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Article by Richard Harris (from NPR).

Carbon dioxide is the gas we most associate with global warming, but methane gas also plays a very important role. For reasons that are not well understood, methane gas stopped increasing in the atmosphere in the 1990s. But now it appears to be once again on the rise. Scientists are trying to understand why — and what to do about it.

Methane gas comes from all sorts of sources including wetlands, rice paddies, cow tummies, coal mines, garbage dumps and even termites. Drew Shindell, at NASA's Goddard Institute in New York, says, "It's gone up by 150 percent since the pre-industrial period. So that's an enormous increase. CO2, by contrast, has gone up by something like 30 percent."

Molecule for molecule, methane is much more effective than carbon dioxide at trapping heat in the atmosphere. And that's just part of the trouble.

"Methane is much more complicated once it gets into the atmosphere than something like carbon dioxide is," Shindell says, "and that's because it reacts with a lot of different important chemicals."

Bad For Climate And Health

For example, methane in the atmosphere also creates ground-level ozone. And ozone isn't only bad for human health; it also contributes to global warming. Shindell recently totaled up all the effects of methane emissions and realized that the heating effect is more than 60 percent that of carbon dioxide's.

"So that tells you that methane is a pretty big player."

Methane in the atmosphere leveled off in the 1990s, so it seemed that efforts to control industrial emissions were keeping this problem gas in check. But since 2007, methane levels have been on the rise again.

Wetlands Cause Vicious Cycle

A study published last week in Science magazine suggests that at least part of this increase is coming from the vast wetlands in Canada, Russia and the Arctic. The methane in wetlands comes from naturally occurring bacteria. But study author Paul Palmer at the University of Edinburgh says the bacteria are producing more methane because the temperature is rising.

"The higher the temperature, the more efficient they are at producing methane," he says. So global warming is causing these wetlands to produce more methane. And the methane is causing more global warming.

"This really does demonstrate the fact that we are having this vicious cycle in the climate system. And we're seeing it now."

It's not yet to the stage where it's a runaway warming effect, Palmer says. But climate scientists are worried that we could hit that tipping point.

There's no obvious way to control methane from natural wetlands other than to keep them from overheating. But at least half of methane emissions are from human activities, ranging from cattle-rearing and natural gas exploration to coal mining.