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

====== Arctic cloud feedback ======

Clouds in polar regions, especially over the Arctic, form at low altitude above or within a stable to neutral boundary layer and are known to co-vary with sea ice variability beneath. Because the clouds reflect sunlight during summer but trap LW radiation throughout the year, seasonality plays an important role in cloud effects on Arctic climate ( [[#Kay--2016b|Kay et al., 2016b]] ). AR5 assessed that Arctic low-cloud amount will increase in boreal autumn and winter in response to declining sea ice in a warming climate, due primarily to an enhanced upward moisture flux over open water. The cloudier conditions during these seasons result in more downwelling LW radiation, acting as a positive feedback on surface warming ( [[#Kay--2009|Kay and Gettelman, 2009]] ). Over recent years, further evidence of the cloud contribution to the Arctic amplification has been obtained ( [[#7.4.4.1|Section 7.4.4.1]] ; [[#Goosse--2018|Goosse et al., 2018]] ). Space-borne lidar (light detection and ranging) observations show that the cloud response to summer sea ice loss is small and cannot overcome the cloud effect in autumn ( [[#Taylor--2015|Taylor et al., 2015]] ; [[#Morrison--2019|Morrison et al., 2019]] ). The seasonality of the cloud response to sea ice variability is reproduced in GCM simulations ( [[#Laîné--2016|Laîné et al., 2016]] ; [[#Yoshimori--2017|Yoshimori et al., 2017]] ). The agreement between observations and models indicates that the Arctic cloud feedback is positive at the surface. This leads to an Arctic cloud feedback at TOA that is ''likely'' positive, but very small in magnitude, as found in some climate models ( [[#Pithan--2014|Pithan and Mauritsen, 2014]] ; [[#Morrison--2019|Morrison et al., 2019]] ). The observational estimates are sensitive to the analysis period and the choice of reanalysis data, and a recent estimate of the TOA cloud feedback over 60°N–90°N using atmospheric reanalysis data and CERES satellite observations suggests a regional value ranging from –0.3 to +0.5 W m <sup>–2</sup> °C <sup>–1</sup> , which corresponds to a global contribution of –0.02 to +0.03 W m <sup>–2</sup> °C <sup>–1</sup> (R. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ). Based on the overall agreement between ESMs and observations, the Arctic cloud feedback is assessed to be small positive and has the value of 0.01 ± 0.05 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation). The assessed range indicates that a negative feedback is almost as probable as a positive feedback, and the assessment that the Arctic cloud feedback is positive is therefore given ''low confidence'' .

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