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

=== 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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