An MIT paper that appeared online Aug. 29 in the Journal of Geophysical Research elucidates how this underground methane in frozen regions would escape and also concludes that methane trapped under the ocean may already be escaping through vents in the sea floor at a much faster rate than previously believed. Some scientists have associated the release, both gradual and fast, of subsurface ocean methane with climate change of the past and future.

“The sediment conditions under which this mechanism for gas migration dominates, such as when you have a very fine-grained mud, are pervasive in much of the ocean as well as in some permafrost regions,” said lead author Ruben Juanes, the ARCO Assistant Professor in Energy Studies in the Department of Civil and Environmental Engineering.

“This indicates that we may be greatly underestimating the methane fluxes presently occurring in the ocean and from underground into Earth’s atmosphere,” said Juanes. “This could have implications for our understanding of the Earth’s carbon cycle and global warming.”

Methane, the primary component of natural gas, is more abundant in the Earth’s atmosphere now than at any time during the past 400,000 years, according to a recent analysis of air bubbles trapped in ice sheets. Over the last two centuries, methane concentrations in the atmosphere have more than doubled. It is estimated that about 60 percent of global methane emissions are tied to human activities like raising livestock and coal-mining, with the rest tied to natural sources such as wetlands, decomposing forests and underground deposits known as methane hydrates.

In the hydrate phase, a methane gas molecule is locked inside a crystalline cage of frozen water molecules. These hydrates exist in a layer of underground rock or oceanic sediments called the hydrate stability zone or HSZ. Methane hydrates will remain stable as long as the external pressure remains high and the temperature low. Beneath the hydrate stability zone, where the temperatures are higher, methane is found primarily in the gas phase mixed with water and sediment.

But the stability of the hydrate stability zone is climate-dependent.

If atmospheric temperatures rise, the hydrate stability zone will shift upward, leaving in its stead a layer of methane gas that has been freed from the hydrate cages. Pressure in that new layer of free gas would build, forcing the gas to shoot up through the HSZ to the surface through existing veins and new fractures in the sediment. A grain-scale computational model developed by Juanes and recent MIT graduate Antone Jain indicates that the gas would tend to open up cornflake-shaped fractures in the sediment, and would flow quickly enough that it could not be trapped into icy hydrate cages en route.

“Previous studies did not take into account the strong interaction between the gas-water surface tension and the sediment mechanics. Our model explains recent experiments of sediment fracturing during gas flow, and predicts that large amounts of free methane gas can bypass the HSZ,” said Juanes.

Using their model, as well as seismic data and core samples from a hydrate-bearing area of ocean floor (Hydrate Ridge, off the coast of Oregon), Juanes and Jain found that methane gas is very likely spewing out of vents in the sea floor at flow rates up to 1 million times faster than if it were migrating as a dissolved substance in water making its way through the oceanic sediment – a process previously thought to dominate methane transport.

“Our model provides a physical explanation for the recent striking discovery by the National Oceanic and Atmospheric Administration of a plume 1,400 meters high at the seafloor off the Northern California Margin,” said Juanes. This plume, which was recorded for five minutes before disappearing, is believed not to be hydrothermal vent, but a plume of methane gas bubbles coated with methane hydrate.

The Jain and Juanes paper in the Journal of Geophysical Research also explains the short-term consequences of injecting carbon dioxide into the ocean’s subsurface, a method proposed by some researchers for reducing atmospheric greenhouse gas. Juanes found that while some of the CO2 would remain trapped as a hydrate, much would likely spew up through fractures just as methane does.

“It is important to keep both methane and carbon dioxide either in the pipeline or underground, because the consequences of escape can be quite dangerous over time,” said Juanes.

This research was funded by the U.S. Department of Energy.

David Chandler, MIT News Office
March 11, 2009

A gas used for fumigation has the potential to contribute significantly to future greenhouse warming, but because its production has not yet reached high levels there is still time to nip this potential contributor in the bud, according to an international team of researchers.

Scientists at MIT, the Scripps Institution of Oceanography in San Diego and other institutions are reporting the results of their study of the gas, sulfuryl fluoride, this month in the Journal of Geophysical Research. The researchers have measured the levels of the gas in the atmosphere, and determined its emissions and lifetime to help gauge its potential future effects on climate.

Sulfuryl fluoride was introduced as a replacement for methyl bromide, a widely used fumigant that is being phased out under the Montreal Protocol because of its ozone-destroying chemistry. Methyl bromide has been widely used for insect control in grain-storage facilities, and in intensive agriculture in arid lands where drip irrigation is combined with covering of the land with plastic sheets to control evaporation.

“Such fumigants are very important for controlling pests in the agricultural and building sectors,” says Ron Prinn, director of MIT’s Center for Global Change Science and a co-author on the new paper. But with methyl bromide being phased out, “industry had to find alternatives, so sulfuryl fluoride has evolved to fill the role,” he says.

Until the new work, nobody knew accurately how long the gas would last in the atmosphere after it leaked out of buildings or grain silos. “Our analysis has shown that the lifetime is about 36 years, or eight times greater than previously thought, with the ocean being its dominant sink,” Prinn says. So it would become “a greenhouse gas of some importance if the quantity of its use grows as people expect.” For now, the gas is only present in the atmosphere in very small quantities of about 1.5 parts per trillion, though it is increasing by about 5 percent per year. Its newly reported 36-year lifetime, along with studies of its infrared-absorbing properties by researchers at NOAA, “indicate that, ton for ton, it is about 4,800 times more potent a heat-trapping gas than carbon dioxide” says Prinn.

Fortunately, though, “we’ve caught it very early in the game,” says Prinn, the TEPCO Professor of Atmospheric Science in MIT’s Department of Earth, Atmospheric and Planetary Sciences. The detection was made through a NASA-sponsored global research program called the Advanced Global Atmospheric Gases Experiment (AGAGE). “In AGAGE, we don’t just monitor the big greenhouse gases that everybody’s heard of,” he says. “This program is also designed to sniff out potential greenhouse and ozone-depleting gases before the industry gets very big.”

The lead author of the research paper is Jens Mühle of Scripps, and besides Prinn, the co-authors include Jin Huang, a research scientist at MIT’s Center for Global Change Science, Ray Weiss of Scripps, who co-directs AGAGE with Prinn, and eight others from Scripps, the University of Bristol in the United Kingdom and the Centre for Australian Weather and Climate Research.

“Unfortunately, it turns out that sulfuryl fluoride is a greenhouse gas with a longer lifetime than previously assumed,” says Mühle. “This has to be taken into account before large amounts are emitted into the atmosphere.”

Prinn adds that “fumigation is a big industry, and it’s absolutely needed to preserve our buildings and food supply.” But identifying the greenhouse risks from this particular compound, before many factories have been built to produce it in very large amounts, would give the industry a chance to find other substitutes at a time when that’s still a relatively easy change to implement. “Given human inventiveness, there are surely other alternatives out there,” says Prinn. He describes this approach as “a new frontier for environmental science — to try to head off potential dangers as early as possible, rather than wait until it’s a mature industry with lots of capital and jobs at stake.”