New observational constraint on cloud feedbacks suggests moderate climate sensitivity

Abstract:

Global climate models (GCMs) predict substantial future warming in response to increasing greenhouse gas concentrations, in part due to decreases in the cover and reflectance of tropical low-level clouds. We use new satellite observations that accurately discriminate stratocumulus from shallow cumulus clouds to separately evaluate their sensitivity to warming. Combined with inferences about climate changes in sea surface temperature (SST) patterns, these results place constraints on the contribution of the tropics to low-cloud feedback. We show that shallow cumulus clouds are relatively insensitive to warming whereas GCMs typically show large positive cloud feedback in regions dominated by these clouds. We also show that stratocumulus clouds are very sensitive to warming and changes in the strength of the tropical inversion layer, which is controlled by changes in the SST gradient across the tropical Pacific. Models fail to reproduce the observed trend of this SST gradient over the past 50-60 years, implying that they do not correctly simulate changes in inversion strength and overestimate the positive stratocumulus contribution to cloud feedback. If weak east-Pacific warming continues in the coming decades, it will generate a weaker low-cloud feedback and, in turn, imply a more moderate climate sensitivity (3.47 ± 0.33 K) than many models currently predict (multimodel median = 3.89 K).