Cloud feedback processes in a general circulation model

The model used is a general circulation model of the atmosphere coupled with a mixed layer model of the oceans. The sensitivity of each version of the model is inferred from the equilibrium response of the model to a doubling of the atmospheric concentration of carbon dioxide. In response to the increase of atmospheric carbon dioxide, cloudiness increases around the tropopause and is reduced in the upper troposphere, thereby raising the height of the cloud layer in the upper troposphere. This implies a reduction of the temperature of the cloud top and, accordingly, of the upward terrestrial radiation from the top of the model atmosphere. On the other hand, the increase of low cloudiness in high latitudes raises the planetary albedo and thus decreases the CO₂ induced warming of climate. However, the contribution of this negative feedback process is much smaller than the effect of the positive feedbakc process involving the change of high cloud. -from Authors