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

==== 4.5.1.2 Annual Mean Atmospheric Temperature ====

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Section 12.4.3.2 of AR5 assessed that there is ''high confidence'' in the overall pattern of projected end of 21st century tropospheric temperature change and that it is ''very likely'' that some of the largest warming will occur in the northern high latitudes. They further assessed that proportionately larger warming is ''likely'' to occur in the tropical upper troposphere than at the tropical surface, but with ''medium confidence'' owing to the relatively large observational uncertainties and contradictory analyses regarding model accuracy in simulating tropical upper tropospheric temperature trends.

CMIP6 projections show warming throughout the troposphere by the end of this century and a mix of warming and cooling in the stratosphere depending on the emissions scenario (Figure 4.22). The patterns of tropospheric temperature change are highly consistent with those derived from earlier generations of climate models as assessed in AR5, AR4 and TAR. In SSP1-2.6, the multi-model mean warming remains below 3°C everywhere in the troposphere except near the surface in the Arctic; this is similar to the findings in AR5 based on CMIP5 models for RCP2.6. In SSP3-7.0, the zonal mean tropospheric warming is also largest in the tropical upper troposphere, reaching more than 5°C, and near the surface in the Arctic where warming exceeds 8°C (Figure 4.22). It is ''likely'' that the warmer projected GSAT in the unconstrained CMIP6 model ensemble contributes to larger warming in the tropical upper troposphere and in the Arctic lower troposphere. This assessment is based on the understanding of polar amplification assessed in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] (Section 7.4.4.1), and at low latitudes is based on the understanding of moist convective processes as well as the relationship between CMIP5- and CMIP6-simulated surface temperatures and tropical upper tropospheric warming over the historical period ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.1.2|Section 3.3.1.2]] ).

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[[File:278b2fb7f99af11bea733043c47c0ce7 IPCC_AR6_WGI_Figure_4_22.png]]

'''Figure 4.22''' '''|''' '''Long-term change of annual and zonal mean atmospheric temperature.''' Displayed are multi-model mean change in annual and zonal mean atmospheric temperature (°C) in 2081–2100 relative to 1995–2014 for '''(left)''' SSP1-2.6 and '''(right)''' SSP3-7.0. The number of models used is indicated in the top right of the maps. Diagonal lines indicate regions where less than 80% of the models agree on the sign of the change and no overlay where 80% or more of the models agree on the sign of the change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Projected stratospheric temperature trends are determined by a balance between the major radiative drivers from ozone recovery, rising CO <sub>2</sub> and other greenhouse gases (including stratospheric water vapour) ( [[#Maycock--2016|Maycock, 2016]] ), as well as future changes in the Brewer –Dobson circulation, which can alter the latitudinal pattern of stratospheric temperature trends ( [[#Fu--2015|Fu et al., 2015]] , 2019). In the lower stratosphere, the CMIP6 models project a weak cooling in the inner tropics in SSP1-2.6 and a warming at other latitudes (Figure 4.22). There is enhanced lower stratospheric warming over the Antarctic pole owing to the effects of ozone hole recovery on polar temperatures ( [[#Maycock--2016|Maycock, 2016]] ; [[#Solomon--2017|Solomon et al., 2017]] ). The projected strengthening of the Brewer –Dobson circulation in the future ( [[#Hardiman--2014|Hardiman et al., 2014]] ) also affects stratospheric temperature trends, with adiabatic cooling at low latitudes and warming in middle and high latitudes ( [[#Fu--2015|Fu et al., 2015]] , 2019). In SSP3-7.0, there is widespread cooling across much of the stratosphere, as expected from the higher GHG emissions, with a smaller warming in the Antarctic lower stratosphere. Owing to the importance of ozone recovery for the radiative balance of the stratosphere, future global and local stratospheric temperature trends do not scale with projected GSAT change.

In summary, new results since AR5 do not generally alter the understanding of projected zonal mean atmospheric temperature changes. There is ''high confidence'' in the overall pattern of projected tropospheric temperature changes given its robustness across many generations of climate models. It is further ''very likely'' that projected long-term tropospheric warming will be larger than the global mean in the Arctic lower troposphere. It is ''likely'' that tropical upper tropospheric warming will be larger than at the tropical surface, however with an uncertain magnitude owing to the potentially large role of natural internal variability and differences across models in the simulated free tropospheric temperature response to a given forcing scenario ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.1.2|Section 3.3.1.2]] ). 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.

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