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Caltech scientists looking to the moon for insights into the Earth's climate say they have observed unexpectedly large fluctuations in the planet's cloud cover over the last two decades.

Contrary to computer models of the climate system, the team found that Earth's average albedo (the amount of light reflected back into space, relative to the amount landing on it) is not constant from one year to the next, nor is it constant over decadal timescales.

The researchers combined eight years of 'earthshine' data - the amount of light Earth reflects on to the dark side of the moon, directly linked to how cloudy the planet is - with twenty years of satellite cloud data. The observation data were collected at the Big Bear Solar Observatory (BBSO) in California.

They found that the Earth's reflectiveness, or the amount of cloud cover, declined steadily from 1985 to 1995, and fell even more dramatically between 1996 and 1997. This low albedo remained roughly constant until 2001. This same period saw a massive increase in solar heating of the planet and as a consequence, an accelerated increase in mean global temperatures.

The last three years, however, have seen this decline largely reversed, and the Earth's albedo is now similar to pre 1995 levels, consistent with much cloudier skies.

Enric Palle, the lead scientist on the project acknowledged that climate is an extremely complex system. He explains that the Earth's surface temperature is determined by a balance between the warming effect of sunlight and the cooling effect of heat radiated back into space. This balance is determined by many factors in addition to albedo, such as the amount of greenhouse gases present in the atmosphere.

"Our results are only part of the story," he said. "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 team has ambitious plans to continue its research, and is working to establish a global network of observing stations that would allow continuous monitoring of the albedo throughout the lunar month, regardless of local weather conditions. BBSO data are already supplemented by observations from Crimea in the Ukraine, and there will soon be observations from Yunnan in China, too.

The team is building a prototype robotic telescope to automate observations. Ultimately the researchers want to build and deploy eight of these fully automated observational posts around the globe. ®

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