Editing IPCC:AR6/WGI/Chapter-4 (section)

== Executive Summary ==

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This chapter assesses simulations of future global climate change, spanning time horizons from the near term (2021–2040), mid-term (2041–2060), and long term (2081–2100) out to the year 2300. Changes are assessed relative to both the recent past (1995–2014) and the 1850–1900 approximation to the pre-industrial period.

'''The projections assessed here are mainly based on a new range of scenarios, the Shared Socio-economic Pathways (SSPs) used in the Coupled Model Intercomparison Project Phase 6 (CMIP6).''' Among the SSPs, the focus is on the five scenarios SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. In the SSP labels, the first number refers to the assumed shared socio-economic pathway, and the second refers to the approximate global effective radiative forcing (ERF) in 2100. Where appropriate, this chapter also assesses new results from CMIP5, which used scenarios based on Representative Concentration Pathways (RCPs). Additional lines of evidence enter the assessment, especially for change in globally averaged surface air temperature (GSAT) and global mean sea level (GMSL), while assessment for changes in other quantities is mainly based on CMIP6 results. Unless noted otherwise, the assessments assume that there will be no major volcanic eruption in the 21st century. {1.6, 4.2.2, 4.3.2, 4.3.4, 4.6.2, Box 4.1, Cross-Chapter Box 4.1, Cross-Chapter Box 7.1, 9.6}

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=== Temperature ===

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'''Assessed future change in GSAT is, for the first time in an IPCC report, explicitly constructed by combining scenario-based projections with observational constraints based on past simulated warming, as well as an updated assessment of equilibrium climate sensitivity (ECS) and transient climate response (TCR).''' Climate forecasts initialized using recent observations have also been used for the period 2019–2028. The inclusion of additional lines of evidence has reduced the assessed uncertainty ranges for each scenario. {4.3.1, 4.3.4, 4.4.1, 7.5}

'''In the near term (2021–2040), a 1.5°C increase in the 2''' '''0-yea''' '''r average of GSAT, relative to the average over the period 1''' '''850–190''' '''0, is''' ''very likely'' '''to occur in scenario SSP5-8.5,''' ''likely'' '''to occur in scenarios SSP2-4.5 and SSP3-7.0, and''' ''more likely than not'' '''to occur in scenarios SSP1-1.9 and SSP1-2.6.''' The threshold-crossing time is defined as the midpoint of the first 20-year period during which the average GSAT exceeds the threshold. In all scenarios assessed here except SSP5-8.5, the central estimate of crossing the 1.5°C threshold lies in the early 2030s. This is in the early part of the ''likely'' range (2030 '''–''' 2052) assessed in the IPCC Special Report on Global Warming of 1.5°C (SR1.5), which assumed continuation of the then-current warming rate; this rate has been confirmed in the AR6. Roughly half of this difference between assessed crossing times arises from a larger historical warming diagnosed in AR6. The other half arises because for central estimates of climate sensitivity, most scenarios show stronger warming over the near term than was assessed as ‘current’ in SR1.5 ( ''medium confidence'' ). When considering scenarios similar to SSP1-1.9 instead of linear extrapolation, the SR1.5 estimate of when 1.5°C global warming is crossed is close to the central estimate reported here. It is ''more likely than not'' that under SSP1-1.9, GSAT relative to 1850–1900 will remain below 1.6°C throughout the 21st century, implying a potential temporary overshoot of 1.5°C global warming of no more than 0.1°C. If climate sensitivity lies near the lower end of the assessed ''very likely'' range, crossing the 1.5°C warming threshold is avoided in scenarios SSP1-1.9 and SSP1-2.6 ( ''medium confidence'' ). {2.3.1, Cross-Chapter Box 2.3, 3.3.1, 4.3.4, Box 4.1, 7.5}

'''By 2030, GSAT in any individual year could exceed 1.5°C relative to 1850–1900 with a likelihood between 40% and 60%, across the scenarios considered here''' ( ''medium confidence'' ''').''' Uncertainty in near-term projections of annual GSAT arises in roughly equal measure from natural internal variability and model uncertainty ( ''high confidence'' ). By contrast, near-term annual GSAT levels depend less on the scenario chosen, consistent with the IPCC Fifth Assessment Report (AR5) assessment. Forecasts initialized from recent observations simulate annual GSAT changes for the period 2019–2028 relative to the recent past that are consistent with the assessed ''very likely'' range ( ''high confidence'' ). {4.4.1, Box 4.1}

'''Compared to the recent past (1995–2014), GSAT averaged over the period 2081–2100 is''' ''very likely'' '''to be higher by 0.2°C–1.0°C in the low-emissions scenario SSP1-1.9 and by 2.4°C–4.8°C in the high-emissions scenario SSP5-8.5.''' For the scenarios SSP1-2.6, SSP2-4.5, and SSP3-7.0, the corresponding ''very'' ''likely'' ranges are 0.5°C–1.5°C, 1.2°C–2.6°C, and 2.0°C–3.7°C, respectively. The uncertainty ranges for the period 2081–2100 continue to be dominated by the uncertainty in ECS and TCR ( ''very high confidence'' ). Emissions-driven simulations for SSP5-8.5 show that carbon-cycle uncertainty is too small to change the assessment of GSAT projections ( ''high confidence'' ). {4.3.1, 4.3.4, 4.6.2, 7.5}

'''The CMIP6 models project a wider range of GSAT change than the assessed range''' ( ''high confidence'' '''); furthermore, the CMIP6 GSAT increase tends to be larger than in CMIP5''' ( ''very high confidence'' ''').''' About half of the increase in simulated warming has occurred because higher climate sensitivity is more prevalent in CMIP6 than in CMIP5; the other half arises from higher ERF in nominally comparable scenarios (e.g., RCP8.5 and SSP5-8.5; ''medium confidence'' ). In SSP1-2.6 and SSP2-4.5, ERF changes also explain about half of the changes in the range of warming ( ''medium confidence'' ). For SSP5-8.5, higher climate sensitivity is the primary reason behind the upper end of the warming being higher than in CMIP5 ( ''medium confidence'' ). {4.3.1, 4.3.4, 4.6.2, 7.5.6}

'''While high-warming storylines – those associated with GSAT levels above the upper bound of the assessed''' ''very likely'' '''range – are by definition''' ''extremely unlikely'' ''', they cannot be ruled out. For SSP1-2.6, such a high-warming storyline implies long-term (2081–2100) warming well above, rather than well below, 2°C''' ( ''high confidence'' ''').''' Irrespective of scenario, high-warming storylines imply changes in many aspects of the climate system that exceed the patterns associated with the central estimate of GSAT changes by up to more than 50% ( ''high confidence'' ). {4.3.4, 4.8}

'''It is''' ''virtually certain'' '''that the average surface warming will continue to be higher over land than over the ocean and that the surface warming in the Arctic will continue to be more pronounced than the global average over the 21st century.''' On average, the surface is expected to warm faster over land than over the ocean by a factor of 1.5 ( ''likely'' range 1.4 to 1.7). The warming pattern ''likely'' varies across seasons, with northern high latitudes warming more during boreal winter than summer ( ''medium confidence'' ). Regions with increasing or decreasing year-to-year variability of seasonal mean temperatures will ''likely'' increase in their spatial extent. {4.3.1, 4.5.1, 7.4.4}

'''It is''' ''very likely'' '''that long-term lower-tropospheric warming will be larger in the Arctic than in the global mean.''' It is ''very likely'' that global mean stratospheric cooling will be larger by the end of the 21st century in a pathway with higher atmospheric CO <sub>2</sub> concentrations. It is ''likely'' that tropical upper tropospheric warming will be larger than at the tropical surface, but with an uncertain magnitude owing to the effects of natural internal variability and uncertainty in the response of the climate system to anthropogenic forcing. {4.5.1, 3.3.1.2}

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=== Precipitation ===

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'''Annual global land precipitation will increase over the 21st century as GSAT increases''' ( ''high confidence'' '''). The''' ''likely'' '''range of change in globally averaged annual land precipitation during 2081–2100 relative to 1995''' '''–2014 is –0.2 to +4.7% in the low-emissions scenario SSP1-1.9 and 0.9–12.9% in the high-emissions scenario SSP5-8.5, based on all available CMIP6 models.''' The corresponding ''likely'' ranges are 0.0–6.6% in SSP1-2.6, 1.5–8.3% in SSP2-4.5, and 0.5–9.6% in SSP3-7.0. {4.3.1, 4.5.1, 4.6.1, 8.4.1}

'''Precipitation change will exhibit substantial regional differences and seasonal contrast as GSAT increases over the 21st century''' ( ''high confidence'' ''').''' As warming increases, a larger land area will experience statistically significant increases or decreases in precipitation ( ''medium confidence'' ). Precipitation will ''very likely'' increase over high latitudes and the tropical oceans, and ''likely'' increase in large parts of the monsoon region, but ''likely'' decrease over large parts of the subtropics in response to greenhouse gas-induced warming. Interannual variability of precipitation over many land regions will increase with global warming ( ''medium confidence'' ). {4.5.1, 4.6.1, 8.4.1}

'''Near-term projected changes in precipitation are uncertain, mainly because of natural internal variability, model uncertainty, and uncertainty in natural and anthropogenic aerosol forcing''' ( ''medium confidence'' ''').''' In the near term, no discernible differences in precipitation changes are projected between different SSPs ( ''high confidence'' ). The anthropogenic aerosol forcing decreases in most scenarios, contributing to increases in GSAT ( ''medium confidence'' ) and global mean land precipitation ( ''low confidence'' ). {4.3.1, 4.4.1, 4.4.4, 8.5}

'''In response to greenhouse gas-induced warming, it is''' ''likely'' '''that global land monsoon precipitation will increase, particularly in the Northern Hemisphere, although Northern Hemisphere monsoon circulation will''' ''likely'' '''weaken.''' In the long term (2081–2100), monsoon rainfall change will feature a north–south asymmetry characterized by a greater increase in the Northern Hemisphere than in the Southern Hemisphere and an east–west asymmetry characterized by an increase in Asian-African monsoon regions and a decrease in the North American monsoon region ( ''medium confidence'' ). Near-term changes in global monsoon precipitation and circulation are uncertain due to model uncertainty and internal variability such as Atlantic Multi-decadal Variability and Pacific Decadal Variability ( ''medium confidence'' ). {4.4.1, 4.5.1, 8.4.1, 10.6.3}

'''It is''' ''likely'' '''that at least one large volcanic eruption will occur during the 21st century. Such an eruption would reduce GSAT for several years, decrease global mean land precipitation, alter monsoon circulation, modify extreme precipitation, and change the profile of many regional climatic impact-drivers.''' A low-likelihood, high-impact outcome would be several large eruptions that would greatly alter the 21st century climate trajectory compared to SSP-based Earth system model projections. {Cross-Chapter Box 4.1}

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=== Large-scale Circulation and Modes of Variability ===

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'''In the near term, the forced change in Southern Annular Mode in austral summer is''' ''likely'' '''to be weaker than observed during the late 20th century under all five SSPs assessed.''' This is because of the opposing influence in the near- to mid-term from stratospheric ozone recovery and increases in other greenhouse gases on the Southern Hemisphere summertime mid-latitude circulation ( ''high confidence'' ). In the near term, forced changes in the Southern Annular Mode in austral summer are therefore ''likely'' to be smaller than changes due to natural internal variability. {4.3.3, 4.4.3}

'''In the long term, the Southern Hemisphere mid-latitude jet is''' ''likely'' '''to shift poleward and strengthen under SSP5-8.5 relative to 1995–2014.''' This is ''likely'' to be accompanied by an increase in the Southern Annular Mode index in all seasons relative to 1995–2014. For SSP1-2.6, CMIP6 models project no robust change in the Southern Annular Mode index in the long term. It is ''likely'' that wind speeds associated with extratropical cyclones will strengthen in the Southern Hemisphere storm track for SSP5-8.5. {4.5.1, 4.5.3}

'''The CMIP6 multi-model ensemble projects a long-term increase in the boreal wintertime Northern Annular Mode index under the high-emissions scenarios of SSP3-7.0 and''' '''SSP5-8.5''' ''', but regional changes may deviate from a simple shift in the m''' '''id-latit''' '''ude circulation.''' Substantial uncertainty and thus ''low confidence'' remain in projecting regional changes in Northern Hemisphere jet streams and storm tracks, especially for the North Atlantic basin in winter; this is due to large natural internal variability, the competing effects of projected upper- and lower-tropospheric temperature gradient changes, and new evidence of weaknesses in simulating past variations in North Atlantic atmospheric circulation on seasonal-to-decadal time scales. One exception is the expected decrease in frequency of atmospheric blocking events over Greenland and the North Pacific in boreal winter in SSP3-7.0 and SSP5-8.5 scenarios ( ''medium confidence'' ). {4.5.1}

'''Near-term predictions and projections of the sub-polar branch of the Atlantic Multi-decadal Variability (AMV) on the decadal time scale have improved in CMP6 models compared to CMIP5''' ( ''high confidence'' ''').''' This is ''likely'' to be related to a more accurate response to natural forcing in CMIP6 models. Initialization contributes to the reduction of uncertainty and to predicting subpolar sea surface temperature. AMV influences on the nearby regions can be predicted over lead times of 5–8 years ( ''medium'' ''confidence'' ). {4.4.3}

'''It is''' ''virtually certain'' '''that the El Niño–Southern Oscillation (ENSO) will remain the dominant mode of interannual variability in a warmer world.''' There is no model consensus for a systematic change in intensity of ENSO sea surface temperature variability over the 21st century in any of the SSP scenarios assessed ( ''medium confidence'' ). However, it is ''very'' ''likely'' that ENSO rainfall variability, used for defining extreme El Niños and La Niñas, will increase significantly, regardless of amplitude changes in ENSO SST variability, by the second half of the 21st century in scenarios SSP2-4.5, SSP3-7.0, and SSP5-8.5. {4.3.3, 4.5.3, 8.4.2}

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=== Cryosphere and Ocean ===

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'''Under the SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios, it is''' ''likely'' '''that the Arctic Ocean in September, the month of annual minimum sea ice area, will become practically ice-free (sea ice area less than 1 million km''' <sup>2</sup> ''') averaged over 2081–2100 and all available simulations.''' Arctic sea ice area in March, the month of annual maximum sea ice area, also decreases in the future under each of the considered scenarios, but to a much lesser degree (in percentage terms) than in September ( ''high confidence'' ). {4.3.2}

'''Under the five scenarios assessed, it is''' ''virtually certain'' '''that global mean sea level (GMSL) will continue to rise through the 21st century.''' For the period 2081–2100 relative to 1995–2014, GMSL is ''likely'' to rise by 0.46–0.74 m under SSP3-7.0 and by 0.30–0.54 m under SSP1-2.6 ( ''medium confidence'' ). For the assessment of change in GMSL, the contribution from land-ice melt has been added offline to the CMIP6-simulated contributions from thermal expansion. {4.3.2. 9.6}

'''It is''' ''very likely'' '''that the cumulative uptake of carbon by the ocean and by land will increase through to the end of the 21st century.''' Carbon uptake by land shows greater increases but with greater uncertainties than for ocean carbon uptake. The fraction of emissions absorbed by land and ocean sinks will be smaller under high emissions scenarios than under low emissions scenarios ( ''high confidence'' ). Ocean surface pH will decrease steadily through the 21st century, except for SSP1-1.9 and SSP1-2.6 where values decrease until around 2070 and then increase slightly to 2100 ( ''high confidence'' ). {4.3.2, 5.4}

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=== Climate Response to Emissions Reduction, Carbon Dioxide Removal and Solar Radiation Modification ===

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'''If strong mitigation is applied from 2020 onward as reflected in SSP1-1.9, its effect on 20-year trends in GSAT would''' ''likely'' '''emerge during the near term (2021–2040), measured against an assumed non-mitigation scenario such as S''' '''SP3-7.''' '''0 and SSP5-8.5. However, the response of many other climate quantities to mitigation would be largely masked by internal variability during the near term, especially on the regional scale''' ( ''high confidence'' ''').''' The mitigation benefits for these quantities would emerge only later during the 21st century ( ''high confidence'' ). During the near term, a small fraction of the surface can show cooling under all scenarios assessed here, so near-term cooling at any given location is fully consistent with GSAT increase ( ''high confidence'' ). Events of reduced and increased GSAT trends at decadal time scales will continue to occur in the 21st century but will not affect the centennial warming ( ''very high confidence'' ). {4.6.3, Cross-Chapter Box 3.1}

'''Because of the near-linear relationship between cumulative carbon emissions and GSAT change, the cooling or avoided warming from carbon dioxide removal (CDR) is proportional to the cumulative amount of CO''' <sub>2</sub> '''removed by CDR''' ( ''high confidence'' ''').''' The climate system response to net negative CO <sub>2</sub> emissions is expected to be delayed by years to centuries. Net negative CO <sub>2</sub> emissions due to CDR will not reverse some climate change, such as sea level rise, at least for several centuries ( ''high confidenc'' e). The climate effect of a sudden and sustained CDR termination would depend on the amount of CDR-induced cooling prior to termination and the rate of background CO <sub>2</sub> emissions at the time of termination ( ''high confidence'' ). {4.6.3, 5.5, 5.6}

'''Solar radiation modification (SRM) could offset some of the effects of anthropogenic warming on global and regional climate, but there would be substantial residual and overcompensating climate change at the regional scale and seasonal time scale''' ( ''high confidence'' '''), and''' '''there is''' ''low confidence'' '''in our understanding of the climate response to SRM, specifically at the regional scale.''' Since AR5, understanding of the global and regional climate response to SRM has improved, due to modelling work with more sophisticated treatment of aerosol-based SRM options and stratospheric processes. Improved modelling suggests that multiple climate goals could be met simultaneously. A sudden and sustained termination of SRM in a high-emissions scenario such as SSP5-8.5 would cause a rapid climate change ( ''high confidence'' ). However, a gradual phase-out of SRM combined with emissions reductions and CDR would ''more'' ''likely than not'' avoid larger rates of warming '''.''' {4.6.3}

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=== Climate Change Commitment and Change Beyond 2100 ===

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'''Earth system modelling experiments since AR5 confirm that the zero CO''' <sub>2</sub> '''emissions commitment (the additional rise in GSAT after all CO''' <sub>2</sub> '''emissions cease) is small''' ( ''likely'' '''less than 0.3°C in magnitude) on decadal time scales, but that it may be positive or negative.''' There is ''low'' ''confidence'' in the sign of the zero CO <sub>2</sub> emissions commitment. Consistent with SR1.5, the central estimate is taken as zero for assessments of remaining carbon budgets for global warming levels of 1.5°C or 2°C. {4.7.2, 5.5.2} .

'''Overshooting specific global warming levels such as 2°C has effects on the climate system that persist beyond 2100''' ( ''medium confidence'' ''').''' Under one scenario including a peak and decline in atmospheric CO <sub>2</sub> concentration (SSP5-3.4-OS), some climate metrics such as GSAT begin to decline but do not fully reverse by 2100 to levels prior to the CO <sub>2</sub> peak ( ''medium confidence'' ). GMSL continues to rise in all models up to 2100 despite a reduction in CO <sub>2</sub> to 2040 levels. {4.6.3, 4.7.1, 4.7.2}

'''Using extended scenarios beyond 2100, projections show''' ''likely'' '''warming by 2300, relative to 1850-1900, of 1.0°C-2.2°C for SSP1-2.6 and 6.6°C-14.1°C for SSP5-8.5.''' By 2300, warming under the SSP5-3.4-OS overshoot scenario decreases from a peak around year 2060 to a level very similar to SSP1-2.6. Precipitation over land continues to increase strongly under SSP5-8.5. GSAT projected for the end of the 23rd century under SSP2-4.5 ( ''likely'' 2.3°C–4.6°C higher than over the period 1850–1900) has not been experienced since the mid-Pliocene, about 3 million years ago. GSAT projected for the end of the 23rd century under SSP5-8.5 ( ''likely'' 6.6°C–14.1°C higher than over the period 1850–1900) overlaps with the range estimated for the Miocene Climatic Optimum (5°C–10°C higher) and Early Eocene Climatic Optimum (10°C–18°C higher), about 15 and 50 million years ago, respectively ( ''medium confidence'' ). {2.3.1.1, 4.7.1}

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