Earth Brightens After Years of Dimming

By RANDOLPH E. SCHMID
Associated Press Writer

May 27, 2004
WASHINGTON

Scientists studying earthshine -- the amount of light reflected by the Earth -- say the planet appeared to dim from 1984 to 2001 and then reversed its trend and brightened from 2001 to 2003.

The shift appears to have resulted from changes in the amount of clouds covering the planet. More clouds reflect more light back into space, potentially cooling the planet, while a dimmer planet with fewer clouds would be warmed by the arriving sunlight.

That means the changes in brightness could signal climate change, though it's too early to tell.

Steven Koonin, a California Institute of Technology physicist and co-author of the paper, said that "at the moment, the cause of these variations is not known, but they imply large shifts in the Earth's radiative budget. Continuing observations ... will be necessary to learn their implications for climate."

"This work is probably going to be used in arguments for and against global warming. Our paper neither proves or disproves the carbon dioxide effect," said Enric Palle, lead author of the report appearing in Thursday's issue of the journal Science.

"Our results are only part of the story, since the Earth's surface temperature is determined by a balance between sunlight that warms the planet and heat radiated back into space, which cools the planet," said Palle, of the Big Bear Solar Observatory in California, operated by the New Jersey Institute of Technology.

Climate change "depends upon many factors in addition to (reflected light), such as the amount of greenhouse gases present in the atmosphere. But these new data emphasize that clouds must be properly accounted for and illustrate that we still lack the detailed understanding of our climate system necessary to model future changes with confidence."

The researchers used two sets of records to establish the amount of light reflected from the Earth.

The records, which partly overlap, include measurements of cloud cover taken by satellites and an analysis of earthshine, which was determined by studying how much it illuminates the dark portion of the moon.

But the use of two separate types of measurements gave pause to James A. Coakley Jr. of Oregon State University, who studies climate change and satellite cloud data.

Observations of "sunlight reflected by the Earth are far from being well understood. At this stage, it's too early to tell how useful such observations might be as a measure of climate variability and climate change," said Coakley, who was not part of the research team.

Philip R. Goode of the New Jersey institute, a co-author of the paper, contended that the moon analysis is in fact quite accurate.

"Our method has the advantage of being very precise because the bright lunar crescent serves as a standard against which to monitor earthshine, and light reflected by large portions of Earth can be observed simultaneously," said Goode.

Earthshine brightening the face of the moon, he noted, was first described by Leonardo da Vinci.

Regular earthshine observations began in 1997, and the researchers suggested that the changes they observed may be part of a natural variation. Continuing the observations through an entire 11-year cycle of solar variability will be important to better understand the changes, they said.

The research was funded by the National Aeronautics and Space Administration.