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. From simple single column models to fully comprehensive three-dimensional climate models, when we increase CO2 to very high levels, we find a peak in the temperature increase per doubling of CO2 at around 310 K. While we understand why this happens in simple models, we lack an understanding for fully comprehensive climate models. Our study uses an intermediate complexity three-dimensional climate model to bridge this gap. We find that the peak in climate sensitivity is not as pronounced as in other studies. We show that this is due to large changes in the atmospheric circulation which causes a moistening of the subtropics and an increase in the climate sensitivity. Climate sensitivity peaks around 310 K in a variety of climate models, ranging from idealized single column models to comprehensive modelsWe use a clear-sky GCM to bridge the gap between comprehensive models and single column models for which we have a theory for this peakAt high CO2, changes in atmospheric circulation cause a moistening of the subtropical regions and increase the climate sensitivity