Correlation Between Cloud Adjustments and Cloud Feedbacks Responsible for Larger Range of Climate Sensitivities in CMIP6

Abstract

AbstractWhile the higher mean Equilibrium Climate Sensitivity (ECS) in CMIP6 has been attributed to more positive cloud feedbacks, it is unclear what causes the greater range of ECS values across CMIP6 models compared to CMIP5. Here, we investigate the relationship between radiative forcing and cloud feedbacks across the two model generations to explain the very high ECS values in some CMIP6 models. The relationship is sensitive to the definition of the forcing, particularly in CMIP6, but fixed‐sea surface temperature simulations suggest the shortwave cloud feedback (λSW,CL) is anticorrelated with the forcing in CMIP5 and either uncorrelated or weakly positively correlated with the forcing in CMIP6. These relationships reflect the cloud adjustment to the forcing, which is anticorrelated with λSW,CL in CMIP5 and positively correlated in CMIP6. Although we are unable to identify a systematic change across the model generations, we do show that the difference is not due to land effects, and that cloud adjustments are generally driven by low and, especially, midlevel clouds. Furthermore, models derived from a small number modeling centers seem to be responsible for much of the difference between the CMIP5 and CMIP6 ensembles. Our analysis is severely limited by the available simulations, highlighting the need for targeted experiments to better understand cloud adjustments and to probe the sources of intermodel differences.