Enric Pallei and colleagues at the New Jersey Institute of Technology and the California Institute of Technology, both in California, used satellite measurements and recordings of sunlight that bounced off Earth and hit the dark side of the moon for their study.

The satellite pictures of global cloud cover suggest a steady decrease in Earth"s reflectance, called albedo, between 1984 to 2000, Pallei's team writes in Friday's issue of the journal Science.

"Using a phenomenon first explained by Leonardo da Vinci, we can precisely measure global climate change and find a surprising story of clouds," researcher Philip Goode said in a statement.

"Our method has the advantage of being accurate 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. It is also an inexpensive method, requiring only a small telescope and a relatively simple electronic detector."

The annual average albedo declined very gradually from 1985 to 1995, then fell sharply in 1995 and 1996. This fit in with satellite data of actual cloud cover, which can be affected by things such as volcanic eruptions.

This could affect the Earth's surface temperature, as cloud cover holds in heat.

"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," Pallei said in a statement.

"This depends upon many factors in addition to albedo, such as the amount of greenhouse gases - water vapor, carbon dioxide, methane - present in the atmosphere. But this new data emphasizes that clouds must be properly accounted for and illustrates that we still lack the understanding of our climate system necessary to model future changes with confidence."