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

====== Extratropical cloud optical depth feedback ======

Mixed-phase clouds that consist of both liquid and ice are dominant over the Southern Ocean (50°S–80°S), which accounts for 20% of the net CRE in the present climate ( [[#Matus--2017|Matus and L’Ecuyer, 2017]] ). It has been argued that the cloud optical depth (opacity) will increase over the Southern Ocean as warming drives the replacement of ice-dominated clouds with liquid-dominated clouds ( [[#Tan--2019|Tan et al., 2019]] ). Liquid clouds generally consist of many small cloud droplets, while the crystals in ice clouds are orders of magnitude fewer in number and much larger, causing the liquid clouds to be optically thicker and thereby resulting in a negative feedback ( [[#Boucher--2013|Boucher et al., 2013]] ). However, this phase-change feedback works effectively only below freezing temperature ( [[#Lohmann--2018|Lohmann and Neubauer, 2018]] ; [[#Terai--2019|Terai et al., 2019]] ) and other processes that increase or decrease liquid water path (LWP) may also affect the optical depth feedback ( [[#McCoy--2019|McCoy et al., 2019]] ).

Due to insufficient amounts of super-cooled liquid water in the simulated atmospheric mean state, many CMIP5 models overestimated the conversion from ice to liquid clouds with climate warming and the resultant negative phase-change feedback ( [[#Kay--2016a|Kay et al., 2016a]] ; [[#Tan--2016|Tan et al., 2016]] ; [[#Lohmann--2018|Lohmann and Neubauer, 2018]] ). This feedback can be constrained using satellite-derived LWP observations over the past 20 years that enable estimates of both long-term trends and the interannual relationship with SST variability ( [[#Gordon--2014|Gordon and Klein, 2014]] ; [[#Ceppi--2016|Ceppi et al., 2016]] ; [[#Manaster--2017|Manaster et al., 2017]] ). The observationally-constrained SW feedback ranges from –0.91 to –0.46 W m <sup>–2</sup> °C <sup>–1</sup> over 40°S–70°S depending on the methodology ( [[#Ceppi--2016|Ceppi et al., 2016]] ; [[#Terai--2016|Terai et al., 2016]] ). In some CMIP6 models, representation of super-cooled liquid water content has been improved, leading to weaker negative optical depth feedback over the Southern Ocean closer to observational estimates ( [[#Bodas-Salcedo--2019|Bodas-Salcedo et al., 2019]] ; [[#Gettelman--2019|Gettelman et al., 2019]] ). This improvement at the same time results in a positive optical depth feedback over other extratropical ocean where LWP decreased in response to reduced stability in those CMIP6 models ( [[#Zelinka--2020|Zelinka et al., 2020]] ). Given the accumulated observational estimates and an improved agreement between ESMs and observations, the extratropical optical depth feedback is assessed to be small negative with ''medium confidence.'' Quantitatively, the global contribution of this feedback is assessed to have a value of –0.03 ± 0.05 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation) by combining estimates based on observed interannual variability and the cloud controlling factors.

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