Mackenzie river delta, Northwest Territories ? Only a squawk from a sandhill crane broke the Arctic silence — and a low gurgle of bubbles, a watery whisper of trouble repeated in countless spots around the polar world.

“On a calm day, you can see 20 or more ‘seeps’ out across this lake,” said Canadian researcher Rob Bowen, sidling his small rubber boat up beside one of them. A tossed match would have set it ablaze.

“It’s essentially pure methane.”

Pure methane, gas bubbling up from underwater vents, escaping into northern skies, adds to the global-warming gases accumulating in the atmosphere. And pure methane escaping in the massive amounts known to be locked in the Arctic permafrost and seabed would spell a climate catastrophe.

Is such an unlocking under way?

Researchers say air temperatures here in northwest Canada, in Siberia and elsewhere in the Arctic have risen more than 4.5 degrees Fahrenheit since 1970 — much faster than the global average. The summer thaw is reaching deeper into frozen soil, at a rate of 1.5 inches a year, and a further 13-degree temperature rise is possible this century, says the authoritative, U.N.-sponsored Intergovernmental Panel on Climate Change (IPCC).

In 2007, air monitors detected a rise in methane concentrations in the atmosphere, apparently from far northern sources. Russian researchers in Siberia expressed alarm, warning of a potential surge in the powerful greenhouse gas, additional warming of several degrees, and unpredictable consequences for Earth’s climate.

Others say massive seeps of methane might take centuries. But the Russian scenario is disturbing enough to have led six U.S. national laboratories last year to launch a joint investigation of rapid methane release. And IPCC Chairman Rajendra Pachauri in July asked his scientific network to focus on “abrupt, irreversible climate change” from thawing permafrost.

The data will come from teams like one led by Scott Dallimore, who with Bowen and others pitched tents here on the remote, boggy fringe of North America, 1,400 miles from the North Pole, to learn more about seeps in the 25,000 lakes of this vast river delta.

A “puzzle,” Dallimore calls it.

“Many factors are poorly studied, so we’re really doing frontier science here,” the Geological Survey of Canada scientist said. “There is a very large storehouse of greenhouse gases within the permafrost, and if that storehouse of greenhouse gases is fluxing to the surface, that’s important to know. And it’s important to know if that flux will change with time.”

Permafrost, tundra soil frozen year-round and covering almost one-fifth of Earth’s land surface, runs anywhere from 160 to 2,000 feet deep in this region. Entombed in that freezer is carbon — plant and animal matter accumulated through millennia.

As the soil thaws, these ancient deposits finally decompose, attacked by microbes, producing carbon dioxide and — if in water — methane. Both are greenhouse gases, but methane is many times more powerful in warming the atmosphere.

Researchers led by the University of Florida’s Ted Schuur last year calculated that the top 10 feet of permafrost alone contain more carbon than is currently in the atmosphere.