UPDATED April 30, 2012, he New York Times.
Clouds and Climate
At any moment, about 60 percent of the earth is covered by clouds, which have a huge influence on the climate. An animated map showing a year of cloud cover suggests the outlines of continents because land and ocean features influence cloud patterns. Related Article »
Radiated heat from land, oceans and low clouds is held near the earth’s surface by greenhouse gases but must eventually escape to space, as shown above. Areas with more cirrus cloud cover tend to emit less heat to space.
Reflected sunlight from low clouds, ice and other bright surfaces returns some of the sun’s energy back into space. The oceans and other dark surfaces reflect less light and are warmed by absorbing sunlight.
Incoming sunlight varies with location and with the seasons. The above map shows the average sunlight falling on the earth’s surface in March, as measured in watts per square meter.
Cirrus clouds are high, thin and cold. Many of these wispy clouds allow sunlight to pass through easily, but they tend to trap rising heat. A future increase in cirrus clouds would probably warm the planet, while a decrease would most likely cool it.
Global Energy Balance To maintain a stable temperature, the earth must balance the incoming energy of sunlight with the outgoing energy of reflected light and radiated heat. Clouds alter the balance in several ways, with an overall cooling effect in today’s climate. Humans are adding greenhouse gases to the atmosphere, causing warming near the surface, and a big question is how clouds will react.