The amount of Arctic sea ice in summer, for example, has shrunk 15 to 20 percent in the past three decades.
(See National Geographic magazine's "Global Warning: Signs From Earth.")
Average temperatures in Alaska, western Canada, and eastern Russia are now 4º to 7ºF (3º to 4ºC) warmer than they were 50 years ago.
Zimov and his colleagues write, "In response to climate warming permafrost sediments have already begun to thaw, with extreme projections that almost all yedoma [an especially carbon-rich form of permafrost] will thaw by the end of the 21st century."
The researchers say most of the carbon frozen beneath the 360,000 square miles (1,000,000 square kilometers) of Siberian and Alaskan permafrost they studied would enter Earth's atmosphere upon thawing.
"We're saying there's this stuff deep in the ice, and that if it is thawed out, it is actually very decomposable. Bacteria and fungi will readily eat it if it's not frozen," Schuur said.
Zimov notes that the once dormant microbes will break down matter buried tens of yards deep, even without oxygen.
"In this case microbes produce not carbon dioxide but methane. Methane production is [a] much slower process than carbon dioxide production," he said.
"But as a greenhouse gas methane is [over] 20 times stronger."
The University of Florida's Schuur and others experts say a big concern is that carbon emissions from thawing permafrost could become self-perpetuating.
Added greenhouse gases would fuel rising temperatures, the scenario goes, which would thaw more permafrost, releasing more greenhouse gases, and so on.
Christopher Field studies the global carbon cycle, warming, and ecosystems at the Carnegie Institute's Department of Global Ecology based at Stanford University.
He says that over the last 150 years the impacts of fossil fuel emissions have been softened somewhat by the absorbing effects of the land and the oceans.
"Only about half of the carbon that's been released from fossil fuel emissions has stayed in the atmosphere. The rest has been stored," he said.
"I think we're seeing now [that] it's kind of a race between the mechanisms that store carbon and the mechanisms that have potential to release carbon."
Field says the permafrost analysis by Zimov and colleagues suggest that carbon-releasing mechanisms are "increasingly likely to win this race."
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