Do Cloud Feedbacks, Different External Forcing, or Something Else Enhance 21st-Century Warming in the Latest Generation of an Earth System Model?

Abstract:

As a group, CMIP6 models exhibit more positive extratropical cloud feedbacks, higher equilibrium climate sensitivity (ECS), and greater transient 21st Century warming than their CMIP5-generation predecessors. However, distinguishing the roles of more positive feedbacks, other structural model changes, and changed external forcing in the magnitude and spatial pattern of 21st Century warming is challenging in the multi-model CMIP6 context. Here we simplify the problem by comparing 21st Century climate projections from CESM1 (CMIP5 generation), CESM1-CLDMOD (CMIP5 generation, plus one parameterization change that enhances ECS via more positive extratropical shortwave cloud feedbacks), and CESM2 (CMIP6 generation - many structural changes, generally leading to more positive cloud feedbacks and higher ECS). CESM1-CLDMOD has more positive extratropical cloud feedbacks than CESM1, but the impact on the 21st Century global warming rate is fairly small under RCP8.5. Although global-scale impact of the enhanced extratropical feedbacks is small in CESM1-CLDMOD, there are some notable regional impacts such as a larger increase of Antarctic snowfall and a higher rate of Arctic sea ice loss.

CESM2 has been run under both RCP8.5 and SSP585. Under SSP585, CESM2 exhibits significantly more warming (in all latitudes but the Arctic) than CESM1 and CESM1-CLDMOD under RCP8.5. Whereas previous analyses have highlighted extratropical cloud feedbacks to explain enhanced warming in the CMIP6 generation of models, we find that CESM2 has strong shortwave feedbacks in the tropics and subtropics. By comparing CESM2’s responses under RCP8.5 and SSP585, we will distinguish structural changes from forcing changes in driving these feedbacks and the enhanced warming.