High Climate Sensitivity in the Community Earth System Model Version 2 (CESM2)
During development of the most recent version of the CESM model, CESM2, the equilibrium climate sensitivity dramatically increased. The goals of this paper are (1) document the climate sensitivity, and (2) quantify the role that changes in different model components and aspects of the representation of clouds played. Using slab ocean and fixed-SST simulations to slowly step through small changes, we evaluate the climate sensitivity in interim versions of the model between CESM1 and 2. We apply the radiative feedback kernel technique to quantify cloud feedbacks.
The equilibrium climate sensitivity in CESM2 is 5.3 K. Contributions to the increase in climate sensitivity from CESM1 to CESM2 come from multiple factors, including changes in cloud feedbacks due to reduced sensitivity to aerosols and also the aerosol forcing itself.
The Community Earth System Model Version 2 (CESM2) has an equilibrium climate sensitivity (ECS) of 5.3 K, which is much higher than the previous version of this model, CESM1. By pulling apart the factors that led to this increase in sensitivity using experiments with interim model configurations, we document that the increase in equilibrium climate sensitivity arises from a combination of factors, including changes in aerosol forcing, cloud feedbacks, and other model components.