Scientists studying global warming in the Arctic have discovered a previously unknown source of methane working its way into the atmosphere, a source that is releasing large amounts of the gas each year.
Methane is, molecule for molecule, a far more potent global-warming gas than carbon dioxide. The newly discovered emissions are welling up from the continental shelf off Siberia's northern coast
They are estimated at nearly 8 million metric tons a year, making them roughly equal to the amount that, until now, scientists had attributed to emissions from all the world's oceans combined, the researchers calculate. Still, the emissions represent no more than about 1 percent of total global emissions.
Their study is set for publication in tomorrow's issue of the journal Science.
It's not clear whether this previously unknown source of atmospheric methane became active recently or represents a long-term source whose existence only now has come to light, several scientists say.
"I don't think we are looking at a future catastrophe, but we may be looking at some acceleration of the increase in methane in the atmosphere because of enhanced release from systems like this," explains Dr. Brook, who was not part of the research team.
Is this a new source?
Scientists have taken careful measurements of atmospheric methane for years. The newly discovered source, the Eastern Siberian Arctic Shelf, may already be accounted for in those measurements.
Yet during the past few years, atmospheric methane levels have increased after a period where concentrations stalled, says Andrew Weaver, a climate scientist at the University of Victoria in Canada. Recent studies have suggested that the resumed increase is tied to methane sources in the northern hemisphere.
The discovery of emissions from Siberia's continental shelf "is such a northern-hemisphere source," says Dr. Weaver, who also did not take part in the study. "This is a really important piece of science," he adds.
But he acknowledges that more work needs to be done to determine if the shelf represents a new source of methane. The international team, led by Natalia Shakhova, a scientist at the University of Alaska at Fairbanks and the Russian Academy of Sciences' Pacific Oceanological Institute, agrees.
But Dr. Shakhova says some evidence suggests a role for global warming in the methane's release.
Atmospheric concentrations of methane – which are already tiny – barely budge over the course of swings between ice ages and so-called interglacial periods, paleoclimate data suggest. Earth is in the midst of one of these periods today.
But methane concentrations in the atmosphere over the Arctic are now roughly three times the typical levels found during interglacial periods, Shakhova says – higher than at any time in the past 400,000 years. Concentrations over the East Siberian Arctic Shelf are higher still.
"That makes us think that the current global change might contribute" to the levels she and her colleagues have measured, accelerating the processes that lead to the methane's release, she says.
The East Siberian Arctic Shelf
The East Siberian Arctic Shelf covers more than 800,000 square miles. It's the largest expanse of continental shelf – and the most shallow – on the planet.
The shelf was part of Siberia's tundra more than 15,000 years ago. Like today's tundra, it had a layer of permafrost below the surface, with related deposits of methane trapped underneath.
When the glaciers melted at the end of the last ice age, the northern reaches of the ancient tundra submerged to become today's continental shelf.
This condition makes these deposits far more sensitive to warming than the methane beneath permafrost on land, researchers say. Even in summer, temperatures in the soil around the permafrost remain well below freezing. But on the shallow continental shelf, the bottom water is only a fraction of a degree below freezing. (It remains liquid because of its salinity.) Even a tiny bit of warming in bottom water can lead to thawing permafrost and the release of methane.
Some scientists remain cautions about interpreting the team' results. Todd Sowers, a paleoclimatologist at Penn State University notes that a range of factors can confound attempts to measure the movement of methane from the sea floor, through the water, and into the atmosphere.
Still, he says, "this is a very important topic. What you'd like to do is go back every 10 years to see if it's changing. That's when you can start saying: These are recent changes."
Shakhova's team "has set up the baseline" for that kind of work "very nicely," he says.