Editing IPCC:AR6/SYR/Longer-Report (section)

==== 3.1.1. Long-term Climate Change ====

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'''The uncertainty range on assessed''' '''future changes in global surface temperature is narrower than in the AR5.''' For the first time in an IPCC assessment cycle, multi-model projections of global surface temperature, ocean warming and sea level are constrained using observations and the assessed climate sensitivity. The ''likely'' range of equilibrium climate sensitivity has been narrowed to 2.5°C to 4.0°C (with a best estimate of 3.0°C) based on multiple lines of evidence '''[[#footnote-045|112]]''' , including improved understanding of cloud feedbacks. For related emissions scenarios, this leads to narrower uncertainty ranges for long-term projected global temperature change than in AR5. { ''WGI A.4, WGI Box SPM.1, WGI TS.3.2, WGI 4.3'' }

'''Future warming depends on future GHG emissions, with cumulative net CO''' '''2''' dominating. The assessed best estimates and ''very likely'' ranges of warming for 2081-2100 with respect to 1850–1900 vary from 1.4 [1.0 to 1.8]°C in the very low GHG emissions scenario (SSP1-1.9) to 2.7 [2.1 to 3.5]°C in the intermediate GHG emissions scenario (SSP2-4.5) and 4.4 [3.3 to 5.7]°C in the very high GHG emissions scenario (SSP5-8.5) '''[[#footnote-044|113]]''' . { ''WGI SPM B.1.1, WGI Table SPM.1, WGI Figure SPM.4'' } . ( ''Cross-Section Box.2 Figure 1'' )

'''Modelled pathways consistent with the continuation of policies implemented by the end of 2020 lead to global warming of 3.2 [2.2 to 3.5]°C (5–95% range) by 2100 (''' '''''medium confidence)''''' '''(see also Section 2.3.1).''' Pathways of &gt;4°C (≥50%) by 2100 would imply a reversal of current technology and/or mitigation policy trends ( ''medium confidence'' ). However, such warming could occur in emissions pathways consistent with policies implemented by the end of 2020 if climate sensitivity or carbon cycle feedbacks are higher than the best estimate ( ''high confidence'' ). { ''WGIII SPM C.1.3'' }

'''Global warming will continue to increase in the near term in nearly all considered scenarios and modelled pathways. Deep, rapid, and sustained GHG emissions reductions, reaching net zero CO''' '''2''' emissions and including strong emissions reductions of other GHGs, in particular CH '''4 , are necessary to limit warming to 1.5°C (&gt;50%) or less than 2°C (&gt;67%) by the end of century (''' '''''high confidence).''''' The best estimate of reaching 1.5°C of global warming lies in the first half of the 2030s in most of the considered scenarios and modelled pathways '''[[#footnote-043|114]]''' . In the very low GHG emissions scenario (SSP1-1.9), CO 2 emissions reach net zero around 2050 and the best-estimate end-of-century warming is 1.4°C, after a temporary overshoot (see Section 3.3.4) of no more than 0.1°C above 1.5°C global warming. Global warming of 2°C will be exceeded during the 21st century unless deep reductions in CO 2 and other GHG emissions occur in the coming decades. Deep, rapid, and sustained reductions in GHG emissions would lead to improvements in air quality within a few years, to reductions in trends of global surface temperature discernible after around 20 years, and over longer time periods for many other climate impact-drivers '''[[#footnote-042|115]]''' ( ''high confidence'' ). Targeted reductions of air pollutant emissions lead to more rapid improvements in air quality compared to reductions in GHG emissions only, but in the long term, further improvements are projected in scenarios that combine efforts to reduce air pollutants as well as GHG emissions ( ''high confidence'' ) '''[[#footnote-041|116]]''' . { ''WGI SPMB.1, WGI SPM B.1.3, WGI SPM D.1, WGI SPM D.2, WGI Figure SPM.4, WGI Table SPM.1,'' . ''WGI Cross-Section Box TS.1; WGIII SPM C.3, WGIII Table SPM.2, WGIII Figure SPM.5, WGIII Box SPM.1 Figure 1, WGIII Table 3.2'' } ( ''Table 3.1, Cross-Section Box.2 Figure 1'' )

'''Changes in short-lived climate forcers (SLCF) resulting from the five considered scenarios lead to an additional net global warming in the near and long term (''' '''''high confidence)''''' '''. Simultaneous stringent climate change mitigation and air pollution control policies limit this additional warming and lead to strong benefits for air quality (''' '''''high confidence)''''' '''.''' In high and very high GHG emissions scenarios (SSP3-7.0 and SSP5-8.5), combined changes in SLCF emissions, such as CH 4 , aerosol and ozone precursors, lead to a net global warming by 2100 of ''likely'' 0.4°C to 0.9°C relative to 2019. This is due to projected increases in atmospheric concentration of CH 4 , tropospheric ozone, hydrofluorocarbons and, when strong air pollution control is considered, reductions of cooling aerosols. In low and very low GHG emissions scenarios (SSP1-1.9 and SSP1-2.6), air pollution control policies, reductions in CH 4 and other ozone precursors lead to a net cooling, whereas reductions in anthropogenic cooling aerosols lead to a net warming ( ''high confidence'' ). Altogether, this causes a ''likely'' net warming of 0.0°C to 0.3°C due to SLCF changes in 2100 relative to 2019 and strong reductions in global surface ozone and particulate matter ( ''high confidence'' ). { ''WGI SPMD.1.7, WGI Box TS.7'' } . ( ''Cross-Section Box.2'' )

'''Continued GHG emissions will further affect all major climate system components, and many changes will be irreversible on centennial to millennial time scales.''' Many changes in the climate system become larger in direct relation to increasing global warming. With every additional increment of global warming, changes in extremes continue to become larger. Additional warming will lead to more frequent and intense marine heatwaves and is projected to further amplify permafrost thawing and loss of seasonal snow cover, glaciers, land ice and Arctic sea ice ( ''high confidence'' ). Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation '''[[#footnote-040|117]]''' , and very wet and very dry weather and climate events and seasons ( ''high confidence'' ). The portion of global land experiencing detectable changes in seasonal mean precipitation is projected to increase ( ''medium confidence'' ) with more variable precipitation and surface water flows over most land regions within seasons ( ''high confidence'' ).and from year to year ( ''medium confidence'' ). Many changes due to past and future GHG emissions are irreversible '''[[#footnote-039|118]]''' on centennial to millennial time scales, especially in the ocean, ice sheets and global sea level (see 3.1.3). Ocean acidification ( ''virtually certain'' ), ocean deoxygenation ( ''high confidence'' ).and global mean sea level ( ''virtually certain'' ).will continue to increase in the 21st century, at rates dependent on future emissions. { ''WGI SPM B.2, WGI SPM B.2.2, WGI SPM B.2.3, WGI SPM B.2.5, WGI SPM B.3, WGI SPM B.3.1,'' . ''WGI SPM B.3.2, WGI SPM B.4, WGI SPM B.5, WGI SPM B.5.1, WGI SPM B.5.3, WGI Figure SPM.8'' } . ( ''Figure 3.1'' )

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'''Figure 3.1: Projected changes of annual maximum daily''' '''temperature, annual mean total column soil moisture CMIP and annual maximum daily''' '''precipitation at''' '''global''' '''warming levels of 1.5°C, 2°C, 3°C, and 4°C relative to 1850-1900. Simulated (a)''' annual maximum temperature change (°C), '''(b)''' annual mean total column soil moisture (standard deviation), '''(c)''' annual maximum daily precipitation change (%). Changes correspond to CMIP6 multi-model median changes. In panels (b) and (c), large positive relative changes in dry regions may correspond to small absolute changes. In panel (b), the unit is the standard deviation of interannual variability in soil moisture during 1850-1900. Standard deviation is a widely used metric in characterising drought severity. A projected reduction in mean soil moisture by one standard deviation corresponds to soil moisture conditions typical of droughts that occurred about once every six years during 1850-1900. The WGI Interactive Atlas ( https://interactive-atlas.ipcc.ch/ ) can be used to explore additional changes in the climate system across the range of global warming levels presented in this figure. ''WGI Figure SPM.5, WGI Figure TS.5, WGI Figure 11.11, WGI Figure 11.16, WGI Figure 11.19'' ( ''Cross-Section Box.2'' )

[https://www.ipcc.ch/figures/figure-3-1 ]

'''With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact-drivers.''' Increases in hot and decreases in cold climatic impact-drivers, such as temperature extremes, are projected in all regions ( ''high confidence'' ). At 1.5°C global warming, heavy precipitation and flooding events are projected to intensify and become more frequent in most regions in Africa, Asia ( ''high confidence'' ), North America ( ''medium'' to ''high confidence'' ).and Europe. ( ''medium confidence'' ). At 2°C or above, these changes expand to more regions and/or become more significant ( ''high confidence'' ), and more frequent and/or severe agricultural and ecological droughts are projected in Europe, Africa, Australasia and North, Central and South America ( ''medium'' to ''high confidence'' ). Other projected regional changes include intensification of tropical cyclones and/or extratropical storms ( ''medium confidence'' ), and increases in aridity and fire weather '''[[#footnote-038|119]]''' ( ''medium'' to ''high confidence'' ). Compound heatwaves and droughts become ''likely'' more frequent, including concurrently at multiple locations ( ''high confidence'' ). { ''WGI SPMC.2, WGI SPM C.2.1, WGI SPM C.2.2, WGI SPM C.2.3, WGI SPM C.2.4, WGI SPM C.2.7'' }

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