Do clouds provide a greenhouse thermostat? – Scripps Institution of Oceanography research

Do clouds provide a greenhouse thermostat? – Scripps Institution of Oceanography research – Brief Article

Richard Monastersky

Earth’s oceans have a natural thermostat that — at least for the present — keeps their surface waters from warming above 32’C. Understanding that thermostat may be critical for predicting how the climate will change, but new research shows that the problem does not yield to a simple explanation.

Last year, hope for a straightforward answer ran high. V Ramanathan and William Collins of the Scripps Institution of Oceanography in La Jolla, Calif., proposed that cirrus clouds regulate ocean surface temperatures in the tropical Pacific (SN: 5/11/91, p.303).

This week, however, a group of atmospheric researchers disputes the cirrus thermostat model. “Our contention is that clouds are really not the story,” says Anthony D. Del Genio of NASA’s Goddard Institute for Space Studies in New York City a coauthor of the study led by Rong Fu of the University of California, Los Angeles. They report their work in the July 30 NATURE.

Ramanathan and Collins presented satellite data suggesting that cirrus clouds helped limit ocean temperatures in 1987, when an El Nino warming brewed in the equatorial Pacific. The data showed that as the sea surface warmed in the central and eastern equatorial Pacific, cirrus clouds reflected more sunlight back into space, cutting down the light reaching the ocean surface.

Fu and her colleagues counter with evidence that sea surface temperatures do not control cirrus cloud formation. Using satellite data for the whole tropical Pacific, they find that cirrus clouds across this broad region remained nearly normal during the 1987 El Nino, even though sea surface temperatures rose by 2 [degrees] C to 3 [degrees] C through the area, says Del Genio. Looking at years without El Nino warmings, the researchers found enhancements in cirrus clouds even when ocean temperatures did not increase, indicating that some other factor controlled the cloud properties.

If cirrus clouds don’t cap ocean temperatures, evaporation may, suggest the researchers. Water evaporating from the ocean releases heat into the atmosphere, cooling off the sea surface, just as evaporating sweat chills a person’s skin. Ocean circulation could also help regulate temperatures by carrying warm water away from the tropics, Del Genio says.

Ramanathan and Collins contend they are not through yet. Collins told SCIENCE NEWS that several groups have papers pending that contradict the conclusions of Fu and her co-workers. Part of the discrepancy arises because Fu’s group looked at the entire tropics, effectively diminishing the importance of the El Nino warming near the equator, Collins says.

The debate is important because the tropical Pacific has Earth’s strongest greenhouse effect and one most sensitive to change. Climate experts need to understand the tropical thermostat in order to predict how it will function when global temperatures increase. A major international experiment planned for later this year in the western Pacific should provide some of the needed information.

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