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 CO(2)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 (lambda(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 lambda(cld). Over low-latitude regions, increasing GMST strengthens lambda(cld)likely through the nonlinear increase in water vapor, which causes low-cloud thinning through thermodynamic and radiative processes.