http://news.excite.com/news/ap/010417/15/earthshine

Earth Glow Used To Monitor Climate

Updated 3:32 PM ET April 17, 2001

By RANDOLPH E. SCHMID, Associated Press Writer

WASHINGTON (AP) - Scientists are using our planet's ghostly reflection on the dark moon to help monitor climate.

The glow, known as earthshine, helps researchers measure how much sunlight the Earth is reflecting, an important factor in determining global change.

During a crescent moon, earthshine can be viewed with the naked eye, looking at the shaded portion of the moon. Leonardo da Vinci first explained the phenomenon, in which the moon acts as a giant mirror, showing the sunlight reflected from Earth.

Scientists from the New Jersey Institute of Technology and the California Institute of Technology report in the May 1 issue of Geophysical Research Letters that they used earthshine to measure the Earth's reflectance. They concluded that the fraction of sunlight the planet reflects is currently 29.7 percent.

"Earth's climate is driven by the net sunlight that it absorbs," says Philip R. Goode, leader of the New Jersey Institute of Technology team.

"We have found surprisingly large - up to 20 percent - seasonal variations in Earth's reflectance," he said.

If Earth reflected even one percent less light on average, the effect could be significant enough to be a concern with regard to global warming, the scientists said. Less light reflected could mean more absorbed on Earth, resulting in warmer temperatures.

In the early 20th century, the French astronomer Andre-Louis Danjon undertook the first quantitative observations of earthshine. But the method lay dormant for nearly 50 years, until Caltech professor Steven E. Koonin described its potential in a 1991 paper.

The new measurements are based on about 200 nights of observations of the shaded area of the moon at regular intervals over a recent two-year period, and another 70 nights during 1994-95.

The study relies on averages over long periods, because the reflectance changes substantially from night to night with changing weather, and even more dramatically from season to season with changing snow and ice cover.

The data suggest that the reflectance has decreased slightly during the past five years, as the Sun's magnetic activity has climbed from solar minimum to maximum during that time.

Many scientists have noted changes in climate that seem to mimic the 11-year solar cycle and this research could support the theory that the sun's magnetic field plays an indirect role in Earth's climate.

-- Tidbit (of@the.day), April 18, 2001