Estimates suggest that about a third of current annual methane emissions come from wetlands, forest and grass fires, oceans, and other natural sources.
The remaining two-thirds reportedly come from the production and transportation of oil and natural gas, coal mining, rotting landfills, sewage, belching livestock, and other human-influenced sources.
Fortunately, chemical reactions powered by ultraviolet radiation remove more than 90 percent of the emitted methane in the atmosphere.
Using data from ground-based air sampling stations around the globe, the researchers found that a complex array of factors controlled year-to-year variability in methane levels.
Among natural sources, this variability was most heavily influenced by high or low levels of wetland emissions linked to prolonged wet or dry periodsnot forest fires, as some studies have suggested.
The scientists also speculate that the slow growth in methane emissions during the 1990s may have been triggered, in part, by the industrial collapse of the former Soviet Union and efforts to reduce landfill methane emissions in North American and Europe.
Starting in 1999, however, human sources of methane started rising again, particularly in northern Asia.
The uptick coincides with China's economic boom and that country's growing fossil fuel consumption. But Bousquet said his study "cannot constitute a proof, but [it] urges [others] to perform direct estimates of emissions in Asia."
Christopher Field studies the global carbon cycle at the Carnegie Institution's Department of Global Ecology at Stanford University in Palo Alto, California.
The ecologist said that the study shows that "the growth rate of methane in the atmosphere is really strongly dependent on what happens in ecosystems" and is likely to increase again because of rising human-made emissions.
(Related: "The Big Thaw" from National Geographic magazine.)
Climate Quick Fix?
Jos Lelieveld, an atmospheric chemist at the Max Planck Institute for Chemistry in Mainz, Germany, says that when it comes to global warming, methane does offer a silver living.
"It will be very difficult to control carbon dioxide in the short-term future. Not only because of energy [consumption] and political consequences but also because the lifetime of carbon dioxide is enormously long," he said.
"And so if climate change is really affecting our daily lives in a really undesirable waylike stronger hurricanes or more droughts in some areasthen there may be the desire to do something on the short term," Lelieveld added.
"One of the few alternatives that we have there is to reduce methane," he said.
The scientist says that given today's high energy prices, methane from landfills, coal mines, and oil and gas production facilities can be cost-effectively captured and burned to produce electricity and carbon dioxide.
Producing carbon dioxide may not seem like a lofty goal, but in this case it may be the lesser of two evils. CO2 traps 95 percent less heat than methane per molecule emitted.
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES