On the rise: Siberian lakes: major sources of methane.

Recent warming in Siberia is liberating carbon that had been locked in the permafrost for millennia. Consequently, Siberian lakes are making a surprisingly large contribution to atmospheric methane, a planet-wanning greenhouse gas, according to a new analysis of the methane bubbling from those lakes.

Previous studies had suggested that most northern Siberian lakes sit atop permafrost containing rich soil. Methane-making microbes in the oxygen-poor lake-bottom sediments render the lakes major methane sources, says Katey M. Walter, an ecologist at the University of Alaska in Fairbanks.

Researchers had assumed that most methane from a northern Siberian lake emanated at the same rate from all points on the lake's surface, says Walter. However, in winter field studies, she and her colleagues noted large volumes of methane gas trapped below surface ice. These gas bubbles supplement the even diffusion.

Earlier studies of lakes in the region had noted bubbles of methane but hadn't considered them important, says Walter.

After identifying several locations where methane bubbles accumulated in the winter, Walter and her colleagues deployed underwater bubble traps to measure the gas produced. They found that over the course of a year, these locations were releasing about 20 times as much methane as was lakewide diffusion.

The new finding boosts the total estimate of annual methane emissions from all northern Siberian lakes to 3.8 million metric tons, an increase of as much as 63 percent over past estimates, the researchers report in the Sept. 7 Nature.

"It's remarkable that methane bubbles from the lakes with such high intensity," says John Hobbie, an ecosystem scientist at the Marine Biological Laboratory in Woods Hole, Mass.

One factor contributing to that intensity is climate warming in northern Siberia between 1974 and 2000, says Walter. During that period, large amounts of carbonrich permafrost melted, which made the lakes expand to cover about 14 percent more area. That expansion alone has boosted methane emissions in the region about 58 percent, suggests the researchers' model of methane emissions.

Walter and her colleagues observed that most of the methane arose from the lake bottom within 15 meters of shore. So, the gas probably comes from sediment-dwelling microbes feasting on organic material that had recently eroded into the expanding lakes. Carbon dating of the methane indicates that the organic material had been locked away for 20,000 to 40,000 years.

About 500 billion metric tons of carbon remain locked in the permafrost soils of northern Siberia, says Ted Schuur, an ecologist at the University of Florida in Gainesville. That's almost two-thirds of the amount now wafting around Earth's atmosphere as carbon dioxide, methane, and other planet-warming gases, he notes.

Walter says that as the warming expected in coming decades makes lakes expand further and permafrost melt even more, microbes could release as much as one-third of northern Siberia's carbon in planet-heating methane. "It's a [climate] time bomb waiting to go off," she says.