Methane Emissions Rising (But You Wouldn't Know It), Study Says

Sean Markey
for National Geographic News
September 27, 2006
Human-caused methane gas, a contributor to global warming, is on the rise in Earth's atmosphere, a new study says. But declines in methane emissions from wetlands are painting a deceptively rosy picture, researchers say.

After rising during the 1980s, the growth of methane in the atmosphere slowed during the 1990s, providing a bright spot in an otherwise gloomy global warming forecast.

And while methane concentrations in Earth's atmosphere have remained stable, the study suggests the news isn't as good as scientists once thought.

(Rekated: "Global Warming Feedback Loop Caused by Methane, Scientists Say" [August 29, 2006].)

Writing in tomorrow's issue of the journal Nature, researchers say that human-caused methane emissions began rising again in 1999, possibly tied to China's booming economy.

The increase has been masked, however, by a drop in methane emissions from wetlands as they dried out, especially in the Northern Hemisphere and possibly due to global warming.

"Atmospheric methane levels may increase again in the near future if wetland emissions return to their mean 1990s levels," the authors write.

Philippe Bousquet, a physicist and chemist at the University of Versailles and the Laboratory of the Sciences of the Climate and the Environment in Gif-sur-Yvette, France, co-authored the study.

"It's important to know if methane is going to stabilize or to increase again, because methane is about 20 times more efficient [at trapping heat] in the atmosphere than CO2 [carbon dioxide, another greenhouse gas]," he said.

"So if it continues to increase, it can have some climate change consequences," he added.

Where Methane Comes From

Since preindustrial times the amount of methane in Earth's atmosphere has nearly tripled.

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 periods—not 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 way—like stronger hurricanes or more droughts in some areas—then 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.

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