After Years of Dimming
Associated Press Writer
May 27, 2004
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
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
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.