As global warming melts the permafrost, bacteria eat the thawed organic matter, then belch methane into the lakes.
Using radiocarbon dating, Walter's team determined that the methane being produced beneath the lakes comes from organic matter that died as long as 42,900 years ago but that is only now decomposing, presumably due to recent temperature increases.
Melting permafrost has caused lakes to increase in area by 14 percent in recent decades, she adds.
"It's a time bomb."
The new paper is important, says Walter Oechel of the Global Change Research Group at California's San Diego State University. It helps quantify the degree of climate-warming feedbacks from Arctic warming, he says.
"They're not only significant but larger than expected," Oechel said.
In addition to increasing methane production from lakebeds, he says, there is also a risk that global warming might lead to the melting of methane-containing ice in the permafrost.
"Those are huge stores of methane," Oechel said. "If those came out, they would swamp the current warming [that is] due to carbon dioxide from all sources."
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