Good news about climate change is especially rare in the Arctic. But now comes news that increases in one greenhouse gas—methane—lead to the dramatic decline of another. Research off the coast of Norway’s Svalbard archipelago suggests that where methane gas bubbles up from seafloor seeps, surface waters directly above absorb twice as much carbon dioxide (CO2) as surrounding waters. The findings suggest that methane seeps in isolated spots in the Arctic could lessen the impact of climate change.
“This is … totally unexpected,” says Brett Thornton, a geochemist at Stockholm University who was not involved in the research. These new findings challenge the popular assumption that methane seeps inevitably increase the global greenhouse gas burden.
Methane is a potent greenhouse gas. Molecule for molecule, it traps nearly 30 times as much heat in the atmosphere as CO2. But scientists know relatively little about its role in the global carbon cycle. Most atmospheric methane comes from biological sources—belching bovines and bacteria feasting on decomposing litter—or from the burning of fossil fuels. In the ocean, methane bubbles up from deep seeps, where it is often stored in icelike crystal lattices of water called hydrates. When those hydrates “melt,” because of changing temperatures and pressures, the methane is released, and it can percolate into the atmosphere above.
To find out just how much methane the Arctic Ocean was contributing to the global balance, biogeochemist John Pohlman of the U.S. Geological Survey in Woods Hole, Massachusetts, set out to measure the gas close to the ocean surface above known methane seeps near Svalbard during the Arctic summer. He and his team were constantly surprised by how little methane they found. But the bigger surprise was that surface water CO2 levels dropped whenever their ship crossed a seep. “[The CO2 data] became the most important part of the story,” Pohlman says.
When combined with other data—sudden drops in water temperature, along with increases in dissolved oxygen and pH at the surface—the lower CO2 levels were telltale signs of bottom water upwelling and photosynthesis, Pohlman says. Pohlman and his team conclude that the same physical forces that are pushing the methane bubbles up are also pumping nutrient-rich cold waters from the sea bed to the surface, fertilizing phytoplankton blooms that soak up CO2, they write today in the Proceedings of the National Academy of Sciences.
Such a “fertilization effect” would be “really surprising,” says Thornton, who has studied methane emissions above seeps in the Laptev and East Siberian seas. “There are lots of nutrients in bottom water and bringing that to the surface could certainly [result in] draw down of CO2.”
In fact, the study finds that in such zones, nearly 1900 times more CO2 is being absorbed than methane emitted. That’s a small but real consolation for those concerned about global warming, Pohlman says. In these limited zones, the atmospheric benefit from CO2 sequestration is about 230 times greater than the warming effect from methane emissions.
But whether the findings apply to ocean seeps in other parts of the world is still a big question. Svalbard is in many ways a bellwether. Some methane seeps occur because the hydrates there are barely stable, and can be upset by slight changes in temperature and pressure. Globally, methane hydrate reservoirs may hold as much as one-third the carbon content of all fossil fuels. And with similar seeps along continental margins worldwide, there has been growing concern that methane emissions will dramatically increase as oceans warm.
But Pohlman says one can’t count on the methane fertilizing effect being the same everywhere. Even in his study area, it’s apt to change with the seasons. He notes that his team’s data were collected in the constant sunlight of Arctic summer. During the dark polar night, photosynthesis would drop to nearly nothing, and methane emissions wouldn’t be offset by declining CO2.