Feedbacks in theclimate system 91Figure 5.14Schematic of the physical processes associated with clouds.
Water vapour feedback
This is the most important.^11 With a warmer atmosphere more evapo-
ration occurs from the ocean and from wet land surfaces. On average,
therefore, a warmer atmosphere will be a wetter one; it will possess a
higher water vapour content. Since water vapour is a powerful green-
house gas, on average a positive feedback results of amagnitude that
models estimate to approximately double the increase in the global
average temperature that would arise with fixed water vapour.^12
Cloud-radiation feedback
This is more complicated as several processes are involved. Clouds in-
terfere with the transfer of radiation in the atmosphere in two ways
(Figure 5.14). Firstly, they reflect a certain proportion of solar radia-
tion back to space, so reducing the total energy available to the sys-
tem. Secondly, they act as blankets to thermal radiation from the Earth’s
surface in a similar way to greenhouse gases. By absorbing thermal
radiation emitted by the Earth’s surface below, and by themselves
emitting thermal radiation, they act to reduce the heat loss to space by the
surface.
The effect that dominates for any particular cloud depends on the
cloud temperature (and hence on the cloud height) and on its detailed
optical properties (those properties which determine its reflectivity to
solar radiation and its interaction with thermal radiation). The latter
depend on whether the cloud is of water or ice, on its liquid or solid