In this presentation, we will show the radiative forcing and feedbacks that govern equilibrium climate sensitivity (ECS) across all CMIP6 models that have performed the abrupt-4xCO2 experiment. We will diagnose the sources of inter-model spread in ECS, highlight reasons for any systematic differences between CMIP5 and CMIP6, and evaluate why several models have much larger ECS than their CMIP5 counterparts. Finally, we will elucidate the cloud changes that comprise cloud feedback across models, with attention to the processes that are large on average and/or show large disagreement across models. At the time of writing, 13 CMIP6 models have been analyzed. On average, their ECS is higher than the CMIP5 multi-model mean. This is solely due to stronger positive cloud feedbacks, as the distributions of non-cloud feedbacks and effective 4xCO2 radiative forcing are nearly identical to those in CMIP5. The strengthened cloud feedback in CMIP6 comes from an enhanced shortwave (SW) component that is only slightly opposed by a weaker longwave component. The increased SW cloud feedback comes in roughly equal measure from a stronger positive cloud amount component and a weaker negative cloud albedo component. Physical reasons for these changes in cloud feedback will be explored in greater detail.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. It is supported by the Regional and Global Climate Modeling Program of the Office of Science at the DOE. IM Release # LLNL-ABS-775079