The Effects of Changing the Solar Constant on the Climate of a General Circulation Model

Abstract

A study is conducted to evaluate the response of a simplified three-dimensional model climate to changes of the solar constant. The model explicitly computes the heat transport by large-scale atmospheric disturbances. It contains the following simplifications: a limited computational domain, an idealized topography, no heat transport by ocean currents, no seasonal variation, and fixed cloudiness. It is found that the temperature of the model troposphere increases with increasing solar radiation. The greatest increase occurs in the surface layer of higher latitudes due to the effects of the snow-cover feedback mechanism as well as the suppression of vertical mixing by a stable lower troposphere. This result is found to be qualitatively similar to that obtained from previous studies with one-dimensional zonal mean models. One of the most interesting features of this investigation is the extreme sensitivity of the intensity of the computed hydrologic cycle to small changes of the solar constant. Current estimates indicate a 27% increase of the former as compared with a 6% increase of the latter. This large intensification of the hydrologic cycle in the model atmosphere results from the increase in the rate of evaporation which is caused by the following changes: 1) reduction of the Bowen ratio due to the nonlinear increase of saturation vapor pressure with increasing temperature at the earth's surface, and 2) decrease in the net upward terrestrial surface radiation resulting from the increase in the moisture content in air and from the reduction of the lapse rate (both of which increase the downward terrestrial radiation and increase the energy available for evaporation). It is shown that the latitude of maximum snowfall retreats poleward as the solar constant is increased. Furthermore, the total amounts of snowfall and snow accumulation decrease markedly with increasing insolation due to the poleward shift of the region of subfreezing surface temperature away from the zone of maximum baroclinic instability.