The role of cloud microphysics in high-cloud feedback effects on climate change

Abstract

THE effect of clouds on the radiation balance of the Earth (cloud forcing) is such that, on average, they cause the mean surface temperature to decrease. When the climate system is perturbed, the cloud forcing may change and this change will exert a feedback on the system. Climate models predict that clouds will have an overall positive feedback effect, that is, their presence will tend to enhance any surface-temperature changes, as in the greenhouse effect for example. The models preferentially increase the amounts of high clouds and their optical depths and decrease the amount of low clouds. Here I look at the temperature dependence of the infrared emittance of high clouds using data from a ground-based lidar/radiometer and in situ aircraft measurements of cloud properties. I conclude that the infrared emittances of high clouds are less sensitive to changes in temperature than predicted by climate models, chiefly because the microphysics of cirrus clouds modifies the relationship between cloud optical depth and cloud ice/liquid water path, in ways not accounted for in the models.