yee@ames.arc.nasa.gov (Peter E. Yee) (02/07/89)
Charles Redmond
Headquarters, Washington, D.C. February 3, 1989
Keith Henry
Langley Research Center, Hampton, Va.
RELEASE: 89-14
EFFECTS OF CLOUDS ON CLIMATE MEASURED BY NASA PROJECT
The most accurate Earth radiation budget measurements ever
made, provided by NASA's Earth Radiation Budget Experiment (ERBE)
satellite, confirm that clouds result in a net cooling of the
Earth. Previously, atmospheric scientists were divided on the
impact of clouds on the Earth's temperature.
These measurements serve as the baseline for climate
modelers to determine whether clouds will partially offset or
enhance a future warming of the Earth due to the greenhouse
effect. The greenhouse effect is the term given to warming of
the Earth's atmosphere and surface resulting from increased
concentrations of gases, such as carbon dioxide, methane, nitrous
oxide and chlorofluorocarbons.
The major uncertainty in the current ability to assess the
future impact of greenhouse gases is the effect of clouds. ERBE
has given climate modelers some very specific numbers to test and
validate their models.
At any given time, clouds cover about 60 percent of the
Earth. Wispy, high-altitude cirrus (ice) clouds generally tend
to warm the surface. They reflect less solar energy back to
space than stratus and cumulus (liquid water) clouds.
Because they are so high in the atmosphere, ice clouds are
more efficient at trapping the infrared heat energy emitted by
the Earth and the atmosphere. Low-level water clouds not only
shade the Earth's surface from the Sun's rays better than high-
altitude ice clouds, they emit almost as much infrared radiation
as a cloud-free Earth. The net effect of clouds on surface
temperatures depends on how these different cloud types are
distributed over the globe.
The first of the three ERBE satellites was launched by the
Space Shuttle Challenger in October 1984. National Oceanic and
Atmospheric Administration (NOAA) weather satellites NOAA-9 and
NOAA-10 launched in December 1984 and September 1986,
respectively, also carried the ERBE instruments. ERBE provided
the first opportunity to measure the three components of the
Earth's radiation balance with identical instruments flying
simultaneously on separate satellites.
ERBE's instruments measured the solar radiation striking the
Earth and the solar radiation reflected by the Earth. These two
measurements determine how much solar energy is absorbed by the
Earth. The absorbed solar radiation heats the Earth's surface.
The infrared radiation emitted by the Earth and its atmosphere
also is measured. This emitted radiation cools the surface of
the Earth.
The ERBE measurements indicate that in April 1985, clouds
reflected back to space considerably more radiation than they
trapped in the atmosphere and the surface. This loss of
radiation due to the clouds is approximately equivalent to a 10
to 30 degrees F. decrease in the global surface temperature.
Thus, barring changes in any other variables, the Earth would be
about 20 degeees F. warmer without any cloud cover. In
comparison, climate modelers believe that a doubling of carbon
dioxide in the atmosphere would result in a warming of about 4 to
8 degrees F.
Global warming would change circulation patterns, altering
the climates and growing seasons of many regions over the
globe. Significant warming also would melt large amounts of ice
and snow in the polar regions, increasing the height of the
world's oceans. Changes in global circulation patterns would
also change the distribution of clouds over the globe. The ERBE
data also revealed for the first time the regional distribution
of cloud effects over the globe.
Langley scientists Patrick Minnis and Edwin F. Harrison
published the first paper in 1984 showing the regional cloud
effects over part of the Earth. They used data from a weather
satellite which only observed North and South America. The ERBE
satellites viewed all of the Earth with sensors much more
accurate than those normally carried on weather satellites.
ERBE is the first step in NASA's long-term program for
studying climate change and is both a national and international
effort supported by other government agencies and universities.
The ERBE satellite instruments were developed at Langley and
the TRW Corporation, Redondo Beach, Calif. Langley is
responsible for data analysis. ERBS was developed at NASA's
Goddard Space Flight Center, Greenbelt, Md., and Ball Aerospace,
Boulder, Colo. NOAA supplied the two weather satellites.eli@spdcc.COM (Steve Elias) (02/07/89)
for more information on this topic, refer to Science, 6 Jan 1989. -- Steve Elias (eli@spdcc.com);((617 239 9406)) *funk() /* C, not lisp */