A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Are there deposits of methane under the sea? Will global warming release the methane to the atmosphere?

August 3, 2007

Dear Cecil:

I was watching an episode of Nature on PBS about sea turtles, and at one point the narrator mentioned vast deposits of methane at the bottom of the ocean in solid form. Then he gave some vague warning that the warming oceans may unleash all this methane into the atmosphere. What's the straight dope on methane lurking beneath the sea? Will it give us an energy-independent utopia or burn us all to a crisp?

Cecil replies:

Right now I'd say things could go either way. That's what makes living in the 21st century fun.

What we're talking about are the strange crystals called methane hydrates, in which molecules of methane (the main component of natural gas) are trapped inside tiny cages of ice. The result is an icelike substance that contains up to 15 percent methane by volume. First observed in the 1800s, methane hydrates were initially regarded mainly as a nuisance that formed in cold natural-gas pipelines. In 1964, however, a Siberian gas rig discovered a large methane hydrate deposit underground, spurring research on whether such deposits could be a good source of natural gas. Deep-sea exploration turned up even greater deposits in the oceans, and with the rise of natural-gas prices and the push to reduce carbon dioxide emissions, methane hydrates are now getting serious attention in the energy industry.

Although it's still a fossil fuel, methane when burned releases much less carbon dioxide than coal or oil per unit of heat. What's more, using technology already in hand, we can strip the carbon out of methane and produce hydrogen, which burns without releasing any carbon dioxide at all. (Granted, the stripping process produces some CO2, so we'd have to figure a way to sequester that.) How much methane is locked in hydrates is uncertain, but researchers from British Petroleum and Texas A&M estimate world underwater reserves at 35 to 177 quadrillion cubic feet. Given that proven world reserves of conventional natural gas amount to just 6 quadrillion cubic feet, even at the low end of the range we're talking about a lot of gas.

Just a couple problems. The first is that no one has figured out how to extract hydrated methane economically, although several countries have research efforts under way. The second is that while methane hydrate is stable as long as it's kept cold and under pressure, vast amounts of methane could be suddenly released into the atmosphere under certain circumstances - for example, if a rise in sea temperature due to, say, global warming caused shallow deposits of methane hydrate to melt. We don't want that to happen, because methane is a potent greenhouse gas. Some believe a methane hydrate release 251 million years ago contributed to the Permian extinction, the largest mass die-off in earth's history, when perhaps 95 percent of all species on the planet were wiped out. And the Late Paleocene Thermal Maximum - a severe temperature spike, possibly triggered by a Caribbean volcanic eruption or even a comet impact, that coincided with another mass extinction 55 million years ago - may have been compounded by a similar release of undersea methane.

Yet another concern with methane hydrates is the danger of massive underwater landslides. When methane hydrate is mixed with sediment on the ocean floor, it prevents the sediment from solidifying properly and leads to instability. Undersea slides are dangerous for two reasons: they can release methane into the atmosphere, already established as a bad thing, and they can cause tsunamis. Methane hydrates may have been behind the Storegga landslide under the North Sea 8,000 years ago that sent a 13-foot tsunami crashing into the coast of Scotland. Scars from underwater slumps along the coasts of the United States are suspected to be the result of dips in sea level disturbing hydrate-rich sediment 20,000 years back. There's not much we can do about naturally occurring landslides, but a few people think that messing with the hydrates via mining is asking for trouble in the form of tsunamis, sea floor subsidence, and damage to underwater cables. There's also the environmental impact to consider. In 1997 scientists found centipedelike critters living in hydrate deposits in the Gulf of Mexico, so get ready for the Greenpeace campaign to save the ice worms.

Other problems arising from a methane release can be faintly comical but still hazardous. Ever been embarrassed as a kid when bubbles in the bathtub revealed you'd let one go? Scale that up a few orders of magnitude and embarrassment would be the least of your worries. While tales of gas bubbles in the Black Sea upending ships and drilling rigs are surely apocryphal, in 2000 a sunken fishing trawler was found at the center of a huge gas eruption site in the North Sea. (Yes, I thought of the Bermuda Triangle too.) You may not have heard much about methane hydrate before now, but odds are you will.

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