Thin, high cirrus clouds may help regulate global temperature and serve as a counter to global warming, theorize a team of scientists led by Richard Lindzen of the Massachusetts Institute of Technology.

Their study in the March 2001 Bulletin of the American Meteorological Society examines the behavior of high cirrus clouds over a large section of the western tropical Pacific Ocean. The scientists say cirrus clouds operate much as the "iris" of an eye regulates the admission of light. The clouds open in response to rising surface temperature, permitting cooling. The clouds close when the surface temperature cools to retain heat.

• The study finds that high cirrus clouds decrease in thickness by about 22 percent per one degree Celsius increase in sea surface temperature; conversely, the clouds thicken when the sea surface temperature is lower.

• That is because thick clouds reflect more sunlight than thin clouds back into space and help mitigate surface warming; thin clouds, conversely, don't deflect as much sunlight but are efficient in trapping heat at the surface.

• Most intriguing, a 22 percent decrease in cirrus cloud cover also leads to a significant decrease in sea surface temperature of about 1.1°C.

According to some climate model forecasts, a doubling of carbon dioxide in the atmosphere would lead to a 1.2°C temperature increase. But the existence of the atmospheric heat "vent" should change that prediction to between 0.57° and 0.83°C.

The study's authors say these findings require climate modelers to scale back by as much as two-thirds the projected warming resulting from a doubling of carbon dioxide.

Source: John Carlisle, "Natural Heat Vent May Counter Global Warming," National Policy Analysis No. 336, May 2001, National Center for Public Policy Research, 777 N. Capitol St. N.E., Suite 803 Washington, D.C. 20002,(202) 371-1400; based on Richard S. Lindzen, Ming-Dah Chou and Arthur Y. Hou, " Does the Earth Have an Adaptive Infrared Iris?" Bulletin of the American Meteorological Society, March 2001.

For NCPPR text http://www.nationalcenter.org/NPA336.html