'Heat vent' may diminish global warming

Climate models may have to be revised

By BBC News Online science editor Dr David Whitehouse

The Pacific Ocean may open a "heat vent" above it that releases enough energy into space to reduce projected climate warming caused by the build-up of greenhouse gases in the atmosphere.

"High clouds over the western tropical Pacific Ocean seem to decrease when sea surface temperatures are higher," said Arthur Hou of the American space agency, Nasa. The mechanism allows the heat to escape, keeping the oceans cool.

To reach this conclusion, researchers analysed satellite observations over a vast ocean region stretching from Australia and Japan to the Hawaiian Islands.

The newly discovered vent, if confirmed, could significantly reduce estimates of future global warming now being put forward by computer models of the Earth's climate.

Cooling effect

Cirrus clouds - high-altitude clouds of ice crystals - typically form as a by-product of the life cycle of towering cumulus clouds created by updrafts of heated, moist air.

Rising sea temperatures trigger the effect

As these cumulus convective clouds grow taller, water droplets inside them collide and combine into raindrops that either fall out of the cloud or continue rising until they freeze into ice crystals forming cirrus clouds.

The researchers noticed that when the temperature of the sea was rising, the coverage of cirrus clouds decreased, allowing the release of more infrared energy into space. This had the effect of cooling the ocean.

For each degree Celsius rise in ocean surface temperature, the ratio of cirrus cloud area to cumulus cloud area over the ocean dropped by 17-27%.

Poor sunshields

"With warmer sea surface temperatures beneath the cloud, the coalescence process that produces precipitation becomes more efficient," explained Richard Lindzen of the Massachusetts Institute of Technology.

"More of the cloud droplets form raindrops and fewer are left in the cloud to form ice crystals. As a result, the area of cirrus cloud is reduced."

Clouds play a critical and complicated role in regulating the temperature of the Earth. Thick, bright, watery clouds like cumulus shield the atmosphere from incoming solar radiation by reflecting much of it back into space.

Thin, icy cirrus clouds are poor sunshields but very efficient insulators that trap energy rising from the Earth's warmed surface. A decrease in cirrus cloud area would have a cooling effect by allowing more heat energy, or infrared radiation, to leave the planet.

If the effect is found to be widespread, the Earth may be much less sensitive to the warming effects of rising greenhouse gas concentrations in the atmosphere.

Climate science

The researchers estimate that this effect could cut by two-thirds the projected increase in global temperatures initiated by a doubling of carbon dioxide in the atmosphere.

The International Panel on Climate Change (IPCC), the United Nations body which has done more than any other agency to publicise what it regards as the certainty of global warming, has itself conceded the importance of understanding cloud behaviour.

How clouds impact on climate is widely regarded as one of the great unknowns in atmospheric science.

Scientists from Nasa's Goddard Space Flight Center in Maryland and the Massachusetts Institute of Technology presented their research in the Bulletin of the American Meteorological Society.