Cloud Cover, Precipitation, and Global Warming
Global temperatures have been rising over the last 20 years. Satellite data show a rise in temperature of the global ocean surface of about 1°C (1.8°F) over the past decade. Any rise in sea-surface temperature will increase the rate of evaporation, and an increase in evaporation will raise the average atmospheric content of water vapor.
Water has several roles in global climate. First, in its vapor state it is one of the greenhouse gases that absorb and emit longwave radiation, enhancing the warming effect of the atmosphere above the Earth's surface. In fact, water is a more powerful greenhouse gas than CO2 . So we can predict that an increase in global water vapor in the atmosphere should enhance warming.
Second, water vapor, when cooled, can form clouds. Will more clouds increase or decrease global temperatures? Cloud droplets and ice crystals reflect a large proportion of incoming shortwave radiation back to space, cooling global temperatures. But cloud particles also absorb longwave radiation from the ground and return that emission as counterradiation. This absorption is an important part of the greenhouse effect, and it is even stronger for water as cloud droplets or ice particles than as water vapor. Which effect, shortwave cooling or longwave warming, will dominate?
The best estimate, at present, is that the average flow of shortwave energy reflected by clouds back to space is about 50 W/m2 (loss), while the longwave warming effect of clouds amounts to about 30 W/m2 (gain). So, at the moment, the net effect of clouds is to cool the planet by about 20 W/m2. But computer models predict that with more clouds, the cooling effect will be reduced somewhat, making temperatures rise even higher.
Precipitation is the third role of water in global climate. With more water vapor and more clouds in the air, more precipitation should result. But what if precipitation increases in arctic and subarctic zones? More snow would increase the Earth's albedo, thus tending to reduce global temperatures. That could reduce ocean evaporation. More snow could also tie up more water in snow packs and glaciers, reducing runoff to the oceans and reducing the rate at which sea level rises.
So the situation is quite complicated. At this time, scientists are unsure exactly how the global climate system will respond to global warming induced by the CO2 increases predicted by the end of the century. But models are beginning to converge on common predictions that show the changes will be significant and important to many regions and human activities.