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Do trees share blame for global warming?

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Cows burp it, pipelines and landfills leak it, and vast amounts lie frozen beneath the ocean floor. Methane is ubiquitous - as fuel for heating and cooking and as a source of concern for atmospheric scientists. Molecule for molecule, methane packs thousands of times more punch as a "greenhouse gas" than carbon dioxide does.

Until now, scientists tracking debits and credits in the globe's methane "budget" figured they had a pretty good handle on where the gas comes from - mostly from microbes breaking down organic material in places where oxygen is relatively scarce.

Enter Frank Keppler. Working with colleagues from Northern Ireland and the Netherlands, Dr. Keppler has discovered that plants may give off significant amounts of methane just by growing. And the amount they give off appears to rise with temperature. The results have stunned many researchers because no one expected methane to form biologically out in the open air, where oxygen abounds.

It's not that there's more methane in the atmosphere, but that some of it is coming from a wholly unexpected source. The results imply that, at best, this new source of methane may need to be taken into account as nations try to curb carbon-dioxide emissions by planting trees. Would increased methane emissions erase the gains against CO2? At worst, the results imply that thawing tundra in the Arctic is not the only worrisome source of methane in a warming world.

The experiments Keppler and his colleagues performed grew out of the team's effort to measure the gases that plants give off only in tiny amounts. When they looked at emissions from dead leaves, "we saw a pattern of methane" along with other gases, says Keppler, a scientist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Others had detected methane from rice plants, but thought the rice merely acted as a minipipeline for methane formed in the muck in which rice grows.

Keppler put fresh and dried plant materials, as well as young plants, in special chambers. He removed possible sources of contamination - including microbes - and found that from bananas and sugar cane to European ash and Spanish moss, the material yielded methane.

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