From an early NOAA website:
CLOUD CLIMATOLOGY: SIMPLE EARLY VIEWS OF CLOUDS
The earliest attempts to predict how changes in cloud cover would affect greenhouse warming concluded that they would have no net effect: clouds would neither speed nor slow a change in climate. That conclusion was based on the belief that any change that made clouds better at cooling the Earth would also make them more efficient at retaining heat near the surface. For example, if cloud cover were to increase (as many thought it would, assuming that warmer temperatures would speed evaporation), the amount of sunlight reaching Earth's surface would decrease, but then the thermal radiation trapped by the cloud might increase by the same amount.
Even such a simple scenario has problems, though. Because the decrease in solar heating would affect surface temperatures, whereas the change in the emission of thermal radiation would affect air temperatures at higher altitudes, additional cloud cover would reduce the temperature contrasts between the surface and the higher altitudes that drive the winds. Any reduction of winds might in turn inhibit the formation of clouds. The early studies did not account for this possibility.
Another idea is that higher atmospheric temperatures could create denser clouds, since greater evaporation rates at higher temperatures would make more water vapor available in the atmosphere for cloud condensation. Because denser clouds reflect more sunlight, there would be an enhanced cooling effect. This would reduce the magnitude of the greenhouse warming. On the other hand, denser clouds might also lead to an increase in precipitation (rainfall and snowfall), possibly from storm clouds, whose tops are especially high and cold. Such clouds, which are particularly good absorbers of thermal radiation, could more than make up for their tendency to block sunshine. In that case the warming would be intensified. Observations have shown, however, that warmer temperatures seems to create less dense, low-level clouds instead. The evidence we have so far suggests that this effect occurs because, as temperature increases, the air near the surface becomes drier, causing the cloud base to rise and reducing the cloud layer thickness. Earlier studies did not consider this possibility.
Such "what-if" discussions can go on indefinitely. All of the changes mentioned above are physically reasonable and there are many more to be considered. The question is: How many and which ones will actually take place when the climate changes and exactly how large will they be? In all likelihood, all of these changes and more would occur together, but we don't know what the
net effect would be.
Another kind of complication is that
clouds come in many forms , depending on the weather conditions that create them. Low, dense sheets of stratocumulus clouds hanging just above the ocean cool more than they heat. They make efficient shields against incoming sunlight, and because they are low — and therefore warm — they radiate upward almost as much thermal radiation as the surface does. In contrast, the thin, wispy cirrus clouds, which soar at 6,000 meters (20,000 feet) and higher, reflect little sunlight, but they are so cold that they absorb most of the thermal radiation that comes their way. Hence they warm more than they cool. The net cooling effect of clouds is the sum of a large number of such specific effects, many of which cancel one another.
Atmospheric scientists have been aware for nearly two decades that the complex effects of clouds on radiation and water exchanges pose a major challenge to the understanding of climatic change. In 1974 an international conference of investigators in Stockholm highlighted the need for greater understanding of clouds as one of the two biggest obstacles to further progress in climate research. The second was inadequate knowledge of ocean currents. Recent comparisons of the predictions made by various computer climate models show that the problem has not gone away. In some models, for instance, clouds decrease the net greenhouse effect, whereas in others they intensify it.
CLOUD CLIMATOLOGY: HOW CLOUDS MIGHT CHANGE WITH GLOBAL WARMING
Although simple relations may hold between climatic conditions and the radiative properties of certain kinds of cloud, predicting how the global distribution of various kinds of clouds would change with global warming is complicated by their interaction with regional wind systems. Consider the roles of clouds in seasonal climatic change. In the midlatitudes, winter brings a substantial decline in solar heating, yet the corresponding drop in air temperature near the surface is between 70 and 80 percent less than what the decline in solar heating would seem to imply. More abundant and thicker winter clouds, with slightly higher tops, trap heat better.
In the tropics, despite significantly greater cloud cover in the rainy season, there is only a small seasonal variation in surface temperature. In part the variation is small because the effects of tropical clouds on thermal and solar radiation nearly cancel one another, but even more important is the controlling influence of heat transports by atmospheric winds.
The quest for more data about clouds and climate continues in parallel with the refinement of climate models. It is a slow-going process: each new piece of information must be incorporated throughout. With certain findings the models themselves may have to be reformulated. But the result should be an increasingly precise understanding of how sensitive the clouds are in response to changes in external forces and what effect those changes would have on global warming. One must hope that the model building and data collection activities will lead to an understanding of climatic change before that change comes to pass.
