By Janet Raloff
Atmospheric scientist Eric Kort was flying over the Arctic Ocean three years ago, monitoring readouts as onboard sensors sniffed the air. Suddenly, as the plane dipped low over some breaks in the sea’s ice cover, those instruments detected the unmistakable whiff of methane, the second most important climate-warming gas associated with human activities.
“This was unexpected,” says Kort, of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. On four more excursions north of the Beaufort and Chukchi seas through April 2010 — always in winter or early spring — the plane’s sensors detected the same taint of methane in very-low-altitude air over broken patches of ice, Kort and collaborators report online April 22 in Nature Geoscience.
The prime suspects are methane-spewing bacteria that live in Arctic surface waters. But the new data call into question microbiology’s understanding of these microbes, says oceanographer David Karl of the University of Hawaii in Honolulu. Normally, these bacteria — in the gut of animals and elsewhere — thrive in conditions free of oxygen. The conundrum here, Karl observes, is that the ocean surface is usually saturated with oxygen.
“This exciting study reminds us how little we know about microbial processes in the sea,” he says.
In their new report, Kort and colleagues calculate the Arctic’s daily methane emissions during the flybys at about 2 milligrams per square meter. “That’s a pretty significant flux to come out of the ocean,” he says.
The methane signal wasn’t accompanied by a similar spike in carbon monoxide or other pollutants associated with either combustion or crude oil. So the gas didn’t appear to come from distant oil-production activities around Prudhoe Bay on the north coast of Alaska, the researchers conclude. Methane also rose in lock step with water vapor, Kort notes, “suggesting both were coming from the same source — the open water below.”
The big question for climate scientists, he says, is how pervasive this seawater flatulence is. If the measurements reflect the Arctic’s marine surface water emissions for much of the year, Kort says, “this could be a pretty substantial methane source.”
Some studies have pointed to the melting of massive subsea deposits, known as gas hydrates, as possible sources of atmospheric methane. But those sources are too close to coastal regions to easily explain the new aerial data, Kort says. In contrast, his group’s findings are consistent with measurements of methane in water from the central Arctic made by Ellen Damm at the Alfred Wegener Institute in Bremerhaven, Germany, and colleagues.
For now, Damm says, there are no confirmed explanations for the mysterious methane releases from Arctic waters seen by Kort’s group during dark months. But she says those data suggest that a seasonal nutrient disruption in the western Arctic Ocean “exerts pressure on the microbial food web” — creating conditions unusually favorable for methane-exhaling bacteria.
