Sensitivity of equilibrium surface temperature of CCM3 to systematic changes in atmospheric CO2

We have redone our computations of the equilibrium response of surface temperature to atmospheric CO2 concentrations using the latest version of the NCAR Community Climate Model (version 3 rather than version 1). Eight GCM simulations with CO2 concentrations varying from 180 to 3000 ppmv were conducted with CCM3 compared to six CO2 concentrations with CCM1. A preliminary examination of the CCM3 simulations showed the same basic non-linear behaviour of temperature to CO2 concentrations obtained previously with CCM1. The magnitude of the sensitivities, however, were much lower in the new CCM3 runs than in the older CCM1 runs. Four possible reasons for the reduced sensitivity in CCM3 are discussed.