Comparison of longwave and shortwave cloud effects on equilibrium surface temperature using a radiative-convective model and 12 years of MISR observations

Abstract

Twelve years of MISR observations have been analyzed for changes in effective cloud height and albedo. Both show a general decrease in global values, modulated in part by variation in the Southern Oscillation Index. The heights have decreased at the rate of 45 ± 19 m/decade, and the green band albedo has decreased at the rate of 0.0028 ±0.0010 /decade. The latitudinal dependence of the changes indicate some regions of likely compensation due to total cloud fraction change, but many other regions where the changes are due to different effects, with high northern latitudes showing reduced albedo due to less sea-ice and snow cover. Many latitudes show a decrease in cloud height without a compensating change in albedo, indicating a reduction in high cloud fraction with little change in total cloud fraction. Put into the context of their effect on equilibrium surface temperature, the cloud height reduction appears to be slightly larger than the albedo effect, and both are at least three times larger than the effect of increasing carbon dioxide over the same 12 years. The net effect is shown to be an insignificant increase in equilibrium temperature over the 12 years of this study, even allowing for positive water vapor feedback.