Changes in cloud distribution may provide a major feedback on climate change. General circulation model simulations show an upward shift of high cloud and a general reduction of free-tropospheric cloud when climate warms. The shift of high cloud seems due to an upward shift of the tropopause. It is argued that the reduction in relative humidity and cloud cover below can be attributed to the increased depth of vertical motions in the warmer climate, which in turn follows from the upward shift of atmospheric radiative cooling as specific humidifies increase. A diagnostic study of the response of a general circulation model is consistent with this mechanism.