Science Briefs

Cloud Climatology

The Role of Clouds in Climate

Clouds have always been signs of the weather to come. Scattered white cumulus clusters sailing across a field of blue promise a dry summer afternoon. Massive dark thunderheads portend crop-damaging wind and rain. A blanket of light gray signals a temperate winter's night. Today meteorologists scan the cloud patterns that move across video monitors, scrutinizing images built from satellite data that make it possible to give daily weather forecasts with much greater accuracy than ever before.

Thus it is ironic that when it comes to forecasting the climate several decades ahead, clouds mainly obscure our vision. Their most important role in climate is to modulate Earth's basic radiation balance: the law of conservation of energy requires that the energy absorbed from the sun balance the energy radiated back into space. Cloud both reflect incoming sunlight and inhibit the radiation of heat from the surface, thereby profoundly affecting both sides of the global energy equation. Clouds would modify any changes in climate as well. The trouble is that the complex ways clouds can respond to other physical variables make it hard to determine their net effect on the radiation balance. Although satellites make it possible to measure the global interactions of clouds and radiation, investigators have only recently shifted attention to the problem.

What makes it so important to disentangle the interactions of clouds and climate? The balance between absorbed solar radiation and emitted heat radiation sets the temperature of Earth. For example, when heat radiation slows, the balance can be maintained only if Earth's temperature rises.