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

==== 7.5.4.2 Emergent Constraints Focused on Cloud Feedbacks and Present-day Climate ====

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A substantial number of emergent constraint studies focus on observables that are related to tropical low-cloud feedback processes ( [[#Volodin--2008|Volodin, 2008]] ; [[#Sherwood--2014|Sherwood et al., 2014]] ; [[#Zhai--2015|Zhai et al., 2015]] ; [[#Brient--2016|Brient and Schneider, 2016]] ; [[#Brient--2016|Brient et al., 2016]] ). These studies yield median ECS estimates of 3.5°C–4°C and in many cases indicate low likelihoods of values below 3°C. The approach has attracted attention since most of the spread in climate sensitivity seen in CMIP5, and earlier climate model ensembles, arises from uncertainty in low-cloud feedbacks ( [[#Bony--2005|Bony and Dufresne, 2005]] ; [[#Wyant--2006|Wyant et al., 2006]] ; [[#Randall--2007|Randall et al., 2007]] ; [[#Vial--2013|Vial et al., 2013]] ). Nevertheless, this approach assumes that all other feedback processes are unbiased ( [[#Klein--2015|Klein and Hall, 2015]] ; [[#Qu--2018|Qu et al., 2018]] ; [[#Schlund--2020|Schlund et al., 2020]] ), for instance the possibly missing negative anvil area feedback or the possibly exaggerated mixed-phase cloud feedback ( [[#7.4.2.4|Section 7.4.2.4]] ). Thus, the subset of emergent constraints that focus on low-level tropical clouds are not necessarily inconsistent with other emergent constraints of ECS. Related emergent constraints that focus on aspects of the tropical circulation and ECS have led to conflicting results ( [[#Su--2014|Su et al., 2014]] ; [[#Tian--2015|Tian, 2015]] ; [[#Lipat--2017|Lipat et al., 2017]] ; [[#Webb--2020|Webb and Lock, 2020]] ), possibly because these processes are not the dominant factors in causing the inter-model spread ( [[#Caldwell--2018|Caldwell et al., 2018]] ).

The fidelity of models in reproducing aspects of temperature variability or the radiation budget has also been proposed as emergent constraints on ECS ( [[#Covey--2000|Covey et al., 2000]] ; [[#Knutti--2006|Knutti et al., 2006]] ; [[#Huber--2010|Huber et al., 2010]] ; [[#Bender--2012|Bender et al., 2012]] ; [[#Brown--2017|Brown and Caldeira, 2017]] ; [[#Siler--2018a|Siler et al., 2018a]] ). Here indices based on spatial or seasonal variability are linked to modelled ECS, and overall the group of emergent constraints yields best estimates of 3.3°C–3.7°C. Nevertheless, the physical relevance of present-day biases to the sum of long-term climate change feedbacks is unclear and therefore these constraints on ECS are not considered reliable.

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