:''' WGII

'''Definition:''' The pelagic zone consists of the entire water column of the open ocean. It is subdivided into the epipelagic zone (<200 m, the uppermost part of the ocean that receives enough sunlight to allow photosynthesis), the mesopelagic zone (200–1000 m depth) and the bathypelagic zone (>1000 m depth). The term ‘pelagic’ can also refer to organisms that live in the pelagic zone.

=== Pelagos ===
'''Working Groups:''' WGII

'''Definition:''' Organisms large and small living in the pelagic zones. Includes plankton (small) and nekton (free swimming, large). See Benthos.

=== Percentile ===
'''Working Groups:''' WGI

'''Definition:''' A partition value in a population distribution that a given percentage of the data values are below or equal to. The 50th percentile corresponds to the median of the population. Percentiles are often used to estimate the extremes of a distribution. For example, the 90th (10th) percentile may be used to refer to the threshold for the upper (lower) extremes.

=== Peri-urban areas ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Dynamic transition zones that have intense interaction between rural and urban economies, activities, households, and lifestyles. Neither fully rural or urban (Seto et al., 2010).

=== Permafrost ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Ground (soil or rock, and included ice and organic material) that remains at or below 0°C for at least two consecutive years (Harris et al., 1988). Note that permafrost is defined via temperature rather than ice content and, in some instances, may be ice-free.

=== Permafrost degradation ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Decrease in the thickness and/or areal extent of permafrost.

=== Permafrost thaw ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Progressive loss of ground ice in permafrost, usually due to input of heat. Thaw can occur over decades to centuries over the entire depth of permafrost ground, with impacts occurring while thaw progresses. During thaw, temperature fluctuations are subdued because energy is transferred by phase change between ice and water. After the transition from permafrost to non-permafrost, ground can be described as thawed.

=== Perturbed parameter ensemble ===
'''Working Groups:''' WGI

'''Definition:''' Parameter ensembles in which model parameters are varied in a systematic manner, aim to assess the uncertainty resulting from internal model specifications within a single model.

=== Phenology ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The relationship between biological phenomena that recur periodically (e.g., development stages, migration) especially related to climate and seasonal changes.

=== Photosynthesis ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The production of carbohydrates in plants, algae and some bacteria using the energy of light. Carbon dioxide (CO 2) is used as the carbon source.

=== Physical climate storyline ===
'''Working Groups:''' WGI

'''Definition:''' A self-consistent and plausible unfolding of a physical trajectory of the climate system, or a weather or climate event, on time scales from hours to multiple decades (Shepherd et al., 2018). Through this, storylines explore, illustrate and communicate uncertainties in the climate system response to forcing and in internal variability.

=== Planetary health ===
'''Working Groups:''' WGII

'''Definition:''' A concept based on the understanding that human health and human civilisation depend on ecosystem health and the wise stewardship of ecosystems.

=== Plankton ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Free-floating organisms living in the upper layers of aquatic systems. Their distribution and migration are primarily determined by water currents. A distinction is made between phytoplankton, which depend on photosynthesis for their energy supply, and zooplankton, which feed on phytoplankton, other zooplankton and bacterioplankton.

=== Planned relocation ===
'''Working Groups:''' WGII

'''Also known as:''' Planned relocation (of humans)

'''Definition:''' A form of human mobility response in the face of sea level rise and related impacts. Planned relocation is typically initiated, supervised and implemented from national to local level and involves small communities and individual assets but may also involve large populations. Also termed resettlement, managed retreat or managed realignment.

=== Plant evaporative stress ===
'''Working Groups:''' WGI

'''Definition:''' Plant evaporative stress in both crops and natural vegetation can result from the combination of a high atmospheric evaporative demand and limited available water to supply this demand by means of evapotranspiration, further enhancing agricultural and ecological drought.

=== Plasticity ===
'''Working Groups:''' WGII

'''Also known as:''' Plasticity (biology)

'''Definition:''' Change in organismal trait values in response to an environmental cue and which does not require change in underlying DNA sequence.

=== Pleistocene ===
'''Working Groups:''' WGI

'''Definition:''' The Pleistocene Epoch is the earlier of two epochs in the Quaternary System, extending from 2.59 Ma to the beginning of the Holocene at approximately 11.7 ka.

=== Pliocene ===
'''Working Groups:''' WGI

'''Definition:''' The Pliocene Epoch is the more recent of two epochs of the Neogene Period within the Cenozoic Era. It extends from 5.33 Ma to the beginning of the Pleistocene Epoch at 2.59 Ma. The Neogene Period precedes the current geological period, the Quaternary Period, which is one of several ice ages that have occurred during Earth’s geological history. It encompasses the mid-Pliocene warm period (MPWP), also known as the Piacenzian warm period, which occurred from approximately 3.3 to 3.0 Ma. The MPWP, in turn, encompasses the interglacial episode, marine isotope stage (MIS) KM5c, which peaked at 3.205 Ma, when orbital forcing was similar to modern (Haywood et al., 2016).

=== Polar amplification ===
'''Working Groups:''' WGI

'''Definition:''' Polar amplification describes the phenomenon where surface temperature change at high latitudes exceeds the global average surface temperature change. The terms Arctic amplification or Antarctic amplification are used when describing the phenomenon occurring at one of the poles.

=== Policies ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Policies (for climate change mitigation and adaptation)

'''Definition:''' Strategies that enable actions to be undertaken to accelerate adaptation and mitigation. Policies include those developed by national and subnational public agencies, and with the private sector. Policies for adaptation and mitigation often take the form of economic incentives, regulatory instruments, and decision-making and engagement processes.

=== Political economy ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The set of interlinked relationships between people, the State, society and markets as defined by law, politics, economics, customs and power that determine the outcome of trade and transactions and the distribution of wealth in a country or economy.

=== Pollen analysis ===
'''Working Groups:''' WGI

'''Definition:''' A technique of both relative dating and environmental reconstruction, consisting of the identification and counting of pollen types preserved in peat, lake sediments and other deposits.

=== Polycentric governance ===
'''Working Groups:''' WGII

'''Definition:''' Polycentric governance involves multiple centres of decision-making with overlapping jurisdictions. While the centres have some degree of autonomy, they also take each other into account, coordinating their actions and seeking to resolve conflicts (Carlisle and Gruby, 2017; Jordan et al., 2018; McGinnis and Ostrom, 2012).

=== Pool, carbon and nitrogen ===
'''Working Groups:''' WGIII

'''Definition:''' A reservoir in the Earth System where elements, such as carbon and nitrogen, reside in various chemical forms for a period of time.

=== Potential evapotranspiration ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The potential rate of water loss from wet soils and from plant surfaces, without any limits imposed by the water supply.

=== Poverty ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A complex concept with several definitions stemming from different schools of thought. It can refer to material circumstances (such as need, pattern of deprivation or limited resources), economic conditions (such as standard of living, inequality or economic position) and/or social relationships (such as social class, dependency, exclusion, lack of basic security or lack of entitlement).

=== Poverty eradication ===
'''Working Groups:''' WGIII

'''Definition:''' A set of measures to end poverty in all its forms everywhere.

=== Poverty trap ===
'''Working Groups:''' WGII

'''Definition:''' Poverty trap is understood differently across disciplines. In the social sciences, the concept, primarily employed at the individual, household or community level, describes a situation in which escaping poverty becomes impossible due to unproductive or inflexible resources. A poverty trap can also be seen as a critical minimum asset threshold, below which families are unable to successfully educate their children, build up their productive assets and get out of poverty. Extreme poverty is itself a poverty trap since poor persons lack the means to participate meaningfully in society. In economics, the term poverty trap is often used at national scales, referring to a self-perpetuating condition where an economy, caught in a vicious cycle, suffers from persistent underdevelopment (Matsuyama, 2008). Many proposed models of poverty traps are found in the literature.

=== Pre-industrial ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Pre-industrial (period)

'''Definition:''' The multi-century period prior to the onset of large-scale industrial activity around 1750. The reference period 1850–1900 is used to approximate pre-industrial global mean surface temperature (GMST).

=== Precipitable water ===
'''Working Groups:''' WGI

'''Definition:''' The total amount of atmospheric water vapour in a vertical column of unit cross-sectional area. It is commonly expressed in terms of the height of the water if completely condensed and collected in a vessel of the same unit cross section.

=== Precipitation deficit ===
'''Working Groups:''' WGII

'''Definition:''' A period with an abnormal precipitation deficit is defined as a meteorological drought.

=== Precursors ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Atmospheric compounds that are not greenhouse gases (GHGs) or aerosols, but that have an effect on GHG or aerosol concentrations by taking part in physical or chemical processes regulating their production or destruction rates.

=== Predictability ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The extent to which future states of a system may be predicted based on knowledge of current and past states of the system. Because knowledge of the climate system’s past and current states is generally imperfect, as are the models that utilize this knowledge to produce a climate prediction, and because the climate system is inherently non-linear and chaotic, predictability of the climate system is inherently limited. Even with arbitrarily accurate models and observations, there may still be limits to the predictability of such a non-linear system (AMS, 2021).

=== Prediction quality/skill ===
'''Working Groups:''' WGI

'''Definition:''' Measures of the success of a prediction against observationally based information. No single measure can summarize all aspects of forecast quality, and a suite of metrics is considered. Metrics will differ for forecasts given in deterministic and probabilistic form.

=== Primary energy ===
'''Working Groups:''' WGIII

'''Definition:''' The energy that is embodied in resources as they exist in nature (e.g., coal, biomass uranium, solar radiation, wind, ocean currents) (Grubler et al. 2012). [Note: Primary energy is defined in several alternative ways. The method used in this report is the direct equivalent method, which counts one unit of secondary energy provided from non-combustible sources as one unit of primary energy. For more details on the methodology, see Section 7 in Working Group III Annex II.]

=== Primary production ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The synthesis of organic compounds by plants and microbes, on land or in the ocean, primarily by photosynthesis using light and 2) carbon dioxide (CO as sources of energy and carbon, respectively. It can also occur through chemosynthesis, using chemical energy, for example, in deep sea vents.

=== Private costs ===
'''Working Groups:''' WGIII

'''Definition:''' Costs carried by individuals, companies or other private entities that undertake an action, whereas social costs include additionally the external costs on the environment and on society as a whole. Quantitative estimates of both private and social costs may be incomplete, because of difficulties in measuring all relevant effects.

=== Probability density function ===
'''Working Groups:''' WGI

'''Also known as:''' Probability density function (PDF)

'''Definition:''' A probability density function is a function that indicates the relative chances of occurrence of different outcomes of a variable. The function integrates to unity over the domain for which it is defined and has the property that the integral over a sub-domain equals the probability that the outcome of the variable lies within that sub-domain. For example, the probability that a temperature anomaly defined in a particular way is greater than zero is obtained from its PDF by integrating the PDF over all possible temperature anomalies greater than zero. Probability density functions that describe two or more variables simultaneously are similarly defined.

=== Procedural justice ===
'''Working Groups:''' WGII

'''Definition:''' Justice in the way outcomes are brought about including who participates and is heard in the processes of decision-making.

=== Process-based model ===
'''Working Groups:''' WGI

'''Definition:''' Theoretical concepts and computational methods that represent and simulate the behaviour of real-world systems derived from a set of functional components and their interactions with each other and the system environment, through physical and mechanistic processes occurring over time.

=== Production-based emissions ===
'''Working Groups:''' WGIII

'''Definition:''' Emissions released to the atmosphere for the production of goods and services by a certain entity (e.g., a person, firm, country, or region).

=== Projection ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A potential future evolution of a quantity or set of quantities, often computed with the aid of a model. Unlike predictions, projections are conditional on assumptions concerning, for example, future socio-economic and technological developments that may or may not be realised.

=== Prosumers ===
'''Working Groups:''' WGIII

'''Definition:''' A consumer that also produces energy and inputs energy to the system, for which it is an active agent in the energy system and market.

=== Proxy ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A proxy climate indicator is any biophysical property of materials formed during the past that is interpreted to represent some combination of climate-related variations back in time. Climate-related data derived in this way are referred to as proxy data, and time series of proxy data are proxy records. Examples of proxy types include pollen assemblages, tree ring widths, speleothem and coral geochemistry, and various data derived from marine sediments and glacier ice. Proxy data can be calibrated to provide quantitative climate information.

== Q ==

=== Quasi-Biennial Oscillation ===
'''Working Groups:''' WGI

'''Also known as:''' Quasi-Biennial Oscillation (QBO)

'''Definition:''' A near-periodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of around 28 months. The alternating wind maxima descend from the base of the mesosphere down to the tropopause and are driven by wave energy that propagates up from the troposphere.

=== Quaternary ===
'''Working Groups:''' WGI

'''Definition:''' The Quaternary Period is the last of three periods that make up the Cenozoic Era (66 Ma to present), extending from 2.58 Ma to the present, and includes the Pleistocene and Holocene Epochs.

== R ==

=== Radiative forcing ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The change in the net, downward minus upward, radiative flux (expressed in W m –2) due to a change in an external driver of climate change, such as a change in the concentration of carbon dioxide (CO2), the concentration of volcanic aerosols or in the output of the Sun. The stratospherically adjusted radiative forcing is computed with all tropospheric properties held fixed at their unperturbed values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical equilibrium. Radiative forcing is called instantaneous if no change in stratospheric temperature is accounted for. The radiative forcing once both stratospheric and tropospheric adjustments are accounted for is termed the ‘effective radiative forcing‘.

=== Rapid dynamical change ===
'''Working Groups:''' WGI

'''Also known as:''' Rapid dynamical change (of glaciers or ice sheets)

'''Definition:''' Changes in glacier or ice sheet mass controlled by changes in flow speed and discharge rather than by accumulation or ablation. This can result in a rate of mass change larger than that due to any imbalance between accumulation and ablation. Rapid dynamical change may be initiated by a climatic trigger, such as incursion of warm ocean water beneath an ice shelf, or thinning of a grounded tide-water terminus, which may lead to reactions within the glacier system that may result in rapid ice loss.

=== Reanalysis ===
'''Working Groups:''' WGI

'''Definition:''' Reanalyses are created by processing past meteorological or oceanographic data using fixed state-of-the-art weather forecasting or ocean circulation models with data assimilation techniques. They are used to provide estimates of variables such as historical atmospheric temperature and wind or oceanographic temperature and currents, and other quantities. Using fixed data assimilation avoids effects from the changing analysis system that occur in operational analyses. Although continuity is improved, global reanalyses still suffer from changing coverage and biases in the observing systems.

=== Reasons for Concern ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Reasons for Concern (RFCs)

'''Definition:''' Elements of a classification framework, first developed in the IPCC Third Assessment Report, which aims to facilitate judgements about what level of climate change may be dangerous (in the language of Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) by aggregating risks from various sectors, considering hazards, exposures, vulnerabilities, capacities to adapt, and the resulting impacts. From Wikipedia The Intergovernmental Panel on Climate Change (IPCC) has organized many of the risks of climate change into five "reasons for concern." The reasons for concern show that these risks increase with increases in the Earth's global mean temperature (i.e., global warming). The IPCC's five reasons for concern are: threats to endangered species and unique systems damages from extreme climate events effects that fall most heavily on developing countries and the poor within countries global aggregate impacts (i.e., various measurements of total social, economic and ecological impacts) large-scale high-impact events. The five reasons for concern are described in more detail in the Wikipedia page.

=== Rebound effect ===
'''Working Groups:''' WGIII

'''Definition:''' Phenomena whereby the reduction in energy consumption or emissions (relative to a baseline) associated with the implementation of mitigation measures in a jurisdiction is offset to some degree through induced changes in consumption, production, and prices within the same jurisdiction. The rebound effect is most typically ascribed to technological energy efficiency improvements.

=== Reconstruction ===
'''Working Groups:''' WGI

'''Also known as:''' Reconstruction (of climate variable)

'''Definition:''' Approach to reconstructing the past temporal and spatial characteristics of a climate variable from predictors. The predictors can be instrumental data if the reconstruction is used to infill missing data or proxy data if it is used to develop paleoclimate reconstructions. Various techniques have been developed for this purpose: linear multivariate regression-based methods and non-linear Bayesian and analogue methods.

=== Reducing Emissions from Deforestation and Forest Degradation ===
'''Working Groups:''' WGIII

'''Also known as:''' Reducing Emissions from Deforestation and Forest Degradation (REDD+)

'''Definition:''' REDD+ refers to reducing emissions from deforestation; reducing emissions from forest degradation; conservation of forest carbon stocks; sustainable management of forests; and enhancement of forest carbon stocks (see UNFCCC decision 1/CP.16, para. 70).

=== Reference period ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A time period of interest, or a period over which some relevant statistics are calculated. A reference period can be used as a baseline period or as a comparison to a baseline period.

=== Reference scenario ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Scenario used as starting or reference point for a comparison between two or more scenarios. Note 1: In many types of climate change research, reference scenarios reflect specific assumptions about patterns of socio-economic development and may represent futures that assume no climate policies or specified climate policies, for example those in place or planned at the time a study is carried out. Reference scenarios may also represent futures with limited or no climate impacts or adaptation, to serve as a point of comparison for futures with impacts and adaptation. These are also referred to as baseline scenarios in the literature. Note 2: Reference scenarios can also be climate policy or impact scenarios, which in that case are taken as a point of comparison to explore the implications of other features, for example, of delay, technological options, policy design and strategy or to explore the effects of additional impacts and adaptation beyond those represented in the reference scenario. Note 3: The term business as usual scenario has been used to describe a scenario that assumes no additional policies beyond those currently in place and that patterns of socio-economic development are consistent with recent trends. The term is now used less frequently than in the past. Note 4: In climate change attribution or impact attribution research reference scenarios may refer to counterfactual historical scenarios assuming no anthropogenic greenhouse gas (GHG) emissions (climate change attribution) or no climate change (impact attribution).

=== Reforestation ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Conversion to forest of land that has previously contained forests but that has been converted to some other use. [Note: For a discussion of the term forest and related terms such as afforestation, reforestation and deforestation, see the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and their 2019 Refinement, and information provided by the United Nations Framework Convention on Climate Change (IPCC, 2006, 2019; UNFCCC 2021a, b).]

=== Refugium ===
'''Working Groups:''' WGII

'''Definition:''' A refugium is a geographic area where a population found safety from some threat to its existence, for example, climate refugia or glacial refugia (refuge from glaciations).

=== Regenerative agriculture ===
'''Working Groups:''' WGIII

'''Definition:''' A universally agreed definition of this relatively new farming approach has yet to be established, but regenerative agriculture broadly refers to the implementation of varying combinations of agricultural management practices, to ensure the continued restoration and enhancement of soil health, biodiversity and ecosystem functioning, in conjunction with profitable agricultural production.

=== Region ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Land and/or ocean area characterised by specific geographical and/or climatological features. The climate of a region emerges from a multi-scale combination of its own features, remote influences from other regions, and global climate conditions.

=== Regional climate messages ===
'''Working Groups:''' WGI

'''Definition:''' Regional climate messages translate climate information synthesized from different lines of evidence into the context of a user vulnerable to climate at regional scales taking into account the values of both the producer and user (Section 10.5 of the AR6 WGI report).

=== Regional climate model ===
'''Working Groups:''' WGI

'''Also known as:''' Regional climate model (RCM)

'''Definition:''' A climate model at higher resolution over a limited area. Such models are used in downscaling global climate results over specific regional domains.

=== Regional sea level change ===
'''Working Groups:''' WGI

'''Definition:''' Change in sea level relative to a datum (such as present-day mean sea level) at spatial scales of about 100 km.

=== Regulation ===
'''Working Groups:''' WGII

'''Definition:''' A rule or order issued by governmental executive authorities or regulatory agencies and having the force of law. Regulations implement policies and are mostly specific for particular groups of people, legal entities or targeted activities. Regulation is also the act of designing and imposing rules or orders. Informational, transactional, administrative and political constraints in practice limit the regulator’s capability for implementing preferred policies.

=== Relative humidity ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The ratio of actual water vapour pressure to that at saturation with respect to liquid water or ice at the same temperature.

=== Relative sea level change ===
'''Working Groups:''' WGI

'''Also known as:''' Relative sea level (RSL) change

'''Definition:''' The change in local mean sea surface height (SSH) relative to the local solid surface, that is, the sea floor, as measured by instruments that are fixed to the Earth’s surface, such as tide gauges. This reference frame is used when considering coastal impacts, hazards and adaptation needs.

=== Remaining carbon budget ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Cumulative global CO2 emissions from the start of 2018 to the time that CO2 emissions reach net-zero that would result in a given level of global warming.

=== Renewable energy ===
'''Working Groups:''' WGIII

'''Also known as:''' Renewable energy (RE)

'''Definition:''' Any form of energy that is replenished by natural processes at a rate that equals or exceeds its rate of use.

=== Reporting ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The process of formal reporting of assessment results to the United Nations Framework Convention on Climate Change (UNFCCC), according to predetermined formats and established standards, especially the Intergovernmental Panel on Climate Change (IPCC) Guidelines and GPG (Good Practice Guidance)’ (UN REDD, 2009).

=== Representative Concentration Pathways ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Representative Concentration Pathways (RCPs)

'''Definition:''' Scenarios that include time series of emissions and concentrations of the full suite of greenhouse gases (GHGs) and aerosols and chemically active gases, as well as land use / land cover (Moss et al.,2008; van Vuuren et al., 2011). The word representative signifies that each RCP provides only one of many possible scenarios that would lead to the specific radiative forcing characteristics. The term pathway emphasises that not only the long-term concentration levels are of interest, but also the trajectory taken over time to reach that outcome (Moss et al., 2010; van Vuuren et al., 2011). • RCP2.6: One pathway where radiative forcing peaks at approximately 3 W m –2 and then declines to be limited at 2.6 W m –2 in 2100 (the corresponding Extended Concentration Pathway, or ECP, has constant emissions after 2100). • RCP4.5 and RCP6.0: Two intermediate stabilisation pathways in which radiative forcing is limited at approximately 4.5 W m –2 and 6.0 W m –2 in 2100 (the corresponding ECPs have constant concentrations after 2150). • RCP8.5: One high pathway which leads to >8.5 W m –2 in 2100 (the corresponding ECP has constant emissions after 2100 until 2150 and constant concentrations after 2250).

=== Representative Key Risks ===
'''Working Groups:''' WGII

'''Also known as:''' Representative Key Risks (RKRs)

'''Definition:''' Representative, thematic clusters of key risks.

=== Reservoir ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A component or components of the climate system where a greenhouse gas (GHG) or a precursor of a greenhouse gas is stored (UNFCCC Article 1.7 (UNFCCC, 1992)).

=== Residual risk ===
'''Working Groups:''' WGII

'''Definition:''' The risk related to climate change impacts that remains following adaptation and mitigation efforts. Adaptation actions can redistribute risk and impacts, with increased risk and impacts in some areas or populations, and decreased risk and impacts in others.

=== Resilience ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The capacity of interconnected social, economic and ecological systems to cope with a hazardous event, trend or disturbance, responding or reorganising in ways that maintain their essential function, identity and structure. Resilience is a positive attribute when it maintains capacity for adaptation, learning and/or transformation (Arctic Council, 2016).

=== Resolution ===
'''Working Groups:''' WGI; WGII

'''Definition:''' In climate models, this term refers to the physical distance (metres or degrees) between each point on the grid used to compute the equations. Temporal resolution refers to the time step or time elapsed between each model computation of the equations.

=== Resource cascade ===
'''Working Groups:''' WGIII

'''Definition:''' Tracking resource use (materials, energy, water, etc.), efficiency and losses through all conversion steps from primary resource extraction to various conversion steps, all the way to final service delivery.

=== Respiration ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The process whereby living organisms convert organic matter to carbon dioxide (CO 2), releasing energy and consuming molecular oxygen.

=== Response time or adjustment time ===
'''Working Groups:''' WGI

'''Also known as:''' Response time or adjustment time (Ta)

'''Definition:''' In the context of climate variations, the response time or adjustment time is the time needed for the climate system or its components to re-equilibrate to a new state, following a forcing resulting from external processes. It is very different for various components of the climate system. The response time of the troposphere is relatively short, from days to weeks, whereas the stratosphere reaches equilibrium on a time scale of typically a few months. Due to their large heat capacity, the oceans have a much longer response time: typically decades, but up to centuries or millennia. The response time of the strongly coupled surface–troposphere system is, therefore, slow compared to that of the stratosphere, and mainly determined by the oceans. The biosphere may respond quickly (e.g., to droughts), but also very slowly to imposed changes. In the context of lifetimes, response time or adjustment time (T a) is the time scale characterizing the decay of an instantaneous pulse input into the reservoir. See Response time or adjustment time (Ta) under Lifetime.

=== Restoration ===
'''Working Groups:''' WGII

'''Definition:''' In the environmental context, restoration involves human interventions to assist the recovery of an ecosystem that has been previously degraded, damaged or destroyed.

=== Return period ===
'''Working Groups:''' WGI; WGII

'''Definition:''' An estimate of the average time interval between occurrences of an event (e.g., flood or extreme rainfall) of (or below/above) a defined size or intensity.

=== Return value ===
'''Working Groups:''' WGI

'''Definition:''' The highest (or, alternatively, lowest) value of a given variable, on average occurring once in a given period of time (e.g., in 10 years).

=== Risk ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The potential for adverse consequences for human or ecological systems, recognising the diversity of values and objectives associated with such systems. In the context of climate change, risks can arise from potential impacts of climate change as well as human responses to climate change. Relevant adverse consequences include those on lives, livelihoods, health and well-being, economic, social and cultural assets and investments, infrastructure, services (including ecosystem services), ecosystems and species. In the context of climate change impacts, risks result from dynamic interactions between climate-related hazards with the exposure and vulnerability of the affected human or ecological system to the hazards. Hazards, exposure and vulnerability may each be subject to uncertainty in terms of magnitude and likelihood of occurrence, and each may change over time and space due to socio-economic changes and human decision-making (see also risk management, adaptation and mitigation). In the context of climate change responses, risks result from the potential for such responses not achieving the intended objective(s), or from potential trade-offs with, or negative side-effects on, other societal objectives, such as the Sustainable Development Goals (SDGs) (see also risk trade-off). Risks can arise, for example, from uncertainty in implementation, effectiveness or outcomes of climate policy, climate-related investments, technology development or adoption, and system transitions.

=== Risk assessment ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The qualitative and/or quantitative scientific estimation of risks.

=== Risk framework ===
'''Working Groups:''' WGI

'''Definition:''' A common framework for describing and assessing risk across all three Working Groups is adopted to promote clear and consistent communication of risks and to better inform risk assessment and decision-making related to climate change.

=== Risk management ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Plans, actions, strategies or policies to reduce the likelihood and/or magnitude of adverse potential consequences, based on assessed or perceived risks.

=== Risk perception ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The subjective judgement that people make about the characteristics and severity of a risk.

=== Risk trade-off ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The change in the portfolio of risks that occurs when a countervailing risk is generated (knowingly or inadvertently) by an intervention to reduce the target risk (Wiener and Graham, 2009).

=== Risk transfer ===
'''Working Groups:''' WGII

'''Definition:''' The process of formally or informally shifting the financial consequences of particular risks from one party to another whereby a household, community, enterprise or state authority will obtain resources from the other party after a disaster occurs, in exchange for ongoing or compensatory social or financial benefits provided to that other party.

=== River discharge ===
'''Working Groups:''' WGI

'''Definition:''' Water flow within a river channel, for example, expressed in m3s–1. A synonym for river streamflow.

=== Rock glacier ===
'''Working Groups:''' WGI

'''Definition:''' A debris landform (mass of rock fragments and finer material that contains either an ice core or an ice-cemented matrix) generated by a former or current gravity-driven creep of permafrost in mountain slopes (Harris et al., 1988; Giardino et al., 2011; IPA-RG, 2020). It is detectable in the landscape due to the occurrence of (i) a steep slope delimiting the terminal part, (ii) generally well-defined lateral margins in a continuation of the front, and (iii) transversal or longitudinal ridges and furrows (ridge and furrow topography). These are geomorphological indicators of the occurrence of permafrost conditions. Although it is an ice storage feature, it is not a type of glacier since it does not originate at the surface by the recrystallization of snow.

=== Runoff ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The flow of water over the surface or through the subsurface, which typically originates from the part of liquid precipitation and/or snow/ice melt that does not evaporate, transpire or refreeze, and returns to water bodies.

== S ==

=== Salt-water intrusion/encroachment ===
'''Working Groups:''' WGII

'''Definition:''' Displacement of fresh surface water or groundwater by the advance of salt water due to its greater density. This usually occurs in coastal and estuarine areas due to decreasing land-based influence (e.g., from reduced runoff or groundwater recharge, or from excessive water withdrawals from aquifers) or increasing marine influence (e.g., relative sea level rise).

=== Sampling uncertainty ===
'''Working Groups:''' WGI

'''Definition:''' Uncertainty arising from incomplete or uneven availability of measurements in either space or time or both.

=== Scenario ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A plausible description of how the future may develop based on a coherent and internally consistent set of assumptions about key driving forces (e.g., rate of technological change, prices) and relationships. Note that scenarios are neither predictions nor forecasts, but are used to provide a view of the implications of developments and actions.

=== Scenario storyline ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' A narrative description of a scenario (or family of scenarios), highlighting the main scenario characteristics, relationships between key driving forces and the dynamics of their evolution.

=== Sea ice ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Ice found at the sea surface that has originated from the freezing of seawater. Sea ice may be discontinuous pieces (ice floes) moved on the ocean surface by wind and currents (pack ice), or a motionless sheet attached to the coast (land-fast ice). Sea ice concentration is the fraction of the ocean covered by ice. Sea ice less than one year old is called first-year ice. Perennial ice is sea ice that survives at least one summer. It may be subdivided into second-year ice and multi-year ice, where multi-year ice has survived at least two summers.

=== Sea ice area ===
'''Working Groups:''' WGI

'''Also known as:''' Sea ice area (SIA)

'''Definition:''' Sea ice area is the area covered by sea ice. In contrast to sea ice extent, it is a linear measure of sea ice coverage that does not depend on grid resolution.

=== Sea ice concentration ===
'''Working Groups:''' WGI

'''Definition:''' Sea ice concentration is the fraction of the ocean covered by ice.

=== Sea ice extent ===
'''Working Groups:''' WGI

'''Also known as:''' Sea ice extent (SIE)

'''Definition:''' Sea ice extent is calculated for gridded data products as the total area of all grid cells with sea ice concentration above a given threshold, usually 15 %. It hence is a grid-dependent, non-linear measure of sea ice coverage.

=== Sea level change ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Sea level change (sea level rise/sea level fall)

'''Definition:''' Change to the height of sea level, both globally and locally (relative sea level change) at seasonal, annual, or longer time scales due to (1) a change in ocean volume as a result of a change in the mass of water in the ocean (e.g., due to melt of glaciers and ice sheets), (2) changes in ocean volume as a result of changes in ocean water density (e.g., expansion under warmer conditions), (3) changes in the shape of the ocean basins and changes in the Earth’s gravitational and rotational fields and (4) local subsidence or uplift of the land. Global mean sea level change resulting from change in the mass of the ocean is called barystatic. The amount of barystatic sea level change due to the addition or removal of a mass of water is called its sea level equivalent (SLE). Sea level changes, both globally and locally, resulting from changes in water density are called steric. Density changes induced by temperature changes only are called thermosteric, while density changes induced by salinity changes are called halosteric. Barystatic and steric sea level changes do not include the effect of changes in the shape of ocean basins induced by the change in the ocean mass and its distribution.

=== Sea level equivalent ===
'''Working Groups:''' WGI

'''Also known as:''' Sea level equivalent (SLE)

'''Definition:''' The SLE of a mass of water, ice, or water vapour is that mass, converted to a volume using a density of 1000 kg m –3, and divided by the present-day ocean surface area of 3.625 × 1000 m 2. Thus, 362.5 Gt of water mass added to the ocean correspond to 1 mm of global mean sea level rise.

=== Sea level rise ===
'''Working Groups:''' WGI

'''Also known as:''' Sea level rise (SLR)

'''Definition:''' Change to the height of sea level, both globally and locally (relative sea level change) (at seasonal, annual or longer time scales) due to (1) a change in ocean volume as a result of a change in the mass of water in the ocean (e.g., due to melt of glaciers and ice sheets), (2) changes in ocean volume as a result of changes in ocean water density (e.g., expansion under warmer conditions), (3) changes in the shape of the ocean basins and changes in the Earth’s gravitational and rotational fields and (4) local subsidence or uplift of the land. Global mean sea level change resulting from change in the mass of the ocean is called barystatic. The amount of barystatic sea level change due to the addition or removal of a mass of water is called its sea level equivalent (SLE). Sea level changes, both globally and locally, resulting from changes in water density are called steric. Density changes induced by temperature changes only are called thermosteric, while density changes induced by salinity changes are called halosteric. Barystatic and steric sea level changes do not include the effect of changes in the shape of ocean basins induced by the change in the ocean mass and its distribution.

=== Sea surface temperature ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Sea surface temperature (SST)

'''Definition:''' The subsurface bulk temperature in the top few metres of the ocean, measured by ships, buoys and drifters. From ships, measurements of water samples in buckets were mostly switched in the 1940s to samples from engine intake water. Satellite measurements of skin temperature (uppermost layer; a fraction of a millimetre thick) in the infrared or the top centimetre or so in the microwave are also used, but must be adjusted to be compatible with the bulk temperature.

=== Semi-arid zone ===
'''Working Groups:''' WGII

'''Definition:''' Areas where vegetation growth is constrained by limited water availability, often with short growing seasons and high interannual variation in primary production. Annual precipitation ranges from 300 to 800 mm, depending on the occurrence of summer and winter rains.

=== Semi-empirical model ===
'''Working Groups:''' WGI

'''Definition:''' Model in which calculations are based on a combination of observed associations between variables and theoretical considerations relating variables through fundamental principles (e.g., conservation of energy). For example, in sea level studies, semi-empirical models refer specifically to transfer functions formulated to project future global mean sea level (GMSL) change, or contributions to it, from future global surface temperature change or radiative forcing.

=== Sendai Framework for Disaster Risk Reduction ===
'''Working Groups:''' WGII

'''Definition:''' The Sendai Framework for Disaster Risk Reduction 2015–2030 outlines seven clear targets and four priorities for action to prevent new, and to reduce existing, disaster risks. The voluntary, non-binding agreement recognises that the State has the primary role to reduce disaster risk but that responsibility should be shared with other stakeholders, including local government and the private sector. Its aim is to achieve ’substantial reduction of disaster risk and losses in lives, livelihoods and health and in the economic, physical, social, cultural and environmental assets of persons, businesses, communities and countries’.

=== Sensible heat flux ===
'''Working Groups:''' WGI

'''Definition:''' The turbulent or conductive flux of heat from the Earth’s surface to the atmosphere that is not associated with phase changes of water; a component of the surface energy budget.

=== Sensitivity ===
'''Working Groups:''' WGII

'''Definition:''' The degree to which a system or species is affected, either adversely or beneficially, by climate variability or change. The effect may be direct (e.g., a change in crop yield in response to a change in the mean, range, or variability of temperature) or indirect (e.g., damages caused by an increase in the frequency of coastal flooding due to sea level rise).

=== Sequestration ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The process of storing carbon in a carbon pool.

=== Sequestration potential ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The quantity of greenhouse gases that can be removed from the atmosphere by anthropogenic enhancement of sinks and stored in a pool. See Mitigation potential for different subcategories of sequestration potential.

=== Service provisioning ===
'''Working Groups:''' WGIII

'''Definition:''' Various services (such as illumination and mobility) can be provided by ‘systems’ through the use of energy, materials, and other resources comprising (i) Resource flows (e.g., energy), (ii) Technologies for resource use and energy conversion (e.g., vehicles and their engines), and (iii) Social/organisational forms of service delivery (e.g., publicly owned companies, or privately owned companies, e-commerce).

=== Services ===
'''Working Groups:''' WGIII

'''Definition:''' Activities that help satisfy human wants or needs. While they usually involve relationships between producers and consumers, services are less tangible and less storable than goods since they represent flows not stocks, and when their regeneration conditions are protected they may be reused over time.

=== Settlements ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Places of concentrated human habitation. Settlements can range from isolated rural villages to urban regions with significant global influence. They can include formally planned and informal or illegal habitation and related infrastructure.

=== Shared socio-economic pathways ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Shared socio-economic pathways (SSPs)

'''Definition:''' Shared Socio-economic Pathways (SSPs) have been developed to complement the Representative Concentration Pathways (RCPs). By design, the RCP emission and concentration pathways were stripped of their association with a certain socio-economic development. Different levels of emissions and climate change along the dimension of the RCPs can hence be explored against the backdrop of different socio-economic development pathways (SSPs) on the other dimension in a matrix. This integrative SSP-RCP framework is now widely used in the climate impact and policy analysis literature, where climate projections obtained under the RCP scenarios are analysed against the backdrop of various SSPs. As several emissions updates were due, a new set of emissions scenarios was developed in conjunction with the SSPs. Hence, the abbreviation SSP is now used for two things: On the one hand SSP1, SSP2, …, SSP5 are used to denote the five socio-economic scenario families. On the other hand, the abbreviations SSP1-1.9, SSP1-2.6, …, SSP5-8.5 are used to denote the newly developed emissions scenarios that are the result of an SSP implementation within an integrated assessment model. Those SSP scenarios are bare of climate policy assumption, but in combination with so-called shared policy assumptions (SPAs), various approximate radiative forcing levels of 1.9, 2.6, …, or 8.5 W m –2 are reached by the end of the century, respectively.

=== Sharing economy. ===
'''Working Groups:''' WGIII

'''Definition:''' A system which allows people to share goods and services by enabling collaborative use, access or ownership.

=== Shelf seas ===
'''Working Groups:''' WGII

'''Definition:''' Relatively shallow water covering the shelf of continents or around islands. The limit of shelf seas is conventionally considered as 200 m water depth at the continental shelf edge, where there is usually a steep slope to the deep ocean floor. During glacial periods, most shelf seas are lost since they become land as the build-up of ice sheets caused a decrease of global sea level.

=== Shifting development pathways ===
'''Working Groups:''' WGIII

'''Also known as:''' Shifting development pathways (SDP)

'''Definition:''' In this report, shifting development pathways describes transitions aimed at re-directing existing developmental trends. Societies may put in place enabling conditions to influence their future development pathways, when they endeavour to achieve certain outcomes. Some outcomes may be common, while others may be context-specific, given different starting points.

=== Shifting development pathways to sustainability ===
'''Working Groups:''' WGIII

'''Also known as:''' Shifting development pathways to sustainability (SDPS)

'''Definition:''' Shifting development pathways to sustainability involves transitions aligned with a shared aspiration in the Sustainable Development Goals (SDGs) agreed globally, though sustainability may be interpreted differently in various contexts as societies pursue a variety of sustainable development objectives.

=== Short-lived climate forcers ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Short-lived climate forcers (SLCFs)

'''Definition:''' A set of chemically reactive compounds with short (relative to carbon dioxide (CO2)) atmospheric lifetimes (from hours to about two decades) but characterised by different physiochemical properties and environmental effects. Their emission or formation has a significant effect on radiative forcing over a period determined by their respective atmospheric lifetimes. Changes in their emissions can also induce long-term climate effects via, in particular, their interactions with some biogeochemical cycles. SLCFs are classified as direct or indirect, with direct SLCFs exerting climate effects through their radiative forcing and indirect SLCFs being the precursors of other direct climate forcers. Direct SLCFs include 4) methane (CH, 3) ozone (O, primary aerosols and some halogenated species. Indirect SLCFs are precursors of ozone or secondary aerosols. SLCFs can be cooling or warming through interactions with radiation and clouds. They are also referred to as near-term climate forcers. Many SLCFs are also air pollutants. A subset of exclusively warming SLCFs is also referred to as short-lived climate pollutants (SLCPs), including methane, ozone, and black carbon (BC).

=== Short-lived climate pollutants ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Short-lived climate pollutants (SLCP)

'''Definition:''' Many SLCFs are also air pollutants.

=== Significant wave height ===
'''Working Groups:''' WGI

'''Definition:''' The average trough-to-crest height of the highest one-third of the wave heights (sea and swell) occurring in a particular time period.

=== Simple climate model ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Simple climate model (SCM)

'''Definition:''' A broad class of lower-dimensional models of the energy balance, radiative transfer, carbon cycle, or a combination of such physical components. SCMs are also suitable for performing emulations of climate-mean variables of Earth system models (ESMs), given that their structural flexibility can capture both the parametric and structural uncertainties across process-oriented ESM responses. They can also be used to test consistency across multiple lines of evidence with regard to climate sensitivity ranges, transient climate responses (TCRs), transient climate response to cumulative CO 2 emissions (TCREs) and carbon cycle feedbacks.

=== Sink ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere (United Nations Framework Convention on Climate Change (UNFCCC) Article 1.8 (UNFCCC, 1992)).

=== Small Island Developing States ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Small Island Developing States (SIDS)

'''Definition:''' Small Island Developing States (SIDS), as recognised by the United Nations Office of the High Representative for the Least Developed Countries, Landlocked Developing Countries and Small Island Developing States (OHRLLS), are a distinct group of developing countries facing specific social, economic and environmental vulnerabilities (UN-OHRLLS, 2011). They were recognised as a special case for both their environment and their development at the Rio Earth Summit in Brazil in 1992. Fifty-eight countries and territories are presently classified as SIDS by the UN OHRLLS, with 38 being UN member states and 20 being Non-UN Members or Associate Members of the Regional Commissions (UN-OHRLLS, 2018).

=== Smart grids ===
'''Working Groups:''' WGIII

'''Definition:''' A smart grid uses information and communications technology to gather data on the behaviours of suppliers and consumers in the production, distribution, and use of electricity. Through automated responses or the provision of price signals, this information can then be used to improve the efficiency, reliability, economics, and sustainability of the electricity network.

=== Snow cover ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Snow cover refers to all the snow that has accumulated on the ground at a given time (UNESCO/IASH/WMO, 1970).

=== Snow cover duration ===
'''Working Groups:''' WGI

'''Also known as:''' Snow cover duration (SCD)

'''Definition:''' How long snow continuously remains on the land surface, or the period between snow-on and snow-off dates.

=== Snow cover extent ===
'''Working Groups:''' WGI

'''Also known as:''' Snow cover extent (SCE)

'''Definition:''' The areal extent of snow covered ground.

=== Snow water equivalent ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Snow water equivalent (SWE)

'''Definition:''' The depth of liquid water that would result if a mass of snow melted completely.

=== Social cost of carbon ===
'''Working Groups:''' WGIII

'''Also known as:''' Social cost of carbon (SCC)

'''Definition:''' The net present value of aggregate climate damages (with overall harmful damages expressed as a number with positive sign) from one more tonne of carbon in the form of carbon dioxide (CO2), conditional on a global emissions trajectory over time.

=== Social costs ===
'''Working Groups:''' WGIII

'''Definition:''' The full costs of an action in terms of social welfare losses, including external costs associated with the impacts of this action on the environment, the economy (GDP, employment) and on the society as a whole.

=== Social-ecological system ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' An integrated system that includes human societies and ecosystems, in which humans are part of nature. The functions of such a system arise from the interactions and interdependence of the social and ecological subsystems. The system’s structure is characterised by reciprocal feedbacks, emphasising that humans must be seen as a part of, not apart from, nature (Berkes and Folke 1998; Arctic Council, 2016).

=== Social group ===
'''Working Groups:''' WGIII

'''Definition:''' A collective of people who share similar characteristics and collectively may have a sense of unity (Forsyth 2010).

=== Social identity ===
'''Working Groups:''' WGIII

'''Definition:''' The portion of an individual’s self-concept derived from perceived membership in a relevant social group (Tajfel and Turner 1986).

=== Social inclusion ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A process of improving the terms of participation in society, particularly for people who are disadvantaged, through enhancing opportunities, access to resources and respect for rights (UN DESA 2016).

=== Social infrastructure ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The social, cultural, and financial activities and institutions as well as associated property, buildings and artefacts and policy domains such as social protection, health and education that support well-being and public life (Frolova et al., 2016; Latham and Layton, 2019).

=== Social justice ===
'''Working Groups:''' WGII

'''Definition:''' Just or fair relations within society that seek to address the distribution of wealth, access to resources, opportunity, and support according to principles of justice and fairness.

=== Social learning ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A process of social interaction through which people learn new behaviours, capacities, values, and attitudes.

=== Social protection ===
'''Working Groups:''' WGII

'''Definition:''' In the context of development aid and climate policy, social protection usually describes public and private initiatives that provide income or consumption transfers to the poor, protect the vulnerable against livelihood risks, and enhance the social status and rights of the marginalised, with the overall objective of reducing the economic and social vulnerability of poor, vulnerable, and marginalised groups (Devereux and Sabates-Wheeler, 2004). In other contexts, social protection may be used synonymously with social policy and can be described as all public and private initiatives that provide access to services, such as health, education or housing, or income and consumption transfers to people. Social protection policies protect the poor and vulnerable against livelihood risks and enhance the social status and rights of the marginalised, as well as prevent vulnerable people from falling into poverty.

=== Societal transformations ===
'''Working Groups:''' WGII

'''Also known as:''' Societal (social) transformations

'''Definition:''' A change in the fundamental attributes of human systems advanced by societal actors

=== Socio-economic scenario ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A scenario that describes a plausible future in terms of population, gross domestic product (GDP), and other socio-economic factors relevant to understanding the implications of climate change.

=== Socio-technical transitions ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Where technological change is associated with social systems and the two are inextricably linked.

=== Soil carbon sequestration ===
'''Working Groups:''' WGIII

'''Also known as:''' Soil carbon sequestration (SCS)

'''Definition:''' Land management changes which increase the soil organic carbon content, resulting in a net removal of carbon dioxide (CO2) from the atmosphere.

=== Soil erosion ===
'''Working Groups:''' WGII

'''Definition:''' The displacement of the soil by the action of water or wind. Soil erosion is a major process of land degradation.

=== Soil moisture ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Water stored in the soil in liquid or frozen form. Root-zone soil moisture is of most relevance for plant activity.

=== Soil organic carbon ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Carbon contained in soil organic matter.

=== Soil organic matter ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The organic component of soil, comprising plant and animal residue at various stages of decomposition, and soil organisms.

=== Soil temperature ===
'''Working Groups:''' WGI

'''Definition:''' The temperature of the soil. This can be measured or modelled at multiple levels within the depth of the soil.

=== Solar activity ===
'''Working Groups:''' WGI

'''Definition:''' General term collectively describing a variety of magnetic phenomena on the Sun such as sunspots, faculae (bright areas), and flares (emission of high-energy particles). It varies on time scales from minutes to millions of years. The solar cycle, with an average duration of 11 years, is an example of a quasi-regular change in solar activity.

=== Solar cycle ===
'''Working Groups:''' WGI

'''Also known as:''' Solar cycle (11-year)

'''Definition:''' A quasi-regular modulation of solar activity with varying amplitude and a period of between 8 and 14 years.

=== Solar energy ===
'''Working Groups:''' WGIII

'''Definition:''' Energy from the Sun. Often the phrase is used to mean energy that is captured from solar radiation either as heat, as light that is converted into chemical energy by natural or artificial photosynthesis, or by photovoltaic panels and converted directly into electricity.

=== Solar radiation ===
'''Working Groups:''' WGI

'''Definition:''' Electromagnetic radiation emitted by the Sun with a spectrum close to that of a black body with a temperature of 5770 K. The radiation peaks in visible wavelengths. When compared to the terrestrial radiation it is often referred to as shortwave radiation.

=== Solar radiation modification ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Solar radiation modification (SRM)

'''Definition:''' Refers to a range of radiation modification measures not related to greenhouse gas (GHG) mitigation that seek to limit global warming. Most methods involve reducing the amount of incoming solar radiation reaching the surface, but others also act on the longwave radiation budget by reducing optical thickness and cloud lifetime.

=== Solubility pump ===
'''Working Groups:''' WGI

'''Definition:''' A physicochemical process that transports dissolved inorganic carbon from the ocean ’s surface to its interior. The solubility pump is primarily driven by the solubility of carbon dioxide (CO 2) (with more CO 2 dissolving in colder water) and the large-scale, thermohaline patterns of ocean circulation.

=== Solution space ===
'''Working Groups:''' WGII

'''Definition:''' The set of biophysical, cultural, socio-economic and political-institutional dimensions within which opportunities and constraints determine why, how, when and who acts to reduce climate risks. Within these dimensions, there are ’hard’ (unsurpassable) limits and ’soft’(surpassable) limits. The boundaries of the solution space are path dependent, contested and in constant flux (Haasnoot et. al. 2020).

=== Source ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Any process or activity which releases a greenhouse gas (GHG), an aerosol or a precursor of a GHG into the atmosphere (United Nations Framework Convention on Climate Change (UNFCCC) Article 1.9 (UNFCCC, 1992)).

=== South American monsoon ===
'''Working Groups:''' WGI

'''Also known as:''' South American monsoon (SAmerM)

'''Definition:''' The South American monsoon (SAmerM) is a regional circulation characterized by inflow of low-level winds from the Atlantic to South America, including Brazil, Peru, Bolivia and northern Argentina, associated with the development of surface pressure gradients (and intense precipitation) during austral summer (December–January–February). During September–October–November, areas of intense convection migrate from northwestern South America to the south. Associated with this regime, an upper-tropospheric anticyclone (a.k.a. the Bolivian High) forms over the Altiplano region during the monsoon onset. The SAmerM then retreats during March–April–May with a northeastward migration of the convection. Further details on how SAmerM is defined and used throughout the Report are provided in Annex V.

=== South Pacific Convergence Zone ===
'''Working Groups:''' WGI

'''Also known as:''' South Pacific Convergence Zone (SPCZ)

'''Definition:''' A band of low-level convergence, cloudiness and precipitation ranging from the west Pacific warm pool south-eastwards towards French Polynesia. It is one of the most significant features of subtropical Southern Hemisphere climate. It shares some characteristics with the Inter-tropical Convergence Zone (ITCZ), but is more extratropical in nature, especially east of the International Date Line.

=== South and South East Asian monsoon ===
'''Working Groups:''' WGI

'''Also known as:''' South and South East Asian monsoon (SAsiaM)

'''Definition:''' The South and South East Asian monsoon (SAsiaM) is characterized by pronounced seasonal reversals of wind and precipitation. The SAsiaM region extends across vast geographical areas and several countries, including India, Bangladesh, Nepal, Myanmar, Sri Lanka, Pakistan, Thailand, Laos, Cambodia, Vietnam and the Philippines. The SAsiaM starts in late May/early June and progresses towards the north-east, ending in late September/early October. During the core monsoon season, maxima of SAsiaM precipitation are located over the west coast, north-east and central north India, Myanmar and Bangladesh, whereas minima are located over north-west and south-eastern India, western Pakistan, and southeastern and northern Sri Lanka. Further details on how SAsiaM is defined and used throughout the Report are provided in Annex V.

=== Southern Annular Mode ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' Southern Annular Mode (SAM)

'''Definition:''' The leading mode of climate variability of Southern Hemisphere sea-level pressure and geopotential height, which is associated with the strength and latitudinal shifts in the mid- to high-latitudes westerly wind belt. The SAM is also known as the Antarctic Oscillation (AAO). A positive SAM phase is defined as lower-than-normal pressures over the polar regions and higher-than-normal pressures in the southern mid-latitudes, with a contraction towards Antarctica and strengthening of the westerly wind belt. The negative SAM phase exhibits positive high latitude pressure anomalies, negative mid-latitude pressure anomalies and a weaker westerly flow expanded towards the equator. See Section AIV.2.2 in Annex IV of the AR6 WGI report.

=== Southern Ocean ===
'''Working Groups:''' WGII

'''Definition:''' The ocean region encircling Antarctica that connects the Atlantic, Indian and Pacific Oceans together, allowing inter-ocean exchange. This region is the main source of much of the deep water of the world’s ocean and also provides the primary return pathway for this deep water to the surface (Marshall and Speer, 2012; Toggweiler and Samuels, 1995). The drawing up of deep waters and the subsequent transport into the ocean interior has major consequences for the global heat, nutrient and carbon balances, as well as the Antarctic cryosphere and marine ecosystems.

=== Spatial and temporal scales ===
'''Working Groups:''' WGII

'''Definition:''' Climate may vary on a large range of spatial and temporal scales. Spatial scales may range from local (less than 100 000 km 2), through regional (100 000 to 10 million km 2) to continental (10 to 100 million km 2). Temporal scales may range from seasonal to geological (up to hundreds of millions of years).

=== Specific humidity ===
'''Working Groups:''' WGI

'''Definition:''' The specific humidity specifies the ratio of the mass of water vapour to the total mass of moist air.

=== Spill-over effect ===
'''Working Groups:''' WGIII

'''Definition:''' The effects of domestic or sector mitigation measures on other countries or sectors. Spill-over effects can be positive or negative and include effects on trade, (carbon) leakage, transfer of innovations, and diffusion of environmentally sound technology and other issues.

=== Stadial or stade ===
'''Working Groups:''' WGI

'''Definition:''' A brief period of regional climatic cooling during a glacial or interglacial interval, often characterized by transient glacial advances. Stadials are generally of short duration (hundreds to a few thousand years) compared to glacial or interglacial intervals (lasting many thousands to tens of thousands of years). One example of a regional stadial event is based on millennial scale cooling recorded by oxygen isotope ratios in Greenland ice cores, the so called “Greenland Stadials” (Johnsen et al., 1992).

=== Standard ===
'''Working Groups:''' WGII

'''Definition:''' Set of rules or codes mandating or defining product performance (e.g., grades, dimensions, characteristics, test methods, and rules for use). Product, technology or performance standards establish minimum requirements for affected products or technologies. Standards impose reductions in greenhouse gas (GHG) emissions associated with the manufacture or use of the products and/or application of the technology.

=== Steric sea level change ===
'''Working Groups:''' WGI

'''Definition:''' Steric sea level change is caused by changes in ocean density and is composed of thermosteric sea level change and halosteric sea level change.

=== Storm surge ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The temporary increase, at a particular locality, in the height of the sea due to extreme meteorological conditions (low atmospheric pressure and/or strong winds). The storm surge is defined as being the excess above the level expected from the tidal variation alone at that time and place.

=== Storm tracks ===
'''Working Groups:''' WGI

'''Definition:''' Originally, a term referring to the tracks of individual cyclonic weather systems, but now often generalized to refer to the main regions where the tracks of extratropical disturbances occur as sequences of low (cyclonic) and high (anticyclonic) pressure systems.

=== Storyline ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A way of making sense of a situation or a series of events through the construction of a set of explanatory elements. Usually, it is built on logical or causal reasoning. In climate research, the term storyline is used both in connection to scenarios as related to a future trajectory of the climate and human systems and to a weather or climate event. In this context, storylines can be used to describe plural, conditional possible futures or explanations of a current situation, in contrast to single, definitive futures or explanations.

=== Stranded assets ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Assets exposed to devaluations or conversion to ‘liabilities’ because of unanticipated changes in their initially expected revenues due to innovations and/or evolutions of the business context, including changes in public regulations at the domestic and international levels.

=== Stratification ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Process of forming of layers of (ocean) water with different properties such as salinity, density and temperature that act as barriers for water mixing. The strengthening of near-surface stratification generally results in warmer surface waters, decreased oxygen levels in deeper water and intensification of ocean acidification (OA) in the upper ocean.

=== Stratosphere ===
'''Working Groups:''' WGI

'''Definition:''' The highly stratified region of the atmosphere above the tropopause, extending to about 50 km altitude.

=== Stratosphere–troposphere exchange ===
'''Working Groups:''' WGI

'''Also known as:''' Stratosphere–troposphere exchange (STE)

'''Definition:''' Stratosphere–troposphere exchange (STE) is understood as the flux of air or trace constituents across the tropopause, including both directions: the stratosphere to troposphere transport (STT) and troposphere to stratosphere transport (TST). STE is one of the key factors controlling the budgets of ozone, water vapour and other substances in both the troposphere and the lower stratosphere.

=== Stratospheric aerosol injection ===
'''Working Groups:''' WGI

'''Also known as:''' Stratospheric aerosol injection (SAI)

'''Definition:''' One of several solar radiation modification (SRM) approaches to increase the planetary albedo. In the approach, it is proposed to inject highly reflective aerosols such as sulphates into the lower stratosphere. This is expected to increase the fraction of solar radiation deflected to space resulting in a planetary cooling.

=== Stratospheric ozone ===
'''Working Groups:''' WGI

'''Definition:''' Stratospheric ozone describes the 3) ozone (O that resides in the stratosphere, the region of the atmosphere which exists between 10 and 50 kilometres above the surface of the earth. Ninety percent of total-column ozone resides in the stratosphere.

=== Stratospheric polar vortex ===
'''Working Groups:''' WGI

'''Definition:''' A large-scale region of cold air poleward of approximately 60 degrees that is contained by a strong westerly jet from the tropopause (8–10 km) to the stratopause (50–60 km) and that forms in each hemisphere during the winter half-year. Planetary waves can temporarily disrupt the vortex, producing easterly winds and rapid warming over polar regions in the stratosphere, and leading to substantial weakening or breakdown of the vortex.

=== Stratospheric sounding unit ===
'''Working Groups:''' WGI

'''Also known as:''' Stratospheric sounding unit (SSU)

'''Definition:''' A three-channel infrared sounder on operational U.S. National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites. The three channels are used to determine profiles of temperature in the stratosphere (AMS, 2021).

=== Streamflow ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Water flow within a river channel, for example, expressed in m 3 s –1. A synonym for river discharge.

=== Stressors ===
'''Working Groups:''' WGII

'''Definition:''' Events and trends, often not climate-related, that have an important effect on the system exposed and can increase vulnerability to climate-related risk.

=== Subduction ===
'''Working Groups:''' WGI

'''Definition:''' Ocean process in which surface waters enter the ocean interior from the surface mixed layer through Ekman pumping and lateral advection. The latter occurs when surface waters are advected to a region where the local surface layer is less dense and therefore must slide below the surface layer, usually with no change in density.

=== Subnational actors ===
'''Working Groups:''' WGIII

'''Definition:''' State/provincial, regional, metropolitan and local/municipal governments as well as non-party stakeholders, such as civil society, the private sector, cities and other subnational authorities, local communities and indigenous peoples.

=== Sudden stratospheric warming ===
'''Working Groups:''' WGI

'''Also known as:''' Sudden stratospheric warming (SSW)

'''Definition:''' A phenomena of rapid warming in the stratosphere at high latitudes (sometimes more than 50°C in 1–2 days) that can cause breakdown of stratospheric polar vortices.

=== Sufficiency ===
'''Working Groups:''' WGIII

'''Definition:''' A set of measures and daily practices that avoid demand for energy, materials, land and water while delivering human well-being for all within planetary boundaries.

=== Sulphur hexafluoride ===
'''Working Groups:''' WGI

'''Also known as:''' Sulphur hexafluoride (SF6)

'''Definition:''' SF 6, a greenhouse gas (GHG), is mainly used in heavy industry to insulate high-voltage equipment and to assist in the manufacturing of cable-cooling systems and semiconductors.

=== Sunspots ===
'''Working Groups:''' WGI

'''Definition:''' Dark areas on the Sun where strong magnetic fields reduce the convection, causing a temperature reduction of about 1500 K compared to the surrounding regions. The number of sunspots is higher during periods of higher solar activity and varies in particular with the solar cycle.

=== Supply-side measures ===
'''Working Groups:''' WGIII

'''Definition:''' Policies and programmes for influencing how a certain demand for goods and/or services is met. In the energy sector, supply-side mitigation measures aim at reducing the amount of greenhouse gas emissions emitted per unit of energy service produced.

=== Surface energy budget ===
'''Working Groups:''' WGI

'''Definition:''' comprises the exchanges of heat at the surface of the Earth associated with both radiative and non-radiative processes. Typical units: W m -2.

=== Surface mass balance ===
'''Working Groups:''' WGI

'''Also known as:''' Surface mass balance (SMB)

'''Definition:''' Surface mass balance refers to the difference between surface accumulation and surface ablation.

=== Surprises ===
'''Working Groups:''' WGI

'''Definition:''' A class of risk that can be defined as low-likelihood but well-understood events and events that cannot be predicted with current understanding (see Section 1.4.4.3 in AR6 WGI Chapter 1).

=== Sustainability ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A dynamic process that guarantees the persistence of natural and human systems in an equitable manner.

=== Sustainable Development Goals ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Sustainable Development Goals (SDGs)

'''Definition:''' The 17 global goals for development for all countries established by the United Nations through a participatory process and elaborated in the 2030 Agenda for Sustainable Development, including ending poverty and hunger; ensuring health and well-being, education, gender equality, clean water and energy, and decent work; building and ensuring resilient and sustainable infrastructure, cities and consumption; reducing inequalities; protecting land and water ecosystems; promoting peace, justice and partnerships; and taking urgent action on climate change.

=== Sustainable development ===
'''Working Groups:''' WGII; WGIII

'''Also known as:''' Sustainable development (SD)

'''Definition:''' Development that meets the needs of the present without compromising the ability of future generations to meet their own needs (WCED, 1987) and balances social, economic and environmental concerns.

=== Sustainable development pathways ===
'''Working Groups:''' WGII

'''Also known as:''' Sustainable development pathways (SDPs)

'''Definition:''' Trajectories aimed at attaining the Sustainable Development Goals (SDGs) in the short term and the goals of sustainable development in the long term. In the context of climate change, such pathways denote trajectories that address social, environmental and economic dimensions of sustainable development, adaptation and mitigation, and transformation, in a generic sense or from a particular methodological perspective such as integrated assessment models and scenario simulations.

=== Sustainable forest management ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The stewardship and use of forests and forest lands in a way, and at a rate, that maintains their biodiversity, productivity, regeneration capacity, vitality and their potential to fulfil, now and in the future, relevant ecological, economic and social functions, at local, national, and global levels, and that does not cause damage to other ecosystems (Forest Europe, 1993).

=== Sustainable intensification ===
'''Working Groups:''' WGIII

'''Also known as:''' Sustainable intensification (of agriculture)

'''Definition:''' Increasing yields from the same area of land while decreasing negative environmental impacts of agricultural production and increasing the provision of environmental services (CGIAR, 2019). [Note: This definition is based on the concept of meeting demand from a finite land area, but it is scale-dependent. Sustainable intensification at a given scale (e.g., global or national) may require a decrease in production intensity at smaller scales and in particular places (often associated with previous, unsustainable, intensification) to achieve sustainability (Garnett et al., 2013).]

=== Sustainable land management ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The stewardship and use of land resources, including soils, water, animals and plants, to meet changing human needs, while simultaneously ensuring the long-term productive potential of these resources and the maintenance of their environmental functions.

=== Swash ===
'''Working Groups:''' WGI

'''Definition:''' Vertical displacement up the shore-face induced by individual waves.

=== Sympagic ===
'''Working Groups:''' WGII

'''Definition:''' Organisms and habitats related to the sea ice, analogous to pelagic (water column) or benthic (seafloor).

=== Systems of Innovation ===
'''Working Groups:''' WGIII

'''Also known as:''' Systems of Innovation (SI)

'''Definition:''' The set of public and private sector organisations (i.e., formally organised entities such as firms and universities; ‘actors’) and institutions, whose activities and interactions generate, modify and deploy new technologies. The SI approach has been used to understand and analyse innovation at the national, regional, and technological levels, and in transnational contexts (Lundvall, 1988, 1992).

== T ==

=== Talik ===
'''Working Groups:''' WGI

'''Definition:''' A layer or body of unfrozen ground in a permafrost area due to a local anomaly in thermal, hydrological, hydrogeological or hydrochemical conditions (IPA, 2005).

=== Technical potential ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The mitigation potential constrained by biogeophysical limits as well as availability of technologies and practices. Quantification of technical potentials takes into account primarily technical considerations, but social, economic and/or environmental considerations are occasionally also included, if these represent strong barriers for the deployment of an option.

=== Technology deployment ===
'''Working Groups:''' WGIII

'''Definition:''' The act of bringing technology into effective application, involving a set of actors and activities to initiate, facilitate and/or support its implementation.

=== Technology diffusion ===
'''Working Groups:''' WGIII

'''Definition:''' The spread of a technology across different groups users/markets over time.

=== Technology transfer ===
'''Working Groups:''' WGIII

'''Definition:''' The exchange of knowledge, hardware and associated software, money and goods among stakeholders, which leads to the spread of technology for adaptation or mitigation. The term encompasses both diffusion of technologies and technological cooperation across and within countries.

=== Teleconnection ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Association between climate variables at widely separated, geographically fixed locations related to each other through physical processes and oceanic and/or atmospheric dynamical pathways. Teleconnections can be caused by several climate phenomena, such as Rossby wave-trains, mid-latitude jet and storm track displacements, fluctuations of the Atlantic Meridional Overturning Circulation (AMOC), fluctuations of the Walker circulation, etc. They can be initiated by modes of climate variability, thus providing the development of remote climate anomalies at various temporal lags.

=== Teleconnection pattern ===
'''Working Groups:''' WGI

'''Definition:''' Spatial structure of climate anomalies that are linked to each other through teleconnection processes or that are the large-scale fingerprint of modes of climate variability. Teleconnection patterns can be visualized using correlation and/or regression maps of climate variables with some climate indices (i.e., those derived from the temporal variation of the main modes of climate variability). They can also be obtained from principal component analysis, singular value decomposition/maximum covariance analysis, clustering based on spatial recurrence criteria, etc. See also Section Atlas.3.1 of the AR6 WGI report and Teleconnection.

=== Temperature overshoot ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' Exceedance of a specified global warming level, followed by a decline to or below that level during a specified period of time (e.g., before 2100). Sometimes the magnitude and likelihood of the overshoot is also characterised. The overshoot duration can vary from one pathway to the next, but in most overshoot pathways in the literature and as referred to as overshoot pathways in the AR6, the overshoot occurs over a period of at least one decade and up to several decades.

=== Terrestrial radiation ===
'''Working Groups:''' WGI

'''Definition:''' Radiation emitted by the Earth’s surface, the atmosphere and clouds. It is also known as thermal infrared or longwave radiation and is to be distinguished from the near-infrared radiation that is part of the solar spectrum. Infrared radiation, in general, has a distinctive range of wavelengths (spectrum) longer than the wavelength of the red light in the visible part of the spectrum. The spectrum of terrestrial radiation is almost entirely distinct from that of shortwave or solar radiation because of the difference in temperature between the Sun and the Earth–atmosphere system.

=== Thermocline ===
'''Working Groups:''' WGI

'''Definition:''' The layer of maximum vertical temperature gradient in the ocean, lying between the surface ocean and the abyssal ocean. In subtropical regions, its source waters are typically surface waters at higher latitudes that have subducted (see Subduction) and moved equatorward. At high latitudes, it is sometimes absent, replaced by a halocline, which is a layer of maximum vertical salinity gradient.

=== Thermokarst ===
'''Working Groups:''' WGI

'''Definition:''' Process by which characteristic landforms result from thawing of ice-rich permafrost or melting of massive ice (IPA, 2005).

=== Thermosteric sea level change ===
'''Working Groups:''' WGI

'''Definition:''' Thermosteric sea level change (where thermosteric sea level rise may also be referred to as thermal expansion) occurs as a result of changes in ocean temperature: increasing temperature reduces ocean density and increases the volume per unit of mass.

=== Tide gauge ===
'''Working Groups:''' WGI

'''Definition:''' A device at a coastal or deep-sea location that continuously measures the level of the sea with respect to the adjacent land. Time averaging of the sea level so recorded gives the observed secular changes of the relative sea level.

=== Tier ===
'''Working Groups:''' WGII

'''Definition:''' In the context of the IPCC Guidelines for National Greenhouse Gas Inventories, a tier represents a level of methodological complexity. Usually three tiers are provided. Tier 1 is the basic method, Tier 2 intermediate and Tier 3 most demanding in terms of complexity and data requirements. Tiers 2 and 3 are sometimes referred to as higher-tier methods and are generally considered to be more accurate (IPCC, 2019).

=== Time of emergence ===
'''Working Groups:''' WGI

'''Also known as:''' Time of emergence (ToE)

'''Definition:''' Time when a specific anthropogenic signal related to climate change is statistically detected to emerge from the background noise of natural climate variability in a reference period, for a specific region (Hawkins and Sutton, 2012).

=== Tipping element ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A component of the Earth system that is susceptible to a tipping point.

=== Tipping point ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A critical threshold beyond which a system reorganises, often abruptly and/or irreversibly.

=== Top-of-atmosphere energy budget ===
'''Working Groups:''' WGI

'''Definition:''' Comprises the energy fluxes associated with incoming solar radiation, reflected solar radiation and emitted thermal radiation. Typical units: W m -2.

=== Total alkalinity ===
'''Working Groups:''' WGI

'''Definition:''' Total Alkalinity (A T) is a measurable parameter of the seawater acid–base system which, when expressed in micromoles per kilogram of seawater, is a conservative variable both on mixing and for changes in temperature and/or pressure. Changes in total alkalinity in the oceans can result from a variety of biogeochemical processes that affect the acid–base composition of the seawater itself. However, its value is not affected by the exchange of carbon dioxide gas between seawater and the atmosphere. Measurements of total alkalinity can thus be used to help study these biogeochemical processes and can also be used to help calculate the state of the seawater acid–base system. Total alkalinity is most commonly measured using an acidimetric titration technique that determines how much acid is required to titrate a seawater sample to a specified equivalence point.

=== Total carbon budget ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' Refers to two concepts in the literature: (i) an assessment of carbon cycle sources and sinks on a global level, through the synthesis of evidence for fossil fuel and cement emissions, emissions and removals associated with land use and land-use change, ocean and natural land sources and sinks of carbon dioxide (CO2), and the resulting change in atmospheric CO2 concentration.This is referred to as the total carbon budget when expressed starting from the pre-industrial period, and as the remaining carbon budget when expressed from a recent specified date.

=== Total solar irradiance ===
'''Working Groups:''' WGI

'''Also known as:''' Total solar irradiance (TSI)

'''Definition:''' The total amount of solar radiation in watts per square metre received outside the Earth’s atmosphere on a surface normal to the incident radiation, and at the Earth’s mean distance from the Sun. Reliable measurements of solar radiation can only be made from space, and the precise record extends back only to 1978. Variations of a few tenths of a percent are common, usually associated with the passage of sunspots across the solar disk. The solar cycle variation of TSI is of the order of 0.1% (AMS, 2021).

=== Total water level ===
'''Working Groups:''' WGI

'''Definition:''' Extreme total water level (ETWL) is the Extreme still water level (ESWL) plus wave setup. When considering coastal impacts, swash is also important, and Extreme coastal water level (ECWL) is used.

=== Trace gas ===
'''Working Groups:''' WGI

'''Definition:''' A minor constituent of the atmosphere, next to nitrogen and oxygen that together make up 99 % of all volume. The most important trace gases contributing to the greenhouse effect are carbon dioxide (CO2), ozone (O3), methane (CH4), nitrous oxide (N2O), perfluorocarbons (PFCs), chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), sulphur hexafluoride (SF6) and water vapour (H2O).

=== Trade-off ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A competition between different objectives within a decision situation, where pursuing one objective will diminish achievement of other objective(s). A trade-off exists when a policy or measure aimed at one objective (e.g., reducing greenhouse gas emissions) reduces outcomes for other objective(s) (e.g., biodiversity conservation, energy security) due to adverse side effects, thereby potentially reducing the net benefit to society or the environment.

=== Traditional biomass ===
'''Working Groups:''' WGIII

'''Definition:''' The combustion of wood, charcoal, agricultural residues and/or animal dung for cooking or heating in open fires or in inefficient stoves as is common in low-income countries.

=== Transformation ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A change in the fundamental attributes of natural and human systems.

=== Transformation pathways ===
'''Working Groups:''' WGIII

'''Definition:''' Trajectories describing consistent sets of possible futures of greenhouse gas (GHG) emissions, atmospheric concentrations, or global mean surface temperatures implied from mitigation and adaptation actions associated with a set of broad and irreversible economic, technological, societal, and behavioural changes. This can encompass changes in the way energy and infrastructure are used and produced, natural resources are managed and institutions are set up and in the pace and direction of technological change.

=== Transformational adaptation ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Adaptation that changes the fundamental attributes of a social-ecological system in anticipation of climate change and its impacts.

=== Transformative change ===
'''Working Groups:''' WGII

'''Definition:''' A system-wide change that requires more than technological change through consideration of social and economic factors that, with technology, can bring about rapid change at scale.

=== Transient climate response ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Transient climate response (TCR)

'''Definition:''' The surface temperature response for the hypothetical scenario in which atmospheric carbon dioxide (CO2) increases at 1% yr -1 from pre-industrial to the time of a doubling of atmospheric CO 2 concentration (year 70).

=== Transient climate response to cumulative CO2 emissions ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Transient climate response to cumulative CO2 emissions (TCRE)

'''Definition:''' The transient surface temperature change per unit cumulative carbon dioxide (CO2) emissions, usually 1000 GtC. TCRE combines both information on the airborne fraction of cumulative CO2 emissions (the fraction of the total CO2 emitted that remains in the atmosphere, which is determined by carbon cycle processes) and on the transient climate response (TCR).

=== Transition ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The process of changing from one state or condition to another in a given period of time. Transition can occur in individuals, firms, cities, regions and nations, and can be based on incremental or transformative change.

=== Tree line ===
'''Working Groups:''' WGII

'''Definition:''' The upper limit of tree growth in mountains or at high latitudes. It is more elevated or more poleward than the forest line.

=== Tree rings ===
'''Working Groups:''' WGI

'''Definition:''' Concentric rings of secondary wood evident in a cross section of the stem of a woody plant. The difference between the dense, small-celled late wood of one season and the wide-celled early wood of the following spring enables the age of a tree to be estimated, and the ring widths or density can be related to climate parameters such as temperature and precipitation.

=== Trend estimates uncertainty ===
'''Working Groups:''' WGI

'''Definition:''' Uncertainty arising from data fitting to a time-series with potential non-linear and autorogressive character.

=== Tropical Atlantic Variability ===
'''Working Groups:''' WGI

'''Also known as:''' Tropical Atlantic Variability (TAV)

'''Definition:''' A generic term to describe the climate variability of the tropical Atlantic which is dominated at interannual to decadal time scales by two main climate modes: the Atlantic Zonal Mode (AZM) and the Atlantic Meridional Mode (AMM). The Atlantic Zonal Mode, also commonly referred to as the Atlantic Niño or Atlantic equatorial mode, is associated with sea surface temperature anomalies near the equator, peaking in the eastern basin, while the Atlantic meridional mode is characterized by an inter-hemispheric gradient of sea surface temperature and wind anomalies. Both modes are associated with significant teleconnections over Africa and South America.

=== Tropical cyclone ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The general term for a strong, cyclonic-scale disturbance that originates over tropical oceans. Distinguished from weaker systems (often named tropical disturbances or depressions) by exceeding a threshold wind speed. A tropical storm is a tropical cyclone with one-minute average surface winds between 18 and 32 m s –1. Beyond 32 m s –1, a tropical cyclone is called a hurricane, typhoon or cyclone, depending on geographic location.

=== Tropopause ===
'''Working Groups:''' WGI

'''Definition:''' The boundary between the troposphere and the stratosphere. It ranges from 8–9 km at high latitudes to 15–16 km in the tropics.

=== Troposphere ===
'''Working Groups:''' WGI

'''Definition:''' The lowest part of the atmosphere, below the tropopause, where clouds and weather phenomena occur. In the troposphere, temperatures generally decrease with height.

=== Tropospheric ozone ===
'''Working Groups:''' WGI

'''Definition:''' Tropospheric ozone acts as a greenhouse gas.

=== Tsunami ===
'''Working Groups:''' WGII

'''Definition:''' A wave, or train of waves, produced by a disturbance such as a submarine earthquake displacing the sea floor, a landslide, a volcanic eruption or an asteroid impact.

=== Tundra ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A treeless biome characteristic of polar and alpine regions.

=== Turnover time ===
'''Working Groups:''' WGI

'''Also known as:''' Turnover time (T)

'''Definition:''' (also called global atmospheric lifetime) is the ratio of the mass M of a reservoir (e.g., a gaseous compound in the atmosphere) and the total rate of removal S from the reservoir: T = M/S. For each removal process, separate turnover times can be defined. In soil carbon biology, this is referred to as mean residence time.

=== Typological regions ===
'''Working Groups:''' WGI

'''Definition:''' Regions of the Earth that share one or more specific features (known as ’typologies’), such as geographic location (e.g., coastal), physical processes (e.g., monsoons), and biological (e.g., coral reefs, tropical forests), geological (e.g., mountains) or anthropogenic (e.g., megacities) formation, and for which it is useful to consider the common climate features. Typological regions are smaller than climatic zones (e.g., a mountain region) and can be discontinuous (e.g., a group of megacities affected by the urban heat island effect, or monsoon regions).

== U ==

=== Uncertainty ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' A state of incomplete knowledge that can result from a lack of information or from disagreement about what is known or even knowable. It may have many types of sources, from imprecision in the data to ambiguously defined concepts or terminology, incomplete understanding of critical processes or uncertain projections of human behaviour. Uncertainty can therefore be represented by quantitative measures (e.g., a probability density function) or by qualitative statements (e.g., reflecting the judgement of a team of experts) (Moss and Schneider, 2000; IPCC, 2004; Mastrandrea et al., 2010).

=== United Nations Convention to Combat Desertification ===
'''Working Groups:''' WGIII

'''Also known as:''' United Nations Convention to Combat Desertification (UNCCD)

'''Definition:''' A legally binding international agreement linking environment and development to sustainable land management, established in 1994. The Convention’s objective is ‘to combat desertification and mitigate the effects of drought in countries experiencing drought and/or desertification’. The Convention specifically addresses the arid, semi-arid and dry sub-humid areas, known as the drylands, and has a particular focus on Africa. As of September 2020, the UNCCD had 197 Parties.

=== United Nations Framework Convention on Climate Change ===
'''Working Groups:''' WGI; WGII; WGIII

'''Also known as:''' United Nations Framework Convention on Climate Change (UNFCCC)

'''Definition:''' The UNFCCC was adopted in May 1992 and opened for signature at the 1992 Earth Summit in Rio de Janeiro. It entered into force in March 1994 and, as of September 2020, had 197 Parties (196 States and the European Union). The Convention’s ultimate objective is the ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system’ (UNFCCC, 1992). The provisions of the Convention are pursued and implemented by two further treaties: the Kyoto Protocol and the Paris Agreement.

=== Uptake ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The transfer of substances (such as carbon) or energy (e.g., heat) from one compartment of a system to another; for example, in the Earth system from the atmosphere to the ocean or to the land.

=== Upwelling region ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A region of an ocean where cold, typically nutrient-rich waters well up from the deep ocean.

=== Urban Systems ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Urban systems refer to two interconnected systems-first, the comprehensive collections of city elements with multiple dimensions and characteristics: a) encompass physical, built, socioeconomic-technical, political, and ecological subsystems; b) integrate social agent/constituency/processes with physical structure and processes; and c) exist within broader spatial and temporal scales and governance and institutional contexts; and second, the global system of cities and towns.

=== Urban ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' The categorisation of areas as 'urban' by government statistical departments is generally based either on population size, population density, economic base, provision of services, or some combination of the above. Urban systems are networks and nodes of intensive interaction and exchange including capital, culture, and material objects. Urban areas exist on a continuum with rural areas and tend to exhibit higher levels of complexity, higher populations and population density, intensity of capital investment, and a preponderance of secondary (processing) and tertiary (service) sector industries. The extent and intensity of these features varies significantly within and between urban areas. Urban places and systems are open with much movement and exchange between more rural areas as well as other urban regions. Urban areas can be globally interconnected facilitating rapid flows between them – of capital investment, of ideas and culture, human migration, and disease.

=== Urban and peri-urban agriculture ===
'''Working Groups:''' WGII

'''Definition:''' The cultivation of crops and rearing of animals for food and other uses within and surrounding the boundaries of cities, including fisheries and forestry (EPRS, 2014).

=== Urban heat island ===
'''Working Groups:''' WGI; WGIII

'''Also known as:''' Urban heat island (UHI)

'''Definition:''' The relative warmth of a city compared with surrounding rural areas, associated with heat trapping due to land use, the configuration and design of the built environment, including street layout and building size, the heat-absorbing properties of urban building materials, reduced ventilation, reduced greenery and water features, and domestic and industrial heat emissions generated directly from human activities.

=== Urbanisation ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' Urbanisation is a multi-dimensional process that involves at least three simultaneous changes: (i) land use change: transformation of formerly rural settlements or natural land into urban settlements; (ii) demographic change: a shift in the spatial distribution of a population from rural to urban areas; and (iii) infrastructure change: an increase in provision of infrastructure services including electricity, sanitation, etc. Urbanisation often includes changes in lifestyle, culture, and behaviour, and thus alters the demographic, economic, and social structure of both urban and rural areas. (Stokes and Seto 2019; Seto et al. 2014; UNDESA 2018)

=== Urbanization ===
'''Working Groups:''' WGI

'''Definition:''' In the WGI report, urbanization is used to mean the process of soil sealing with the change of natural land cover to built environment and urban areas, together with its associated albedo changes, and increased surface runoff and elevated warming.

== V ==

=== Values and beliefs ===
'''Working Groups:''' WGII

'''Definition:''' Fundamental attitudes about what is important, good, and right; strongly held principles or qualities intrinsically valuable or desirable, often enshrined in laws, traditions, and religions. Examples include human rights, subsistence, and equitable distribution of costs and benefits of climate policies (Hulme, 2009, 2018; Nakashima et al., 2012; UNFCCC, 1992; UN Universal Declaration of Human Rights, 1948).

=== Variable renewable energy ===
'''Working Groups:''' WGIII

'''Also known as:''' Variable renewable energy (VRE)

'''Definition:''' Renewable energy sources such as wind and solar energy whose output is determined by weather, in contrast to ‘dispatchable’ generators that adjust their output as a reaction to economic incentives. Variable renewables have also been termed intermittent, fluctuating, or non-dispatchable. (Hirth, 2013)

=== Vector-borne disease ===
'''Working Groups:''' WGII

'''Definition:''' Illnesses caused by parasites, viruses and bacteria that are transmitted by various vectors (e.g. mosquitoes, sandflies, triatomine bugs, blackflies, ticks, tsetse flies, mites, snails and lice)(UNEP 2018)

=== Ventilation ===
'''Working Groups:''' WGI; WGII

'''Also known as:''' Ventilation (ocean)

'''Definition:''' The exchange of ocean properties with the atmospheric surface layer such that property concentrations are brought closer to equilibrium values with the atmosphere (AMS, 2000), and the processes that propagate these properties into the ocean interior.

=== Verification ===
'''Working Groups:''' WGIII

'''Definition:''' ‘The process of formal verification of reports, for example, the established approach to verify national communications and national inventory reports to the UNFCCC’ (UN REDD, 2009).

=== Vertical land motion ===
'''Working Groups:''' WGI

'''Also known as:''' Vertical land motion (VLM)

'''Definition:''' The change in height of the land surface or the sea floor and can have several causes in addition to elastic deformation associated with contemporary changes in gravity, rotation and viscoelastic solid Earth deformation (GRD) and viscoelastic deformation associated with glacial isostatic adjustment (GIA). Subsidence (sinking of the land surface or sea floor) can, for instance, occur through compaction of alluvial sediments in deltaic regions, removal of fluids such as gas, oil, and water, or drainage of peatlands. Tectonic deformation of the Earth’s crust can occur as a result of earthquakes and volcanic eruptions.

=== Very short-lived halogenated substances ===
'''Working Groups:''' WGI

'''Also known as:''' Very short-lived halogenated substances (VSLSs)

'''Definition:''' Very short-lived halogenated substances (VSLSs) are considered to include source gases (very short-lived halogenated substances present in the atmosphere in the form they were emitted from natural and anthropogenic sources), halogenated product gases arising from source gas degradation, and other sources of tropospheric inorganic halogens. VSLSs have tropospheric lifetimes of around 0.5 years or less.

=== Volatile organic compounds ===
'''Working Groups:''' WGI

'''Also known as:''' Volatile organic compounds (VOCs)

'''Definition:''' Important class of organic chemical air pollutants that are volatile at ambient air conditions. Other terms used to represent VOCs are hydrocarbons (HCs), reactive organic gases (ROGs) and non-methane volatile organic compounds (NMVOCs). NMVOCs are major contributors – together with nitrogen oxides (NOx), and carbon monoxide (CO) – to the formation of photochemical oxidants such as ozone (O 3).

=== Vulnerability ===
'''Working Groups:''' WGI; WGII; WGIII

'''Definition:''' The propensity or predisposition to be adversely affected. Vulnerability encompasses a variety of concepts and elements, including sensitivity or susceptibility to harm and lack of capacity to cope and adapt.

=== Vulnerability index ===
'''Working Groups:''' WGII

'''Definition:''' A metric characterising the vulnerability of a system. A climate vulnerability index is typically derived by combining, with or without weighting, several indicators assumed to represent vulnerability.

== W ==

=== Walker circulation ===
'''Working Groups:''' WGI

'''Definition:''' Direct thermally driven zonal overturning circulation in the atmosphere over the tropical Pacific Ocean, with rising air in the western and sinking air in the eastern Pacific.

=== Water-borne diseases ===
'''Working Groups:''' WGII

'''Definition:''' Illnesses transmitted through contact with, or consumption of, unsafe or contaminated water. (UNEP, 2018)

=== Water cycle ===
'''Working Groups:''' WGI

'''Definition:''' The cycle in which water evaporates from the ocean and the land surface, is carried over the Earth in atmospheric circulation as water vapour, condenses to form clouds, precipitates over the ocean and land as rain or snow, which on land can be intercepted by trees and vegetation, potentially accumulating as snow or ice, provides runoff on the land surface, infiltrates into soils, recharges groundwater, discharges into streams, and ultimately, flows into the oceans as rivers, polar glaciers and ice sheets, from which it will eventually evaporate again. The various systems involved in the hydrological cycle are usually referred to as hydrological systems.

=== Water mass ===
'''Working Groups:''' WGI

'''Definition:''' A body of ocean water with identifiable properties (temperature, salinity, density, chemical tracers) resulting from its unique formation process. Water masses are often identified through a vertical or horizontal extremum of a property such as salinity. North Pacific Intermediate Water (NPIW) and Antarctic Intermediate Water (AAIW) are examples of water masses.

=== Water security ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability (UN-Water, 2013).

=== Water-use efficiency ===
'''Working Groups:''' WGII

'''Definition:''' Carbon gain by photosynthesis per unit of water lost by evapotranspiration. It can be expressed on a short-term basis as the ratio of photosynthetic carbon gain per unit transpirational water loss, or on a seasonal basis as the ratio of net primary production or agricultural yield to the amount of water used.

=== Wave setup ===
'''Working Groups:''' WGI

'''Definition:''' Time-mean sea level elevation due to wave energy dissipation.

=== Weathering ===
'''Working Groups:''' WGI; WGII

'''Definition:''' The gradual removal of atmospheric 2) carbon dioxide (CO through dissolution of silicate and carbonate rocks. Weathering may involve physical processes (mechanical weathering) or chemical activity (chemical weathering).

=== Well-being ===
'''Working Groups:''' WGII; WGIII

'''Definition:''' A state of existence that fulfils various human needs, including material living conditions, meaningful social and community relationships and quality of life, as well as the ability to pursue one’s goals, to thrive, and feel satisfied with one’s life. Ecosystem well-being refers to the ability of ecosystems to maintain their diversity and quality.

=== Well-mixed greenhouse gas ===
'''Working Groups:''' WGI

'''Definition:''' A greenhouse gas (GHG) that has an atmospheric lifetime long enough (greater than several years) to be homogeneously mixed in the troposphere, and as such the global average mixing ratio can be determined from a network of surface observations. For many well-mixed greenhouse gases, measurements made in remote regions differ from the global mean by < 15%.

=== West African monsoon ===
'''Working Groups:''' WGI

'''Also known as:''' West African monsoon (WAfriM)

'''Definition:''' The West African monsoon (WAfriM) is a seasonal reversal in wind and precipitation whose domain includes Benin, Burkina-Faso, northern Cameroon, Cape Verde, northern Central African Republic, Chad, Gambia, Ghana, Guinea, Guinea Bissau, Ivory Coast, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone and Togo. The WAfriM is characterized by the northward progression from May to September of moist low-level south-westerlies from the Gulf of Guinea. In May and June, rainfall essentially remains along the Guinean coast with a maximum occurring near 5°N, followed by a sudden decrease of rainfall, marking the ‘short dry season‘ in the Guinean coast and the monsoon onset in the Sahel. Then rainfall continues to progress northward up to about 18–20°N, with a maximum near 12°N in late August/September, until it retreats starting from October towards the Guinean coast for a second maximum. Further details on how WAfriM is defined and used throughout the Report are provided in Annex V.

=== Wetland ===
'''Working Groups:''' WGI; WGII

'''Definition:''' Land that is covered or saturated by water for all or part of the year (e.g., peatland).

=== Wind energy ===
'''Working Groups:''' WGIII

'''Definition:''' Kinetic energy from airflow arising from the uneven heating of the Earth’s surface. The wind’s kinetic energy is converted to mechanical shaft energy and electricity by a wind turbine, a rotating machine. A wind farm, wind project, wind park, or wind power plant is a group of wind turbines interconnected to a common utility system through a system of transformers, distribution lines, and (usually) one substation.

== Y ==

=== Younger Dryas ===
'''Working Groups:''' WGI

'''Definition:''' The period from approximately 12.9 to 11.7 ka (thousand years before 1950), during the last deglacial transition, characterized by a temporary return to colder conditions in many locations, especially around the North Atlantic.

== Z ==

=== Zero emissions commitment ===
'''Working Groups:''' WGI; WGIII

'''Definition:''' The zero emissions commitment is an estimate of the subsequent global warming that would result after anthropogenic emissions are set to zero. It is determined by both inertia in physical climate system components (ocean, cryosphere, land surface) and carbon cycle inertia. In its widest sense it refers to emissions of each climate forcer including greenhouses gases, aerosols and their precursors. The climate response to this can be complex due to the different time scale of response of each climate forcer. A specific subcategory of zero emissions commitment is the Zero CO2 Emissions Commitment which refers to the climate system response to CO2 emissions after setting these to net zero. The CO2-only definition is of specific use in estimating remaining carbon budgets.

== D ==

=== Displacement ===
'''Working Groups:''' WGII

'''Definition:''' The involuntary movement, individually or collectively, of persons from their country or community, notably for reasons of armed conflict, civil unrest, or natural or human-made disasters (adapted from IOM, 2011).

== I ==

=== Impact assessment ===
'''Definition:''' The practice of identifying and evaluating, in monetary and/or non-monetary terms, the effects of climate change on natural and human systems.

== P ==

=== PH ===
'''Working Groups:''' WGI; WGII

'''Definition:''' A dimensionless measure of the acidity of a dilute solution (e.g., seawater) based on the activity, or effective concentration, of hydrogen ions (H +) in the solution. pH is measured on a logarithmic scale where pH = –log 10 (H +). Thus, a pH decrease of 1 unit corresponds to a 10-fold increase in the acidity, or the activity of H+.