Highlights

👀Scientists’ ability to observe the physical climate system has continued to improve and expand.

📈Since the last IPCC report, new techniques have provided greater confidence in attributing changes in extreme events to human-caused climate change.

🔬The latest generation of climate models is better at representing natural processes, and higher-resolution models that better capture smaller-scale processes and extreme events have become available.

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Framing and Context of the WGI Report

The WGI contribution to the IPCC Sixth Assessment Report (AR6) assesses new scientific evidence relevant for a world whose climate system is rapidly changing, overwhelmingly due to human influence. The five IPCC assessment cycles since 1990 have comprehensively and consistently laid out the rapidly accumulating evidence of a changing climate system, with the Fourth Assessment Report (AR4, 2007) being the first to conclude that warming of the climate system is unequivocal. Sustained changes have been documented in all major elements of the climate system, including the atmosphere, land, cryosphere, biosphere and ocean. Multiple lines of evidence indicate the unprecedented nature of recent large-scale climatic changes in context of all human history, and that they represent a millennial-scale commitment for the slow-responding elements of the climate system, resulting in continued worldwide loss of ice, increase in ocean heat content, sea level rise and deep ocean acidification.

Since the IPCC Fifth Assessment Report (AR5), the international policy context of IPCC reports has changed. The UN Framework Convention on Climate Change (UNFCCC, 1992) has the overarching objective of preventing ‘dangerous anthropogenic interference with the climate system’. Responding to that objective, the Paris Agreement (2015) established the long-term goals of ‘holding the increase in global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels’ and of achieving ‘a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century’. Parties to the Agreement have submitted Nationally Determined Contributions (NDCs) indicating their planned mitigation and adaptation strategies. However, the NDCs submitted as of 2020 are insufficient to reduce greenhouse gas emission enough to be consistent with trajectories limiting global warming to well below 2°C above pre-industrial levels.

This report provides information of potential relevance to the 2023 global stocktake. The 5-yearly stocktakes called for in the Paris Agreement will evaluate alignment among the Agreement’s long-term goals, its means of implementation and support, and evolving global efforts in climate change mitigation (efforts to limit climate change) and adaptation (efforts to adapt to changes that cannot be avoided). In this context, WGI assesses, among other topics, remaining cumulative carbon emission budgets for a range of global warming levels, effects of long-lived and short-lived climate forcers, projected changes in sea level and extreme events, and attribution to anthropogenic climate change.

Understanding of the fundamental features of the climate system is robust and well established. Scientists in the 19th-century identified the major natural factors influencing the climate system. They also hypothesized the potential for anthropogenic climate change due to carbon dioxide (CO2) emitted by fossil fuel combustion. The principal natural drivers of climate change, including changes in incoming solar radiation, volcanic activity, orbital cycles, and changes in global biogeochemical cycles, have been studied systematically since the early 20th century. Other major anthropogenic drivers, such as atmospheric aerosols (fine solid particles or liquid droplets), land-use change and non-CO2 greenhouse gases, were identified by the 1970s. Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact. Past projections of global surface temperature and the pattern of warming are broadly consistent with subsequent observations (limited evidence, high agreement), especially when accounting for the difference in radiative forcing scenarios used for making projections and the radiative forcings that actually occurred.

Global surface temperatures increased by about 0.1°C (likely range –0.1°C to +0.3°C) between the period around 1750 and the 1850–1900 period, with anthropogenic factors responsible for a warming of 0.0°C–0.2°C (likely range). This assessed change in temperature before 1850–1900 is not included in the AR6 assessment of global warming to date, to ensure consistency with previous IPCC assessment reports, and because of the lower confidence in the estimate. There was likely a net anthropogenic forcing of 0.0–0.3 Wm-2 in 1850–1900 relative to 1750, with radiative forcing from increases in atmospheric greenhouse gas concentrations being partially offset by anthropogenic aerosol emissions and land-use change. Net radiative forcing from solar and volcanic activity is estimated to be smaller than ±0.1 Wm-2 for the same period.

Natural climate variability can temporarily obscure or intensify anthropogenic climate change on decadal time scales, especially in regions with large internal interannual-to-decadal variability. At the current level of global warming, an observed signal of temperature change relative to the 1850–1900 baseline has emerged above the levels of background variability over virtually all land regions. Both the rate of long-term change and the amplitude of interannual (year-to-year) variability differ from global to regional to local scales, between regions and across climate variables, thus influencing when changes become apparent. Tropical regions have experienced less warming than most others, but also exhibit smaller interannual variations in temperature. Accordingly, the signal of change is more apparent in tropical regions than in regions with greater warming but larger interannual variations.

The AR6 has adopted a unified framework of climate risk, supported by an increased focus in WGI on low-likelihood, high-impact events. Systematic risk framing is intended to aid the formulation of effective responses to the challenges posed by current and future climatic changes and to better inform risk assessment and decision-making. AR6 also makes use of the ‘storylines’ approach, which contributes to building a robust and comprehensive picture of climate information, allows a more flexible consideration and communication of risk, and can explicitly address low-likelihood, high-impact events.

The construction of climate change information and communication of scientific understanding are influenced by the values of the producers, the users and their broader audiences. Scientific knowledge interacts with pre-existing conceptions of weather and climate, including values and beliefs stemming from ethnic or national identity, traditions, religion or lived relationships to land and sea. ​​Science has values of its own, including objectivity, openness and evidence-based thinking. Social values may guide certain choices made during the construction, assessment and communication of information.

Data, Tools and Methods Used across the WGI

Capabilities for observing the physical climate system have continued to improve and expand overall, but some reductions in observational capacity are also evident. 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 owing to a host of pressures, including increasing temperatures.

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 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.

Since AR5, new techniques have provided greater confidence in attributing changes in climate 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 CO2) 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.

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 the 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 the 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. 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.

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 carbon emissions and global warming. Scenarios have a long history in the IPCC as a method for systematically examining possible futures. A new set of scenarios, derived from the Shared Socio-economic Pathways (SSPs), is used to synthesize knowledge across the physical sciences, impact, and 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 emission pathways, including new low-emissions pathways. The feasibility or likelihood of individual scenarios is not part of this assessment, which focuses on the climate response to possible, prescribed emission 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.