IPCC AR6 Glossary

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

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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

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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

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Working Groups: WGII

Also known as: Access (to food)

Definition: See Access under Food Security

Access to modern energy services

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Working Groups: WGIII

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

Acclimatisation

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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

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Anthropocene

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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

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Working Groups: WGI; WGII; WGIII

Definition: Resulting from or produced by human activities.

Anthropogenic emissions

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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

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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.]

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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

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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

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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.

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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

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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

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Working Groups: WGI; WGIII

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

Baseline/reference

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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

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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

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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

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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).

C

CMIP3, CMIP5 and CMIP6

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Definition: Coupled Model Intercomparison Project Phase 3, Phase 5 and Phase 6.

CO2 equivalent emission

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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

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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

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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

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Working Groups: WGI

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

Capacity building

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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

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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.

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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

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Working Groups: WGIII

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

Chronology

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Working Groups: WGI

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

Circular economy

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

Climate resilient development pathways

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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

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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

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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

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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).

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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

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Working Groups: WGI

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

Cloud-resolving models

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Conference of the Parties

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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

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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

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Working Groups: WGI; WGIII

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

D

Dansgaard-Oeschger events

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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

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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

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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

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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).

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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

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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

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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

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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.

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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

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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

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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.

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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

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Working Groups: WGII

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

Driver

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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

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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

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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.

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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

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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

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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

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Working Groups: WGI

Definition: See Climate feedback.

Earth system model

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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.

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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

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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

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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

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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.

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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

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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.

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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

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Working Groups: WGII

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

Energy access

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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).

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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

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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

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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

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Extratropical jets

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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

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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

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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

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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

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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

Fugitive emissions

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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

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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

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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

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Global mean surface air temperature

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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

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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

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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

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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

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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

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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.

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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

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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

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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

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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

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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.]

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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

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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

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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

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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

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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

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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

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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

Hydroclimate

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Working Groups: WGI

Definition: Part of the climate pertaining to the hydrology of a region.

Hydrofluorocarbons

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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

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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

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Working Groups: WGI; WGII

Definition: A period with large runoff and water deficits in rivers, lakes and reservoirs.

Hydrological sensitivity

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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

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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

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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

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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.

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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

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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.

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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

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Working Groups: WGIII

Definition: Grasslands on which human interventions are carried out, such as grazing domestic livestock or hay removal.

Managed land

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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

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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

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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.

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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

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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

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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

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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

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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

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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).

Montreal Protocol

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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

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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

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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

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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

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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

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Working Groups: WGI; WGII

Definition: The dynamic physical, physicochemical and biological components of the Earth system that would operate independently of human activities.

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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

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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

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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

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Non-CO2 emissions and radiative forcing

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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

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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

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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

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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

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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.

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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.

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Working Groups: WGII

Definition: A worldwide outbreak of a disease in humans in numbers clearly in excess of normal (WHO, 2020).

Pareto optimum

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Working Groups: WGIII

Definition: A state in which no one’s welfare can be increased without reducing someone one’s welfare. Wikipedia

Participatory governance

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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

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

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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

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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

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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

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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

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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

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Probability density function

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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

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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.

Regional sea level change

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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.

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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.

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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.

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Working Groups: WGIII

Definition: A system which allows people to share goods and services by enabling collaborative use, access or ownership.

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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

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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

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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

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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

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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.

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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.

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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

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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

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Working Groups: WGII; WGIII

Definition: Where technological change is associated with social systems and the two are inextricably linked.

Soil carbon sequestration

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Solar energy

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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

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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

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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

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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

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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).

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

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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

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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

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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

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Sufficiency

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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

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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

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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

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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

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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

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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

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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).

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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

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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

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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

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Working Groups: WGI

Definition: Vertical displacement up the shore-face induced by individual waves.

Sympagic

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Working Groups: WGII

Definition: Organisms and habitats related to the sea ice, analogous to pelagic (water column) or benthic (seafloor).

Systems of Innovation

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Working Groups: WGIII

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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

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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

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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

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Working Groups: WGI

Definition: Uncertainty arising from data fitting to a time-series with potential non-linear and autorogressive character.

Tropical Atlantic Variability

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Urbanisation

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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)

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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

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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.

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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

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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

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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

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Working Groups: WGI; WGII

Definition: Land that is covered or saturated by water for all or part of the year (e.g., peatland).

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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+.

IPCC:AR6/Glossary

IPCC AR6 Glossary

1

1.5°C pathway

1.5°C warmer worlds

13C

14C

2

2030 Agenda for Sustainable Development

A

Ablation

Abrupt change

Abrupt climate change

Acceptability of policy or system change

Access

Access to modern energy services

Acclimatisation

Accumulation

Active layer

Acute food insecurity

Adaptation

Adaptation Fund

Adaptation behaviour

Adaptation deficit

Adaptation gap

Adaptation limits

Adaptation needs

Adaptation opportunity

Adaptation options

Adaptation pathways

Adaptive capacity

Adaptive governance

Adaptive management

Added value

Additionality

Adjustments

Advection

Adverse side-effect

Aerosol

Aerosol–cloud interaction

Aerosol effective radiative forcing

Aerosol optical depth

Aerosol–radiation interaction

Afforestation

Agreement

Agricultural and ecological drought

Agriculture, Forestry and Other Land Use

Agroecology

Agroforestry

Air mass

Air pollution

Airborne fraction

Albedo

Alkalinity

Altimetry

Annular modes

Anomaly

Antarctic Ice Sheet

Anthropocene

Anthropogenic

Anthropogenic emissions

Anthropogenic removals

Anthropogenic subsidence

Apparent hydrological sensitivity

Arctic oscillation

Arid zone

Aridity

Artificial ocean upwelling

Assets

Atlantic Meridional Mode

Atlantic Meridional Overturning Circulation

Atlantic Multi-decadal Oscillation

Atlantic Multi-decadal Variability

Atlantic Zonal Mode

Atmosphere

Atmospheric boundary layer

Atmospheric rivers

Attribution

Australian and Maritime Continent monsoon

Autonomous adaptation