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NASA Scientist Predicts Less Climate Cooling from Clouds
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 Oct. 1 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.
Clouds not only yield rain, but also play an important role in regulating the Earth's temperature. The determining characteristic is called albedo. It's a measure of how much radiation, or light, is reflected from a body. Similar to how a white shirt helps keep a person cooler in the summer than a black shirt, the cumulatively vast area of cloud cover around the world reflects large amounts of solar radiation falling on the planet's surface. If the reflective or insulative properties of clouds changed significantly, sunlight that otherwise would have been reflected back into space would get absorbed by the darker, denser mass of ocean and land.
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 °F (1.5 °C) — should be increased by at least 1 °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 °F (1.5-4.5 °C). The IPCC will be issuing its updated assessment early next year.
The finding is based on more than 3000 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.
References
Del Genio, A.D., and A.B. Wolf 2000. The temperature dependence of the liquid water path of low clouds in the southern Great Plains. J. Climate 13, 3465-3486, doi:10.1175/1520-0442(2000)013<3465:TTDOTL>2.0.CO;2.
Tselioudis, G., W.B. Rossow, and D. Rind 1992. Global patterns of cloud optical thickness variation with temperature. J. Climate 5, 1484-1495.
This article was derived from a NASA Goddard Space Flight Center Top Story.
