METHANE SAVIORS?: Methanotrophs like the ones pictured here might be able to prevent a massive "burp" of methane from the Arctic Ocean.
Courtesy of the U.S. Department of Energy

Methane trapped in Arctic ice (and elsewhere) could be rapidly released into the atmosphere as a result of global warming in a possible doomsday scenario for climate change, some scientists worry. After all, methane is 72 times more powerful as a greenhouse gas than carbon dioxide over a 20-year timescale. But research announced at the annual meeting of the American Geophysical Union this December suggests that marine microbes could at least partially defeat the methane "time bomb" sitting at the bottom of the world's oceans.

The conventional wisdom for decades has been that methane emanating from the seafloor could be consumed by a special class of bacteria called methanotrophs. It has long been known, for instance, that these organisms at the bottom of the Black Sea consume methane produced in its deep oxygen-free waters.

What has not been clear is whether these bacteria would be of any use in the event that a special class of ice at the bottom of the ocean is destabilized by a warmer climate. This ice, known as clathrates, or methane hydrates, consists of a cage of water molecules surrounding individual molecules of methane, and it exists under conditions of low temperature and high pressure. These conditions can be found on the continental shelf the world over, but there is an extra large quantity of seafloor suitable for methane hydrates in the Arctic because of its low temperatures and a seafloor plateau that happens to be at the optimum depth for clathrate formation. The Arctic also happens to be more vulnerable to climate change because parts of the poles are warming at least twice as fast as the rest of the world.

To investigate this Arctic ice more carefully, Scott Elliott, a biogeochemist at Los Alamos National Laboratory, used the Coyote supercomputer to model the complex interplay of physical and biological systems that govern the fate of methane released from Arctic clathrates during the first few decades of projected future global warming.

Elliott's model includes the activity of methanotrophs. In accordance with conventional wisdom, his virtual bacteria can keep up with small to medium-size failures of the clathrates and subsequent releases of methane gas. As the "burps" of methane increase in size in response to warming seas, however, his model also shows that in some areas of the Arctic, the methanotrophs could potentially run out of the nutrients required to metabolize methane, including oxygen, nitrate, iron and copper.

But, even if the methanotrophs in the Arctic run out of the nutrients required to digest methane—especially if the waters in which they normally live become anoxic (low in the oxygen modern life-forms need to survive)—a second phenomenon demonstrated in Elliott's models may yet prevent methane from percolating all the way to the surface of the ocean, and then into the atmosphere.

"It happens that the Arctic Ocean is capped with a relatively fresh layer of seawater," Elliott says. Freshwater from the many rivers that empty into the Arctic float atop the denser ocean brine. In Elliott's simulations, methane hits this fresh water "cap" and cannot escape into the atmosphere. Instead, it "hangs out in the Arctic Ocean until it flows out into the deep, abyssal Atlantic Ocean," Elliott says. "The time constants in deep oceans are many hundreds of years—that's long enough for methanotrophs to consume all the methane. The model says that right now we have multiple layers of security."