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

==== 3.3.1.2 Upper-air Temperature ====

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([[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] assessed that the troposphere has warmed since at least the 1950s, that it is ''virtually certain'' that the stratosphere has cooled, and that there is ''medium confidence'' that the upper troposphere in the tropics has warmed faster than the near-surface since at least 2001 ([[IPCC:Wg1:Chapter:Chapter-2#2.3.1.2|Section 2.3.1.2]]). The AR5 assessed that anthropogenic forcings, dominated by greenhouse gases, ''likely'' contributed to the warming of the troposphere since 1961 and that anthropogenic forcings, dominated by the depletion of the ozone layer due to ozone-depleting substances, ''very likely'' contributed to the cooling of the lower stratosphere since 1979. Since AR5, understanding of observational uncertainties in the radiosonde and satellite data has improved with more available data and longer coverage, and differences between models and observations in the tropical atmosphere have been investigated further.

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===== 3.3.1.2.1 Tropospheric temperature =====

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The AR5 assessed with ''low confidence'' that most, though not all, CMIP3 ([[#Meehl--2007|Meehl et al., 2007]]) and CMIP5 ([[#Taylor--2012|Taylor et al., 2012]]) models overestimated the observed warming trend in the tropical troposphere during the satellite period 1979–2012, and that a third to a half of this difference was due to an overestimate of the SST trend during this period ([[#Flato--2013|Flato et al., 2013]]). Since AR5, additional studies based on CMIP5 and CMIP6 models show that this warming bias in tropospheric temperatures remains. Recent studies have investigated the role of observational uncertainty, the model response to external forcings, the influence of the time period considered, and the role of biases in SST trends in contributing to this bias.

Several studies since AR5 have continued to demonstrate an inconsistency between simulated and observed temperature trends in the tropical troposphere, with models simulating more warming than observations ([[#Mitchell--2013|Mitchell et al., 2013]] , 2020; [[#Santer--2017a|Santer et al., 2017a]] , b; [[#McKitrick--2018|McKitrick and Christy, 2018]] ; [[#Po-Chedley--2021|Po-Chedley et al., 2021]]). [[#Santer--2017b|Santer et al. (2017b)]] used updated and improved satellite retrievals to investigate model performance in simulating the tropical mid- to upper-troposphere trends, and removed the influence of stratospheric cooling by regression. These factors were found to reduce the size of the discrepancy in mid- to upper-tropospheric temperature trends between models and observations over the satellite era, but a discrepancy remained. [[#Santer--2017a|Santer et al. (2017a)]] found that during the late 20th century, the discrepancies between simulated and satellite-derived mid- to upper-tropospheric temperature trends were consistent with internal variability, while during most of the early 21st century, simulated tropospheric warming was significantly larger than observed, which they related to systematic deficiencies in some of the external forcings used after year 2000 in the CMIP5 models. However, in CMIP6, differences between simulated and observed upper-tropospheric temperature trends persist despite updated forcing estimates ([[#Mitchell--2020|Mitchell et al., 2020]]). Figure 3.10 shows that CMIP6 models forced by combined anthropogenic and natural forcings overestimate temperature trends compared to radiosonde data ([[#Haimberger--2012|Haimberger et al., 2012]]) throughout the tropical troposphere ([[#Mitchell--2020|Mitchell et al., 2020]]). Over the 1979–2014 period, models are more consistent with observations in the lower troposphere, and least consistent in the upper troposphere around 200 hPa, where biases exceed 0.1°C per decade. Several studies using CMIP6 models suggest that differences in climate sensitivity may be an important factor contributing to the discrepancy between the simulated and observed tropospheric temperature trends ([[#McKitrick--2020|McKitrick and Christy, 2020]] ; [[#Po-Chedley--2021|Po-Chedley et al., 2021]]), though it is difficult to deconvolve the influence of climate sensitivity, changes in aerosol forcing and internal variability in contributing to tropospheric warming biases ([[#Po-Chedley--2021|Po-Chedley et al., 2021]]). Another study found that the absence of a hypothesized negative tropical cloud feedback could explain half of the upper troposphere warming bias in one model ([[#Mauritsen--2015|Mauritsen and Stevens, 2015]]).

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[[File:f84a8ba815752cf82c4431be8ae09afe IPCC_AR6_WGI_Figure_3_10.png]]

'''Figure 3.10 | Observed and simulated tropical mean temperature trends through the atmosphere.''' Vertical profiles of temperature trends in the tropics (20°S–20°N) for three periods: '''(a)''' 1979–2014, '''(b)''' 1979–1997 (ozone depletion era) and '''(c)''' 1998–2014 (ozone stabilization era). The black lines show trends in the Radiosonde Innovation Composite Homogenization (RICH) 1.7 (long dashed) and Radiosonde Observation Correction using Reanalysis (RAOBCORE) 1.7 (dashed) radiosonde datasets ([[#Haimberger--2012|Haimberger et al., 2012]]), and in the ERA5/5.1 reanalysis (solid). Grey envelopes are centred on the RICH 1.7 trends, but show the uncertainty based on 32 RICH-observations members of version 1.5.1 of the dataset, which used version 1.7.3 of the RICH software but with the parameters of version 1.5.1. ERA5 was used as reference for calculating the adjustments between 2010 and 2019, and ERA-Interim was used for the years before that. Red lines show trends in CMIP6 historical simulations from one realization of each of 60 models. Blue lines show trends in 46 CMIP6 models that used prescribed, rather than simulated, sea surface temperatures (SSTs). Figure is adapted from [[#Mitchell--2020|Mitchell et al. (2020)]] , their Figure 1. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

[[#Mitchell--2013|Mitchell et al. (2013)]] and [[#Mitchell--2020|Mitchell et al. (2020)]] found a smaller discrepancy in tropical tropospheric temperature trends in models forced with observed SSTs (see also Figure 3.10a), and CMIP5 models and observations were found to be consistent below 150 hPa when viewed in terms of the ratio of temperature trends aloft to those at the surface ([[#Mitchell--2013|Mitchell et al., 2013]]). [[#Flannaghan--2014|Flannaghan et al. (2014)]] and [[#Tuel--2019|Tuel (2019)]] showed that most of the tropospheric temperature trend difference between CMIP5 models and the satellite-based observations over the 1970–2018 period is due to respective differences in SST warming trends in regions of deep convection, and [[#Po-Chedley--2021|Po-Chedley et al. (2021)]] showed that CMIP6 models with a more realistic SST simulation in the central and eastern Pacific show a better performance than other models. Though systematic biases still remain, this indicates that the bias in tropospheric temperature warming in models is in part linked to surface temperature warming biases, especially in the lower troposphere.

In summary, studies continue to find that CMIP5 and CMIP6 model simulations warm more than observations in the tropical mid- and upper-troposphere over the 1979–2014 period ([[#Mitchell--2013|Mitchell et al., 2013]] , 2020; [[#Santer--2017a|Santer et al., 2017a]] , b; [[#Suárez-Gutiérrez--2017|Suárez-Gutiérrez et al., 2017]] ; [[#McKitrick--2018|McKitrick and Christy, 2018]]), and that overestimated surface warming is partially responsible ([[#Mitchell--2013|Mitchell et al., 2013]] ; [[#Po-Chedley--2021|Po-Chedley et al., 2021]]). Some studies point to forcing errors in the CMIP5 simulations in the early 21st century as a possible contributor ([[#Mitchell--2013|Mitchell et al., 2013]] ; [[#Sherwood--2015|Sherwood and Nishant, 2015]] ; [[#Santer--2017a|Santer et al., 2017a]]), but CMIP6 simulations use updated forcing estimates yet generally still warm more than observations. Although accounting for internal variability and residual observational errors can reconcile models with observations to some extent ([[#Suárez-Gutiérrez--2017|Suárez-Gutiérrez et al., 2017]] ; [[#Mitchell--2020|Mitchell et al., 2020]]), some studies suggest that climate sensitivity also plays a role ([[#Mauritsen--2015|Mauritsen and Stevens, 2015]] ; [[#McKitrick--2020|McKitrick and Christy, 2020]] ; [[#Po-Chedley--2021|Po-Chedley et al., 2021]]). Hence, we assess with ''medium confidence'' that CMIP5 and CMIP6 models continue to overestimate observed warming in the upper tropical troposphere over the 1979–2014 period by at least 0.1°C per decade, in part because of an overestimate of the tropical SST trend pattern over this period.

The AR5 assessed as ''likely'' that anthropogenic forcings, dominated by greenhouse gases, contributed to the warming of the troposphere since 1961 ([[#Bindoff--2013|Bindoff et al., 2013]]). Since then, there has been further progress in detecting and attributing tropospheric temperature changes. [[#Mitchell--2020|Mitchell et al. (2020)]] used CMIP6 models to find that the main driver of tropospheric temperature changes is greenhouse gases. Previous detection of the anthropogenic influence on tropospheric warming may have overestimated uncertainties: [[#Pallotta--2020|Pallotta and Santer (2020)]] found that CMIP5 climate models overestimate the observed natural variability in global mean tropospheric temperature on time scales of 5–20 years. Nevertheless, [[#Santer--2019|Santer et al. (2019)]] found that stochastic uncertainty is greater for tropospheric warming than stratospheric cooling because of larger noise and slower recovery time from the Mount Pinatubo eruption in the troposphere. The detection time of the anthropogenic signal in the tropospheric warming can be affected by both the model climate sensitivity and the model response to aerosol forcing. Volcanic forcing is also important, as models that do not consider the influence of volcanic eruptions in the early 21st century overestimate the observed tropospheric warming since 1998 ([[#Santer--2014|Santer et al., 2014]]). Changes in the amplitude of the seasonal cycle of tropospheric temperatures have also been attributed to human influence. [[#Santer--2018|Santer et al. (2018)]] found that satellite data and climate models driven by anthropogenic forcing show consistent amplitude increases at mid-latitudes in both hemispheres, amplitude decreases at high latitudes in the Southern Hemisphere, and small changes in the tropics.

In summary, these studies confirm the dominant role of human activities in tropospheric temperature trends. We therefore assess that it is ''very likely'' that anthropogenic forcing, dominated by greenhouse gases, was the main driver of the warming of the troposphere since 1979.

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===== 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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