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1993-05-03
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PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Diane Ainsworth
FOR IMMEDIATE RELEASE September 12, 1991
The Upper Atmosphere Research Satellite (UARS), launched
Thursday, Sept. 12 by the space shuttle Discovery, carries one
experiment that will begin to measure changes in the amount of
sunlight falling on Earth's surface, oceans and atmosphere.
The experiment, known as the Active Cavity Radiometer
Irradiance Monitor (ACRIM-II), was built at NASA's Jet Propulsion
Laboratory. ACRIM's investigation of the sun's influence on
Earth's atmosphere and climate will begin this fall and continue
through the duration of the satellite's life span.
"The Earth's climate is determined by how much of the sun's
radiant energy -- the primary source of heat and light -- falls
on the planet's surface, oceans and atmosphere," said Dr. Richard
Willson, principal investigator of the ACRIM-II instrument.
"Changes in the amount of sunlight reaching Earth's
atmosphere are important in understanding climate trends that are
caused by natural phenomena and those that are the result of an
increasingly industrialized society."
Radiation from the sun is the dominant influence on
atmospheric and climatic processes, said Willson, who works in
JPL's Atmospheric and Cometary Sciences Section.
The total output of energy by the sun -- called solar
luminosity -- was long thought to be constant, but is now known
to vary.
1
"Sunspots, for instance, cause a decrease in total solar
output because the spots are cooler than the sun's undisturbed
photosphere," Willson said.
"The relative energetics of this decrease are monumental on
the human scale," he said. "The first sunspot effect we observed
in 1980 decreased the total amount of energy striking Earth by
0.1 percent over a one-week period. That decrease is equal to
the total amount of energy produced and used by the worldwide
population in one year."
Changes in the amount of sunlight reaching Earth's
atmosphere were first documented after launch of the first Active
Cavity Radiometer Irradiance Monitor (ACRIM I) experiment on
NASA's Solar Maximum Mission in 1980.
ACRIM I was the first flight experiment dedicated to the
task of monitoring both short- and long-term variations of the
sun's output on climate change. ACRIM I provided a nearly
continuous record of those changes, detecting variations ranging
from time scales of just minutes to the nine-year lifetime of the
satellite.
The second-generation ACRIM experiment will also tell
scientists more about concentrations of the so-called "greenhouse
gases" -- carbon dioxide, methane and nitrous oxide -- that have
built up in Earth's atmosphere and could give rise to a world
several degrees hotter.
Changes in the sun's 11-year cycles of activity have a
significant effect on the greenhouse warming trend, Willson
pointed out.
"We discovered the sun's ability to mask the greenhouse
effect in the mid-1980s, when activity on the sun was low and
solar luminosity decreased," he said. "The degree to which a
decrease in total solar output might diminish the greenhouse
effect, or an increase exacerbate it, is an important issue in
predicting and preparing for climate fluctuations in the next
century."
ACRIM II is a component of the U.S. Global Change Research
Program, designed to address the need to compile a long-term
database on the impact of solar energy on Earth's climate.
The Upper Atmosphere Research Satellite, managed by the
Goddard Space Flight Center in Greenbelt, Md., is the first in a
series of satellites in NASA's Mission to Planet Earth, a program
to study the natural and human-induced changes in Earth's
atmosphere from space.
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#1388/JPL-PIO