David E. Steitz

Headquarters, Washington, DC

(Phone: 202/358-1730)

Allen Kenitzer

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/286-2806)

RELEASE: 00-151

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 Journal of

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,”
says 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 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 include a combination of
three new satellites, in addition to those in operation today.
This will provide details on the three-dimensional structure of
our atmosphere to better understand the role of clouds and
aerosols on the Earth’s energy balance and climate,” Asrar notes.
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.