Ocean Clouds May Limit Warming

NEW YORK —  Oceans might be able to limit how warm they get at the surface by creating high-altitude clouds to provide some shade, researchers say.

As a result, oceans may not get any warmer than about 89 degrees Fahrenheit as a monthly average, the study says.

Scientists said it is not clear whether that effect would act against global warming, the expected result of a buildup in the atmosphere of heat-trapping gases.

The work is presented in Thursday’s issue of the British journal Nature by Veerhabdran Ramanathan and William Collins of the Scripps Institution of Oceanography in La Jolla.

The cloud-shading effect had been suggested before, but the study provides experimental evidence, Collins said in a telephone interview.

The scientists studied the response of the tropical Pacific Ocean in 1987 to an El Nino effect, a recurring event in which the tropical Pacific warms temporarily. They used data from satellites and ships.

Their analysis found evidence of what they called a “super greenhouse effect,” in which the atmosphere trapped heat more efficiently when the ocean warmed beyond about 80 degrees Fahrenheit as a monthly average. The reason for that is unclear, Collins said.

They also found that beyond that temperature, the ocean’s heat triggered a process in which moisture-rich air rose to create highly reflective cirrus clouds. The clouds provided shade for the ocean below, limiting further heating of the ocean, the scientists said.