Possible reasons for low climate-model sensitivity to increased carbon dioxide concentrations

The sensitivities of two climate-model versions—INMCM4 which participated in the Coupled Model Intercomparison Project, Phase 5 (CMIP5), and a new INMCM5 version with increased vertical and horizontal resolutions in its atmospheric block—to the quadrupled concentration of CO₂ are studied. When the CO₂ concentration is quadrupled, the equilibrium increase in surface temperature amounts to about 4.2 K for INMCM4, which is lower than that for other models that participated in the CMIP5. When the CO₂ concentration increases, the cloud radiative forcing in the model decreases; in this case, one portion of this decrease occurs during the first year after the concentration of CO₂ is quadrupled and the other portion almost linearly depends on the value of global warming. The results of additional numerical experiments with the model show that a rapid decrease in cloud-radiative forcing results from variations in stratification in the atmospheric surface boundary layer and associated increased cloudiness. The portion of a linear decrease in cloud-radiative forcing with increased temperature is associated with an increase in the water content of model clouds at higher temperatures. The elimination of these two mechanisms allows one to increase the model sensitivity to the quadrupled concentration of CO₂ up to 5.2 K.