Wetlands provide clue to greenhouse gas.

Two researchers say they have answered a question that stumped scientists for years: What key factor determines how much methane -- a greenhouse gas that warms Earth by trapping heat -- the different types of wetlands emit? To find out, the researchers slogged through subarctic peat bogs in Canada and subtropical swamps in Florida, monitoring atmospheric gas levels. Their answer: Methane emissions depend on the total amount of carbon dioxide exchanged between the atmosphere and plants.

Identifying this factor provides a tool to measure methane emissions on a global scale and to pinpoint major sources of emissions, says Gary J. Whiting, a biologist at Christopher Newport University in Newport News, Va., who led the study. These measurements should prove valuable because, although the amount of atmospheric methane has more than doubled in the past 100 years, scientists do not know exactly where it all originates. They do know, however, that too much methane can contribute to global warming.

Whiting and Jeffrey P. Chanton, a chemical oceanographer at Florida State University in Tallahassee, report their findings in the Aug. 26 NATURE.

"They established a quantitative link between the total amount of plant growth and the amount of methane produced," says Robert Harriss, an earth systems scientist at the University of New Hampshire in Durham. Remote-sensing satellites can use this formula to indirectly map methane emissions, he explains. Satellites can directly measure the amount of biomass growing in an ecosystem. Correlating this figure to the amount of carbon dioxide exchanged and will reveal the volume of methane emissions from natural sources.

Methane comes from two major sources: natural, including wetlands, oceans, and termites; and human activities, including coal mining, landfills, rice farming. Natural wetlands, which make up just 5 percent of Earth's land surface, play a disproportionately large role in methane emissions. Bacteria that decompose organic material into methane thrive in such flooded, oxygen-starved soils.

Scientists estimate that wetlands contribute up to one-half the methane emitted into the atmosphere--a total of between 100 and 200 million metric tons a year. "Satellite-generated methane maps can be used to narrow our uncertainty," Harriss says.

The same technique could help analyze rice paddy emissions, Whiting says. As rice became more important in feeding the world, farmers converted more land to these shallow fields of grain, which now emit up to 150 million metric tons of methane each year.

Whiting and Chanton tallied their gas measurements on a daily basis; seasonal and yearly tabulations would provide an even more accurate formula, they say. Harriss adds that measurements of vast, remote wetlands -- the Siberian lowlands and the deep-peat swamps of Borneo and Sumatra--would complete the picture.