Editing IPCC:AR6/WGI/Chapter-1 (section)

=== Data, Tools and Methods Used across the WGI Report ===

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'''Capabilities for observing the physical climate system have continued to improve and expand overall, but some reductions in observational capacity are also evident''' ( ''high confidence'' ''').''' Improvements are particularly evident in ocean observing networks and remote-sensing systems, and in paleoclimate reconstructions from proxy archives. However, some climate-relevant observations have been interrupted by the discontinuation of surface stations and radiosonde launches, and delays in the digitisation of records. Further reductions are expected to result from the COVID-19 pandemic. In addition, paleoclimate archives such as mid-latitude and tropical glaciers, as well as modern natural archives used for calibration (e.g., corals and trees), are rapidly disappearing due to a host of pressures, including increasing temperatures ( ''high confi'' ''dence'' ). {1.5.1}

'''Reanalyses have improved since AR5 and are increasingly used as a line of evidence in assessments of the state and evolution of the climate system''' ( ''high confidence'' ) '''.''' Reanalyses, where atmosphere or ocean forecast models are constrained by historical observational data to create a climate record of the past, provide consistency across multiple physical quantities and information about variables and locations that are not directly observed. Since AR5, new reanalyses have been developed with various combinations of increased resolution, extended records, more consistent data assimilation, estimation of uncertainty arising from the range of initial conditions, and an improved representation of the ocean. While noting their remaining limitations, the WGI report uses the most recent generation of reanalysis products alongside more standard observation-based datasets. {1.5.2, Annex 1}

'''Since AR5, new techniques have provided greater confidence in attributing changes in climate and weather extremes to climate change.''' Attribution is the process of evaluating the relative contributions of multiple causal factors to an observed change or event. This includes the attribution of the causal factors of changes in physical or biogeochemical weather or climate variables (e.g., temperature or atmospheric CO <sub>2</sub> ) as done in WGI, or of the impacts of these changes on natural and human systems (e.g., infrastructure damage or agricultural productivity), as done in WGII. Attributed causes include human activities (such as emissions of greenhouse gases and aerosols, or land-use change), and changes in other aspects of the climate, or natural or human systems. {Cross-Working Group Box 1.1}

'''The latest generation of complex climate models has an improved representation of physical processes, and a wider range of Earth system models now represent biogeochemical cycles. Since AR5, higher-resolution models that better capture smaller-scale processes and extreme events have become available.''' Key model intercomparisons supporting this Assessment include the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the Coordinated Regional Climate Downscaling Experiment (CORDEX), for global and regional models respectively. Results using CMIP Phase 5 (CMIP5) simulations are also assessed. Since AR5, large ensemble simulations, where individual models perform multiple simulations with the same climate forcings, are increasingly used to inform understanding of the relative roles of internal variability and forced change in the climate system, especially on regional scales. The broader availability of ensemble model simulations has contributed to better estimations of uncertainty in projections of future change ( ''high confidence'' ). A broad set of simplified climate models is assessed and used as emulators to transfer climate information across research communities, such as for evaluating impacts or mitigation pathways consistent with certain levels of future warming. {1.4.2, 1.5.3, 1.5.4, Cross-Chapter Box 7.1}

'''Assessments of future climate change are integrated within and across the three IPCC Working Groups through the use of three core components: scenarios, global warming levels, and the relationship between cumulative CO''' <sub>2</sub> '''emissions and global warming.''' Scenarios have a long history in the IPCC as a method for systematically examining possible futures. A new set of illustrative scenarios that cover the range of possible future developments of anthropogenic drivers of climate change found in the literature, derived from the Shared Socio-economic Pathways (SSPs), is used to synthesize knowledge across the physical sciences and impact, adaptation and mitigation research. The core set of SSP scenarios used in the WGI report, SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5, cover a broad range of emissions pathways, including new low-emissions pathways. They start in 2015 and include scenarios with high and very high greenhouse gas (GHG) emissions (SSP3-7.0 and SSP5-8.5) and CO <sub>2</sub> emissions that roughly double from current levels by 2100 and 2050, respectively; scenarios with intermediate GHG emissions (SSP2-4.5) and CO <sub>2</sub> emissions remaining around current levels until the middle of the century; and scenarios with very low and low GHG emissions and CO <sub>2</sub> emissions declining to net zero around or after 2050, followed by varying levels of net negative CO <sub>2</sub> emissions (SSP1-1.9, SSP1-2.6). Emissions vary between scenarios depending on socio-economic assumptions, levels of climate change mitigation and, for aerosols and non-methane ozone precursors, air pollution controls. Alternative assumptions may result in similar emissions and climate responses, but the socio-economic assumptions and the feasibility or likelihood of individual scenarios are not part of this assessment, which focuses on the climate response to possible, prescribed emissions futures. Levels of global surface temperature change (global warming levels), which are closely related to a range of hazards and regional climate impacts, also serve as reference points within and across IPCC Working Groups. Cumulative carbon emissions, which have a nearly linear relationship to increases in global surface temperature, are also used. {1.6.1–1.6.4, Cross-Chapter Box 1.5, Cross-Chapter Box 11.1}

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