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Defusing the Methane Greenhouse Time Bomb

Could methane-digesting bacteria and an Arctic cap of fresh water prevent a climate catastrophe?
methanotrophs



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."

Elliot cautions that there is a large degree of uncertainty in the results generated by his model, which is the first attempt ever made to incorporate the biological activity of methanotrophs into a regional climate model.

Vincent Gauci, lecturer in Earth systems and ecosystem science at The Open University in England, agrees that the uncertainties in the model prevent it from being used to conclude whether or not the methane released from deep-sea clathrates will enter the atmosphere, especially in the event of a "catastrophic submarine slope failure," in which large volumes of clathrate spontaneously collapse and release their stored methane. This is an outrageously complex problem," Elliott says.

Dave Valentine, associate professor of microbial geochemistry at the University of California, Santa Barbara, who heard Elliott's talk, notes that paleoclimatologists have yet to definitively answer whether or not there is evidence that methane from clathrates ever reached the atmosphere in the past, which would support the conclusions of Elliott's model. University of Chicago geophysicist David Archer's The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate (Princeton University Press, 2008), notes that the "jury is still out" on what the role was, if any, of destabilizing methane hydrates in past global warming events and mass extinctions.

Even if methane doesn't reach the atmosphere, Elliott's model suggests it could still have dire effects on the Arctic environment: As it is oxidized by methanotrophs, it will acidify the Arctic Ocean and turn the water into a anoxic "dead zone" analogous to the oxygen-free dead zones that show up in the Gulf of Mexico every year as a result of farmland fertilizer runoff carried by the Mississippi River. "It would mean those nutrients [oxygen] are not available to other organisms," Elliott says. "In other words, maybe we're safe, but other organisms are not."

On a local level these changes would be equal to or even greater than the acidification of the ocean that is already occurring because of rising levels of atmospheric carbon dioxide. "It would be a very serious environmental issue—but regional, not global," Elliott says.

Future iterations of Elliott's model will have to include a class of methane-digesting bacteria not included in its first version, says Rick Colwell, an Oregon State University marine microbiologist specializing in methanotrophs who attended a recent presentation by Elliott. These yet-to-be-modeled bacteria operate only in anaerobic conditions that are usually found only in ocean sediments. If conventional, oxygen-dependent methanotrophs deplete the water column of oxygen, it could create conditions favorable for anaerobic methane-digesting bacteria to carry on the work of digesting the methane—flopping these parts of the ocean back to conditions that last prevailed 250 million years ago, during the most devastating mass extinction ever to befall life on Earth.

University of Washington in Seattle paleontologist Peter Ward has hypothesized that this event, known as the Great Dying, was the result of runaway global warming that turned the majority of the world's oceans anoxic throughout their entire depths, leading to a large release of hydrogen sulfide gas, a by-product of the metabolism of anaerobic bacteria. Elliott would not speculate whether or not the phenomena he modeled could have been part of that event, which in Ward's hypothesis was most likely caused by a different source of carbon all together: CO2 vented from massive volcanic eruptions in a region that is now part of Siberia.

"You could refer to these [anaerobic methane-eating bacteria] as a 'biofilter'—they would consume some of the methane that is moving into the water," Colwell says. Already, he adds, these bacteria perform this role in anoxic environments like the depths of the Black Sea.

Also, recent results from the Svalbard Islands north of Norway suggest that methane may not always rise from the water column in the way that Elliott's model assumes. In most models, including Elliott's, methanotrophs in the water were able to digest methane because it diffused into the water. Around Spitsbergen, however, 250 plumes rising from the bottom of the ocean included large bubbles which could ascend much higher up the water column before dispersing, increasing the danger that they could reach the atmosphere intact.

Ultimately, Elliott says, he and his team cannot eliminate the possibility that methanotrophs in the Arctic could be overwhelmed by large burps of methane gas from clathrates. Valentine speculates that the limiting nutrient will be oxygen, but Elliott's model raises some other potentially interesting possibilities.

When asked whether or not fertilizing the Arctic Ocean with some of the missing nutrients that could enhance the productivity of methanotrophs, such as iron, Elliott speculates that "I would bet that someone will very soon discuss the potential for engineering this situation. This becomes an opportunity for the geoengineering types to become creative."

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