The Role of Clouds
Research in the last few years has shown that clouds are a major
variable in Earth's climate system. The climate is very sensitive to
small (10-30 percent) changes in clouds. This sensitivity is very important
in trying to figure out what man-made changes, such as increasing carbon
dioxide, will do to Earth's climate. Depending how these changes affect
clouds, their influence on climate could be either enhanced or damped out.
Let's look at how clouds affect Earth's climate:
Earth's Energy Budget summarizes what we know now about the parts of the Earth's temperature controls. All the energy comes from the sun. An equal amount of energy must go back into space or Earth's temperature will change. The picture shows that clouds make the biggest contribution to reflected energy (think about how bright clouds can appear -- this is reflected energy), and also to emitted heat (clouds act like radiators in the atmosphere -- though much colder than a radiator in a building). You can think of clouds as one thermostat that sets Earth's temperature. If, for example, you increase the average thickness of low clouds a little (i.e., make them more reflective), the Earth's temperature will decrease a little. The effect is as if you turned down the thermostat a little.
We have learned that clouds can act to either warm or cool the Earth. High clouds are often thin and not very reflective. They let lots of the sun's warmth in. They also sit high in the sky, where the air temperature is quite cold; so they do not emit a lot of heat. On balance, high clouds tend to warm the Earth. Low clouds are often quite thick and reflect lots of sunlight back to space. They are also lower in the atmosphere where the air is warmer so they emit more heat. On balance, low clouds tend to cool the Earth. See FAQ for a more detailed discussion of these effects.
A picture summarizes these cloud features.
Further reading on the role of clouds is available from the NASA Fact Sheet on The Importance of Understanding Clouds.