The study, led by Britton Stephens of the National Center for Atmospheric Research, concluded that intact tropical forests are removing an unexpectedly high proportion of carbon dioxide from the atmosphere, thereby partially offsetting carbon entering the air through industrial emissions and deforestation.

"This research fills in another piece of the complex puzzle on how the Earth system functions," said Cliff Jacobs of the National Science Foundation. "These findings will be viewed as a milestone in discoveries about our planet's 'metabolism.'"

Stephens and colleagues analyzed air samples collected by aircraft around the world for decades and found some 40 percent of the carbon dioxide assumed to be absorbed by northern forests is instead being taken up in the tropics.

"Our study will provide researchers with a much better understanding of how trees and other plants respond to industrial emissions of carbon dioxide, which is a critical problem in global warming," Stephens said. "This will help us better predict climate change and identify possible strategies for mitigating it."

The study is reported in Science magazine.