Climate sensitivity peaks around 310K in a wide variety of climate models, ranging from idealized single column models to fully comprehensive climate models. Although an explanation for this peak has been developed using single column models with fixed relative humidity and line-by-line radiation, the relevance of this theory for explaining the peak in comprehensive climate models is unclear. In this paper we increase CO2 using a clear-sky 3-dimensional climate model with a radiation scheme that maintains accuracy for high CO2 and temperature levels. The Equilibrium Climate Sensitivity (ECS) of our model shows a plateau around 310K with the moistening of the subtropical regions caused by a slowdown in atmospheric circulation increasing the ECS at very high CO2 values. Though the changes in CO2 and temperature presented here are extreme, this study shows the potential importance of changes in atmospheric circulation and relative humidity in quantitative assessments of climate sensitivity.