On the Increase of Climate Sensitivity and Cloud Feedback With Warming in the Community Atmosphere Models

Modeling and paleoclimate proxy-based studies suggest that equilibrium climate sensitivity (ECS) depends on the background climate state, though the reason is not thoroughly understood. Here we study the state dependence of ECS over a large range of global mean surface temperature (GMST) in the Community Atmosphere Model (CAM) Versions 4, 5, and 6 by varying atmospheric CO₂ concentrations. We find a robust increase of ECS with GMST in all three models, albeit at different rates, which is primarily attributed to strengthening of the shortwave cloud feedback (λ_cld) at both high and low latitudes. Over high latitudes, increasing GMST leads to a reduction in the cloud ice fraction, weakening the (negative) cloud-phase feedback due to the phase transition of cloud ice to liquid and thereby strengthening λ_cld. Over low-latitude regions, increasing GMST strengthens λ_cld likely through the nonlinear increase in water vapor, which causes low-cloud thinning through thermodynamic and radiative processes.