CO₂-induced climate change related to the effect of cumulus convection on the vertical profile of water vapor was studied using a vertical one-dimensional radiative convective model with the Arakawa-Schubert cumulus parameterization that accounts for the drying effect of cumulus convection. Annual mean tropical solar forcing is used because cumulus convection is most active in the tropics. Since moisture and heat transport by large-scale motion (LS forcing) are important to moisture and heat balance in the tropics, two types of doubling CO₂ experiments were conducted with and without considering LS forcing.
The increase in specific humidity is small above the middle troposphere without LS forcing and the surface temperature increase is much smaller than that obtained in a three-dimensional CO₂ experiment. When LS forcing is specified, however, water vapor increases significantly throughout the troposphere and the surface temperature increase is enhanced to a similar degree as in a three-dimensional experiment. LS forcing of heat enhances cumulus convection under control CO₂ concentration, while LS forcing of moisture contributes to enhance changes due to CO₂ doubling. Thus, the magnitude of cumulus convection response to change in radiative forcing is very sensitive to LS forcing. This means that it is important to incorporate the effects of moisture and heat transport by large-scale motion when investigating cumulus convection response to climate changes.