(COMTEX) Earth Brightens After Years of Dimming ( AP Online )
WASHINGTON, May 27, 2004 (AP Online via COMTEX) -- 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.
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On the Net:
Science: sciencemag.org
By RANDOLPH E. SCHMID
Associated Press Writer
Copyright 2004 Associated Press, All rights reserved
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