Editing IPCC:AR6/WGI/Chapter-3 (section)

===== 3.3.1.2.2 Stratospheric temperature =====

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The AR5 concluded that the CMIP5 models simulated a generally realistic evolution of lower-stratospheric temperatures ([[#Bindoff--2013|Bindoff et al., 2013]] ; [[#Flato--2013|Flato et al., 2013]]), which was better than that of the CMIP3 models, in part because they generally include time-varying ozone concentrations, unlike many of the CMIP3 models. Nonetheless, it was noted that there was a tendency for the simulations to underestimate stratospheric cooling compared to observations. [[#Bindoff--2013|Bindoff et al. (2013)]] concluded that it was ''very likely'' that anthropogenic forcing, dominated by stratospheric ozone depletion by chemical reactions involving trace species known as ozone-depleting substances (ODS), had contributed to the cooling of the lower stratosphere since 1979. Increased greenhouse gases cause near-surface warming but cooling of stratospheric temperatures.

For the lower stratosphere, a debate has been ongoing since AR5 between studies finding that models underestimate the cooling of stratospheric temperature ([[#Santer--2017b|Santer et al., 2017b]]), in part because of underestimated stratospheric ozone depletion ([[#Eyring--2013|Eyring et al., 2013]] ; [[#Young--2013|Young et al., 2013]]), and studies finding that lower stratospheric temperature trends are within the range of observed trends ([[#Young--2013|Young et al., 2013]] ; [[#Maycock--2018|Maycock et al., 2018]]). Different observational data and different time periods explain the different conclusions. [[#Aquila--2016|Aquila et al. (2016)]] used forced chemistry-climate models with prescribed SST to investigate the influence of different forcings on global stratospheric temperature changes. They found that in the lower stratosphere, the simulated cooling trend due to increasing greenhouse gases was roughly constant over the satellite era, while changes in ODS concentrations amplified that stratospheric cooling trend during the era of increasing ozone depletion up until the mid-1990s, with a flattening of the temperature trend over the subsequent period over which stratospheric ozone has stabilized ([[IPCC:Wg1:Chapter:Chapter-2#2.2.5.2|Section 2.2.5.2]]). [[#Mitchell--2020|Mitchell et al. (2020)]] showed that while models simulate realistic trends in tropical lower-stratospheric temperature over the whole 1979–2014 period when compared with radiosonde data, they tend to overestimate the cooling trend over the ozone depletion era (1979–1997) and underestimate it over the ozone stabilization era (1998–2014; Figure 3.10b,c). They speculate that those disagreements are due to poor representations of stratospheric ozone forcing.

Upper stratospheric temperature changes were not assessed in the context of attribution or model evaluation in AR5, but this is an area where there has been considerable progress over recent years ([[IPCC:Wg1:Chapter:Chapter-2#2.3.1.2.1|Section 2.3.1.2.1]]). Simulated temperature changes in chemistry-climate models show good consistency with the reprocessed dataset from NOAA STAR but are less consistent with the revised UK Met Office record ([[#Karpechko--2018|Karpechko et al., 2018]]). The latter still shows stronger cooling than simulated in chemistry-climate models ([[#Maycock--2018|Maycock et al., 2018]]). Reanalyses, which assimilate AMSU and SSU datasets, indicate an upper-stratospheric cooling from 1979 to 2009 of about 3°C at 5 hPa and 4°C at 1 hPa that agrees well with the cooling in simulations with prescribed SST and using CMIP5 forcings ([[#Simmons--2014|Simmons et al., 2014]]). [[#Mitchell--2016|Mitchell (2016)]] used regularized optimal fingerprinting techniques to carry out an attribution analysis of annual mid- to upper-stratospheric temperature in response to external forcings. They found that anthropogenic forcing has caused a cooling of approximately 2°C–3°C in the upper stratosphere over the period of 1979–2015, with greenhouse gases contributing two thirds of this change and ozone depletion contributing one third. They found a large upper-stratospheric temperature change in response to volcanic forcing (0.4°C–0.6°C for Mount Pinatubo) but that change is still smaller than the lower-stratospheric signal. [[#Aquila--2016|Aquila et al. (2016)]] found that the cooling of the middle and upper stratosphere after 1979 is mainly due to changes in greenhouse gas concentrations. Volcanic eruptions and the solar cycle were found not to affect long-term stratospheric temperature trends but to have short-term influences.

In summary, based on the latest updates to satellite observations of stratospheric temperature, we assess that simulated and observed trends in global mean temperature through the depth of the stratosphere are more consistent than based on previous datasets, but some differences remain (''medium confidence''). Studies published since AR5 increase our confidence in the simulated stratospheric temperature response to greenhouse gas and ozone changes, and support an assessment that it is ''extremely likely'' that stratospheric ozone depletion due to ozone-depleting substances was the main driver of the cooling of the lower stratosphere between 1979 and the mid-1990s, as expected from physical understanding. Similarly, revised observations and new studies support an assessment that it is ''extremely likely'' that anthropogenic forcing, both from increases in greenhouse gas concentrations and depletion of stratospheric ozone due to ozone-depleting substances, was the main driver of upper-stratospheric cooling since 1979.

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