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

==== 4.4.1.1 Average Global Surface Air Temperature ====

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The AR5 assessed that it is ''likely'' that GSAT will increase in the range 0.3°C–0.7°C over the period 2016–2035 relative to 1986–2005 ( ''medium confidence'' ), and that there were not large differences in the GSAT change among different RCPs in this period ( [[#Kirtman--2013|Kirtman et al., 2013]] ). The AR5 further assessed that it is ''more likely than not'' that the mean GSAT for the period 2016–2035 will be more than 1°C above the mean for 1850–1900, and it is ''very unlikely'' that it will be more than 1.5°C above the 1850–1900 mean ( ''medium confidence'' ). It was shown that in the period 2016–2035, differences in GSAT across RCP scenarios for a single climate model are typically smaller than differences between climate models under a single RCP scenario, indicating that model structural uncertainty is larger than scenario uncertainty over that period ( [[#Hawkins--2009|Hawkins and Sutton, 2009]] ).

Near-term (2021–2040) GSAT changes relative to 1995–2014 exhibit only minor dependence on SSP scenario, consistent with AR5 (Table 4.5). Averaged over the twenty years of the near term and across all scenarios, GSAT is ''very likely'' to be higher than over 1995–2014 by 0.4°C–1.0°C (Table 4.5), with most of the uncertainty arising from that in ECS and TCR ( ''high confidence'' ) ( [[#4.3.4|Section 4.3.4]] ; e.g., [[#Lehner--2020|Lehner et al., 2020]] ). The assessed near-term warming is thus larger than in AR5 by 0.1°C to 0.2°C. This upward revision has the same magnitude as the ad-hoc downward adjustment to near-term projected GSAT change that was performed in AR5 ( [[#box-4.1|Box 4.1]] ; [[#Kirtman--2013|Kirtman et al., 2013]] ).

Averaged near-term GSAT is ''as likely as not'' at least 1.5°C higher than during 1850–1900, across the five SSP scenarios used here (Table 4.5 and [[#4.3.4|Section 4.3.4]] ). This much higher likelihood of near-term warming reaching 1.5°C than in AR5 arises both because surface warming has continued since AR5 (the period 1995–2014 was warmer by 0.16°C than 1986–2005; Cross-Chapter Box 2.3, Table 1), and because of methodological and dataset updates (the AR6 assessment of 1986–2005 GSAT change relative to 1850–1900 is 0.08°C higher than in the AR5; Cross-Chapter Box 2.3, Table 1).

For annual mean GSAT, uncertainty in near-term projections arises in roughly equal measure from internal variability and model uncertainty ( ''high confidence'' ) ( [[#box-4.1|Box 4.1]] ). Forecasts initialized from recent observations simulate GSAT changes for the period 2019–2028 relative to the recent past that are consistent with the assessed ''very likely'' range in annual mean GSAT ( ''high confidence'' ) (Box 4.1, Figure 1, and Table 4.5). Because annual mean GSAT shows a higher level of internal variability than the 20-year mean, individual years are expected to cross the 1.5°C earlier than the assessed GSAT does. For example, [[#Smith--2018|Smith et al. (2018)]] apply a multi-model decadal-forecast ensemble to assess the likelihood that global warming of 1.5°C higher than over 1850–1900 will be temporarily exceeded in the near future.

When we repeat the uncertainty quantification for GSAT as in [[#4.3.4|Section 4.3.4]] but with the corresponding higher level of internal variability for annual instead of 20-year averages added in quadrature, we can estimate the likelihood that an individual year would cross the GSAT 1.5°C threshold. By 2030, GSAT in any individual year could exceed 1.5°C relative to 1850–1900 with a likelihood between 40 and 60 percent, across the scenarios considered here ( ''medium confidence'' ).

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