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.
In the near term (2021–2040), a 1.5°C increase in the 20-year average of GSAT, relative to the average over the period 1850–1900, 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 about ten years earlier than the midpoint of the likely range (2030–2052) assessed in the SR1.5, which assumed continuation of the then-current warming rate; this rate has been confirmed in the AR6. Roughly half of the ten-year 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. 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.
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. Uncertainty in near term projections of annual GSAT arises in roughly equal measure from natural internal variability and model uncertainty. By contrast, near-term annual GSAT levels depend less on the scenario chosen, consistent with the 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
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-emission scenario SSP1-1.9 and by 2.4°C–4.8°C in the high emission 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. Emissions-driven simulations for SSP5-8.5 show that carbon-cycle uncertainty is too small to change the assessment of GSAT projections.
The CMIP6 models project a wider range of GSAT change than the assessed range; furthermore, the CMIP6 GSAT increase tends to be larger than in CMIP5. 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). In SSP1-2.6 and SSP2-4.5, ERF changes also explain about half of the changes in the range of warming. For SSP5-8.5, higher climate sensitivity is the primary reason behind the upper end of the warming being higher than in CMIP5.
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. 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%.
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. The warming pattern likely varies across seasons, with northern high latitudes warming more during boreal winter than summer. Regions with increasing or decreasing year-to-year variability of seasonal mean temperatures will likely increase in their spatial extent.
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 CO2 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.
Annual global land precipitation will increase over the 21st century as GSAT increases. The likely range of change in globally averaged annual land precipitation during 2081–2100 relative to 1995–2014 is –0.2–4.7% in the low-emission scenario SSP1-1.9 and 0.9–12.9% in the high-emission 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.
Precipitation change will exhibit substantial regional differences and seasonal contrast as GSAT increases over the 21st century. As warming increases, a larger land area will experience statistically significant increases or decreases in precipitation. 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.
Near-term projected changes in precipitation are uncertain, mainly because of natural internal variability, model uncertainty, and uncertainty in natural and anthropogenic aerosol forcing. In the near term, no discernible differences in precipitation changes are projected between different SSPs. The anthropogenic aerosol forcing decreases in most scenarios, contributing to increases in GSAT and global-mean land precipitation.
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. 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.
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.
Large-scale Circulation and Modes of Variability
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. 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.
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.
The CMIP6 multi-model ensemble projects a long-term increase in the boreal wintertime Northern Annular Mode index under the high-emission scenarios of SSP3-7.0 and SSP5-8.5, but regional changes may deviate from a simple shift in the mid-latitude 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 timescales. 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.
Near-term predictions and projections of the sub-polar branch of the Atlantic Multi-decadal Variability (AMV) on the decadal timescale have improved in CMP6 models compared to CMIP5. 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.
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 (SST) variability over the 21st century in any of the SSP scenarios assessed. 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.
Cryosphere and Ocean
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 km2 ) 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.
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. 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.
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 emission scenarios than under low emission scenarios. 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.
Climate Response to Emission Reduction, Carbon Dioxide Removal, and Solar Radiation Modification
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 SSP3-7.0 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. The mitigation benefits for these quantities would emerge only later during the 21st century. 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. Events of reduced and increased GSAT trends at decadal timescales will continue to occur in the 21st century but will not affect the centennial warming.
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 CO2 removed by CDR. The climate system response to net negative CO2 emissions is expected to be delayed by years to centuries. Net negative CO2 emissions due to CDR will not reverse some climate change, such as sea level rise, at least for several centuries. 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 CO2 emissions at the time of termination.
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 timescale, and there is low confidence in our understanding of the climate response to SRM, specifically at the regional scale. Since the 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-emission scenario such as SSP5-8.5 would cause a rapid climate change. However, a gradual phase-out of SRM combined with emissions reductions and CDR would more likely than not avoid larger rates of warming.
Climate Change Commitment and Change Beyond 2100
Earth system modelling experiments since AR5 confirm that the zero CO2 emissions commitment (the additional rise in GSAT after all CO2 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 CO2 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.
Overshooting specific global warming levels such as 2°C has effects on the climate system that persist beyond 2100. Under one scenario including a peak and decline in atmospheric CO2 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 CO2 peak. GMSL continues to rise in all models up to 2100 despite a reduction in CO2 to 2040 levels.
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 (2.3−4.6°C) 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 (6.6−14.1°C) overlaps with the range estimated for the Miocene Climatic Optimum (5°C−10°C) and Early Eocene Climatic Optimum (10°C−18°C), about 15 and 50 million years ago, respectively.