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.

"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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