Editing IPCC:AR6/WGI/Chapter-7 (section)

====== High-cloud altitude feedback ======

It has long been argued that cloud-top altitude rises under global warming, concurrent with the rising of the tropopause at all latitudes ( [[#Marvel--2015|Marvel et al., 2015]] ; [[#Thompson--2017|Thompson et al., 2017]] ). This increasing altitude of high-clouds was identified in early generation GCMs and the tropical high-cloud altitude feedback was assessed to be positive with ''high confidence'' in AR5 ( [[#Boucher--2013|Boucher et al., 2013]] ). This assessment is supported by a theoretical argument called the ‘fixed anvil temperature mechanism’, which ensures that the temperature of the convective detrainment layer does not change when the altitude of high-cloud tops increases with the rising tropopause ( [[#Hartmann--2002|Hartmann and Larson, 2002]] ). Because the cloud-top temperature does not change significantly with global warming, cloud LW emission does not increase even though the surface warms, resulting in an enhancement of the high-cloud greenhouse effect (a positive feedback; [[#Yoshimori--2020|Yoshimori et al. (2020)]] ). The upward shift of high-clouds with surface warming is detected in observed interannual variability and trends in satellite records for recent decades ( [[#Chepfer--2014|Chepfer et al., 2014]] ; [[#Norris--2016|Norris et al., 2016]] ; [[#Saint-Lu--2020|Saint-Lu et al., 2020]] ). The observational detection is not always successful ( [[#Davies--2017|Davies et al., 2017]] ), but the cloud altitude shifts similarly in many CRM experiments ( [[#Khairoutdinov--2013|Khairoutdinov and Emanuel, 2013]] ; [[#Tsushima--2014|Tsushima et al., 2014]] ; [[#Narenpitak--2017|Narenpitak et al., 2017]] ). The high-cloud altitude feedback was estimated to be 0.5 W m <sup>–2</sup> °C <sup>–1</sup> based on GCMs in AR5, but is revised, using a recent re-evaluation that excludes aliasing effects by reduced low-cloud amounts, downward to 0.22 ± 0.12 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation; [[#Zhou--2014|Zhou et al., 2014]] ; [[#Zelinka--2020|Zelinka et al., 2020]] ). In conclusion, there is ''high confidence'' in the positive high-cloud altitude feedback simulated in ESMs as it is supported by theoretical, observational, and process modelling studies.

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