Global warming is causing methane gas to bubble up from Siberian lakes at up to six times the rate previously thought, a new study suggests.

The result: even more global warming.

As the planet warms, frozen soil called permafrost in high-latitude regions such as Siberia and Alaska thaws out.

One consequence: Carbon in the permafrost is released into the atmosphere.

Another is that thawing permafrost along the margins of a lake dislodges frozen plant and animal remains, causing them to sink to the bottom of the lakes, where they decompose and produce methane, a potent greenhouse gas.

Some of the methane diffuses through the lake water and into the air, but the majority of it escapes by bubbling to the surface.

Until now, this bubbling process, called ebullition, has been hard to measure because of the difficulty involved in determining where and when the bubbles will occur.

In the new study, researchers used a combination of aerial surveys, remote sensors and year-round measurements of two Siberian lakes where methane bubbling was known to occur.

To identify bubble hot spots, they surveyed the lakes in autumn, when bubbles rising to the surface freeze in place, leaving behind visible trails.

Extrapolating their data to Siberia's other lakes, the researchers estimate that more than 4 million tons of methane is being released into the atmosphere each year -- between 10 and 63 percent higher than previous estimates.

The study, led by Katey Walter at the University of Alaska at Fairbanks, is detailed in the Sept. 7 issue of the journal Nature.

["The effects can be huge," Walter told the Associated Press. "It's coming out a lot, and there's a lot more to come out."]

Other studies have calculated that about 500 billion tons of carbon is locked up in Siberia's permafrost and that up to 90 percent of it could be released if the region continues to warm as expected.

If this happens, the coming decades will see an increase in the number and size of methane-releasing thaw lakes, scientists say.

["It's kind of like a slow-motion time bomb," Ted Schuur, professor of ecosystem ecology at the University of Florida, told the AP. "There's these big surprises out there that we don't even know about."]

While methane from Siberian lakes is a relatively modest contributor to climate change compared to human greenhouse emissions by industry and automobiles, it helps intensify a positive feedback mechanism for global warming.

The cycle works like this: Thawing permafrost dumps tons of previously frozen organic material into lake bottoms, producing methane.

The methane finds its way into the atmosphere, where it works with other greenhouse gases to trap the sun's rays.

The planet warms even more, causing permafrost to thaw at a quicker pace.

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