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

=== Framing and Context of the WGI Report ===

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'''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 the context of all human history, and that these changes 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. {1.2.1, 1.3, Box 1.2, Appendix 1.A}

'''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|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 emissions enough to be consistent with trajectories limiting global warming to well below 2°C above pre-industrial levels ( ''high confiden'' ''ce'' ). {1.1, 1.2}

'''This report provides information of potential relevance to the 2023 global stocktake.''' The five-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 emissions budgets for a range of global warming levels, effects of long-lived and short-lived climate forcers, observed climate changes and their attribution to human forcing, and projected changes in sea level and climate extremes. {Cross-Chapter Box 1.1}

'''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 (CO <sub>2</sub> ) 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-CO <sub>2</sub> 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. {1.3.1–1.3.6}

'''Global surface temperatures increased by about 0.1°C''' ( ''likely'' '''range –0.1°C to +0.3°C,''' ''medium confidence'' ''') 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,''' ''medium confidence'' ''').''' 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 W m <sup>–2</sup> in 1850 '''–''' 1900 relative to 1750 ( ''medium confidence'' ), 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 W m <sup>–2</sup> for the same period. {Cross-Chapter Box 1.2, 1.4.1, Cross-Chapter Box 2.3}

'''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''' ( ''high confidence'' ''').''' Both the rate of long-term change and the amplitude of interannual (year-to-year) variability differ between global, regional and 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 ( ''high confidence'' ). {1.4.2, FAQ 1.2}

'''AR6 has adopted a unified framework of climate risk, supported by an increased focus in WGI on low-likelihood, high-impact outcomes.''' 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 for a more flexible consideration and communication of risk, and can explicitly address low-likelihood, high-impact outcomes. {1.1.2, 1.4.4, Cross-Chapter Box 1.3}

'''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 ( ''high confidence'' ). 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 ( ''high confidence'' ). {1.2.3, Box 1.1}

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