Scientists studying 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 change 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.
The researchers, from the New Jersey Institute of Technology and the California Institute of Technology, 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." Earthshine, the reflection from Earth, 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 of Technology, 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 DaVinci.
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.
Steven Koonin, a CalTech physicist and co-author of the paper, said that the cause of the variations wasn't known and that continuing observations may help determine implications for the Earth's climate.
The research was funded by the National Aeronautics and Space Administration.