 |
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 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.
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.
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.
Back
to Top
|
