State-Dependence of the Equilibrium Climate Sensitivity in a Clear-Sky GCM

The climate sensitivity peaks around 310 K in a wide variety of climate models, ranging from idealized single column models to fully comprehensive climate models. Here, we increase CO₂ using a clear-sky three-dimensional atmospheric model with a radiation scheme which maintains accuracy for high CO₂ and temperature levels. In contrast, the Equilibrium Climate Sensitivity (ECS) of our model plateaus around 310 K. We show that this is due to the moistening of the subtropical regions caused by a slowdown in atmospheric circulation, which increases the ECS at very high CO₂ values. When relative humidity is fixed, the ECS peak is consistent with single column model results. This work does not rule out that clouds or other complex processes impact the ECS in comprehensive climate models. Though the changes in CO₂ here are extreme, this study underlines the importance of changes in atmospheric circulation and relative humidity in quantitative assessments of climate sensitivity.