RELEASE: H00-151

Don't count on clouds to come to the rescue if the Earth's  current climate-warming trend continues. That's according to new  NASA research published in the October 1st issue of the American  Meteorological Society's Journal of Climate.

Heating and cooling of the Earth are influenced by cloud cover.  Clouds can act as a natural sun shield by reflecting light back  into space. But clouds can also coat the skies like a blanket,  trapping warmth.

Precisely how these competing attributes will change in response  to a warmer atmosphere is not well understood. Anthony Del Genio  of NASA's Goddard Institute for Space Studies in New York City,  analyzed three years of observations of low clouds over land, a  type of cloud thought likely to contribute to future cooling. 

Some climate theories predict that a warmer atmosphere would  evaporate more water, and this additional water vapor would form  thicker clouds. However, Del Genio's research found that when air  temperatures were higher, clouds were thinner and thus less  capable of reflecting sunlight. These thinner clouds occurred  regardless of weather conditions, season, or time of day. 

"The bottoms of the clouds rise with warmer temperatures and the  clouds become thinner," Del Genio explains. "When low clouds are  present, warmer air flowing over land tends to be drier. As a  parcel of dry air rises, it has to rise farther before it  saturates with enough water to form the cloud base."

How much warmer will the climate become? Del Genio believes a  theory that rising carbon dioxide levels would have only a slight  impact on global temperatures is flawed because it doesn't take  into account real-world cloud behavior. 

"The minimum amount of warming predicted by scientists - 3 degrees  Fahrenheit (1.5 degrees Celsius) - should be increased by at least  1 degree F as a result of the new findings," Del Genio asserts.  The current range of 21st century warming, according to the  Intergovernmental Panel on Climate Change (IPCC), is 3-8 degrees F  (1.5-4.5 degrees C). The IPCC will be issuing its updated  assessment early next year.

The finding is based on more than 3,000 individual cloud  "snapshots" collected between 1994 and 1997 at the Department of  Energy's (DOE) Atmospheric Radiation Measurement Southern Great  Plains field station in Oklahoma. Using a unique suite of ground- based and satellite instruments, each snapshot records the air  temperature, the height of the bottom and top of the cloud, and  the amount of liquid water in the cloud. The more liquid water in  a cloud and the thicker the cloud, the more opaque it is and the  more sunlight it reflects.

"Use of these data from the Department of Energy by NASA  researchers demonstrates the value of the United States Global  Change Research Program for studies of our global environment,"  said Dr. Ghassem Asrar, Associate Administrator for NASA's Office  of Earth Sciences, NASA Headquarters, Washington, DC. "This  program allows NASA to share in the wealth of data our sister  agencies gather, complementing satellite, air and ground data for  use by the whole Earth Sciences community. "

The relationship between cloud thinning and temperature was  initially observed in 1992 over much of the world with long-term  satellite observations. George Tselioudis, William Rossow and  David Rind of the NASA Goddard Institute for Space Studies  published the observation using the NASA-funded International  Satellite Cloud Climatology Project (ISCCP) database, a global  composite of cloud observations from international weather  satellites.

"Our new research demonstrates that the global observations of  cloud thinning with warming in the ISCCP data are valid in at  least one location," says Del Genio. "And the satellite data  suggest that this is not a phenomenon peculiar to the U.S. Great  Plains, but one that occurs in many parts of the world." Support  for the analysis of the research was provided by the Department of  Energy Atmospheric Radiation Measurement Program.

"For example, our plans for this decade includes a combination of  three new satellites, in addition to those operation today. This  will provide details on the three-dimensional structure of our  atmosphere so as to better understand the role of clouds and  aerosols on the Earth's energy balance and climate," Asrar said.  Future observations from NASA's PICASSO-CENA spacecraft, scheduled  for launch in 2003, will collect global measurements of cloud base  heights and may shed light on whether clouds in other parts of the  world also become physically thinner with warming.

This research is part of NASA's Earth Science Enterprise; a long- term research program designed to study the Earth's land, oceans,  air, ice and life as a total system. Information and images are  available at URLs: 

http://ametsoc.org/AMS/ 

NOTE TO EDITORS: For a copy of the complete research paper, "The Temperature Dependence of the Liquid Water Path of Low Clouds in the Southern Great Plains", by Anthony Del Genio and Audrey Wolf," contact any of the above mentioned press officers.