Somehow, North America seems to be soaking up a lot of the carbon scientists had expected to find in the atmosphere.
Carbon dioxide in the air has been increasing, but not as rapidly as expected by researchers studying the threat of global warming. So they have been trying to figure out where the excess was going.A report in Friday's edition of the journal Science concludes that much of this missing carbon is being absorbed in North America - possibly by regrowth over abandoned farms and previously logged forests.
The absorption could total between 1.2 billion and 2.2 billion tons annually, a "substantial portion" of the carbon being added to the atmosphere, according to the government and academic researchers.
Even before it was published, however, the paper was generating controversy.
"I don't believe this result," said David Schimel, an ecologist at the National Center for Atmospheric Research in Boulder, Colo. Other studies indicate the amount of carbon taken up by North America can be no more than 700 million tons, he said.
Environmentalists worry that groups opposed to the 1997 Kyoto climate treaty will use the findings to argue that the United States does not need to reduce its emissions of so-called greenhouse gases.
"There is a huge concern that this result will be misinterpreted," Schimel said in a telephone interview.
Pieter Tans, one of the scientists who worked on the paper, admitted that the uncertainties are still large. "This is not ironclad. We say in the paper the evidence is still somewhat tentative," he said.
But, he said, "we do think that we have used good models. We think we've used data in a proper way."