Abstract

Equilibrium climate sensitivity (ECS), defined as the increase in global mean surface temperature resulting from a doubling of atmospheric CO2 compared to pre-industrial levels, has exhibited persistent uncertainty of more than 3 degrees over the past 45 years. Cloud feedback is widely recognized as a primary contributor to the diversity in ECS estimates across climate models. Previous studies have identified several emergent constraints of ECS based on the sensitivity of marine low clouds to sea surface temperature (SST) in the present-day climate; however, these constraints yield conflicting results regarding the likelihood of various ECS values. For instance, our earlier research (Zhai et al. 2015; Jiang et al. 2023) indicated that seasonal variations in subtropical and extratropical marine low cloud amounts suggest strong positive feedback, corresponding to a high ECS. In contrast, Furtado et al. (2023) proposed that while the seasonal cycle of subtropical low clouds implies strong positive feedback, the seasonal cycle of mid-latitude clouds suggests stronger negative feedback, resulting in a lower ECS. In the tropics, ECS inferences derived from the sensitivity of stratocumulus and shallow cumulus clouds to SST also point to differing ECS values. To reconcile these conflicting findings, we analyze the seasonal, interannual, and decadal variations of different low cloud types across various regions and decompose their contributions to total cloud feedback and ECS. This talk will provide a comprehensive assessment of low cloud feedback and its implications for ECS.