Clouds keep ocean temperatures down.

Wispy cirrus clouds high in the sky's frozen reaches seem insubstantial to the eye, but these collections of ice particles act like a thermostat that keeps temperatures in the Pacific ocean from reaching into the red zone, according to two atmospheric researchers.

In most parts of the globe, the ocean surface does not warm above roughly 31 [degrees]C, and scientists have long sought to understand what process sets that limit. Looking for an answer, V. Ramanathan and William Collins of the Scripps Institution of Oceanography in La Jolla, Calif. began studying the El Nino phenomenon, a natural warming in the tropical Pacific ocean that occurs every two to six years. Focusing on an El Nino in 1987, the researchers compared measurements of sea surface temperatures with satellite observations of energy entering and exiting Earth's atmosphere. In the May 2 NATURE, they describe a complex series of changes that -- for the present -- cap the Pacific's temperature.

When the ocean surface warms at the start of the El Nino, evaporation increases, causing the atmosphere's water-vapor content to rise dramatically. The extra vapor strengthens Earth's greenhouse effect, causing ocean surface temperatures to climb even more. If allowed to continue, the researchers say, this self-perpetuating "super greenhouse" could dramatically warm the ocean surface. Ramanathan and Collins find, however, that the additional water vapor and increased convection create massive cirrus clouds that block sunlight and shut down the Pacific super greenhouse.

The new-found cirrus effect could play an important role in the future as greenhouse-gas pollution warms the planet. But because so many interacting elements weave together in the climate system, scientists cannot tell whether such clouds will weaken or boost the warming, says Andrew J. Heymsfield of the National Center for Atmospheric Research in Boulder, Colo.