For JCLI CCSM Special Issue The Climate Sensitivity of the Community Climate System Model: CCSM3

The climate sensitivity of the Community Climate System model is described in terms of the equilibrium change in surface temperature due to a doubling of carbon dioxide in a slab ocean versions of the Community Atmosphere Model (CAM) and the transient climate response, which is the surface temperature change at the point of doubling of carbon dioxide in a 1 % per year CO2 simulation with the fully coupled CCSM. For a fixed atmospheric horizontal resolution across model versions, we show that the equilibrium sensitivity has monotonically increased across CSM1.4, CCSM2, to CCSM3 from: 2.01, 2.27 to 2.47 °C, respectively. The transient climate response for these versions is: 1.44, 1.09 to 1.48°C, respectively. Using climate feedback analysis, we show that both clear sky and cloudy sky processes have contributed to the changes in transient climate response. The dependence of these sensitivities on horizontal resolution is also explored. The equilibrium sensitivity of the high resolution version of CCSM3 is 2.71 °C, while the equilibrium response for the low resolution model is 2.32 °C. We show that the shortwave cloud response of the high resolution version of the CCSM3 is anomalous compared to the low and moderate resolution versions.