Published online 12 December 2006 | Nature | doi:10.1038/news061211-6

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Buried deposits of greenhouse gases may be more unstable than thought.

Bubbling up: shallow deposits of methane ice could release greenhouse gas more easily. Alamy

Geologists have discovered underwater deposits of hydrates — icy deposits of frozen methane gas — at far shallower depths under the ocean floor than expected. The finding suggests that, in a globally warmed world, the hydrates could melt suddenly and release their gas into the atmosphere, thus warming the planet even more.

Hydrates are cage-like structures in which molecules of water surround frozen molecules of gas. When dug up and brought to the surface, they release fizzy bubbles of methane into the atmosphere.

As a greenhouse gas, methane has 20 times the global warming potential of carbon dioxide. Large amounts of the world's carbon is locked up in methane hydrates, both in polar permafrost regions and buried in marine sediments worldwide. So scientists have long worried about a potentially catastrophic melting of these hydrates, triggered by an underwater landslide or warming of the ocean waters above them, that could send temperatures soaring.

Some researchers have suggested that a great global-warming episode 55 million years ago could have been caused by a catastrophic release of methane hydrates from the sea floor.

Shallow grave

The deeper the hydrates lie, the greater the pressure above them, and the less likely they are to destablise and release gas. But now, scientists drilling into the ocean floor have found hydrates off the coast of Vancouver Island, Canada, just 60 to 120 metres under the sea floor. That's less than half the depth they are typically predicted to exist at, given basic equations about the temperatures and pressures under which hydrates are usually stable.

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"This methane can potentially release quickly, in geologic terms. Not in our lifetime, but definitely faster than people had predicted earlier," says Michael Riedel of McGill University in Montreal, who co-led the 2005 expedition that uncovered the shallow hydrates.

Core surprise

The scientists made the discovery last year, while working through the Integrated Ocean Drilling Program to explore the coast off Vancouver Island, where hydrates had been found before.

To their surprise, the five cores they drilled, down to 350 metres depth, contained hydrates very near the surface and distributed across a wide area. Hydrates have been found to occur at shallow depts to a limited extent, even outcropping on the sea floor in isolated spots thanks to unusual local conditions. But the new work indicates that hydrates can be stable in larger swaths, over larger areas, than known before.

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Geologists may now have to recalculate just how much hydrate exists in the world, says Riedel, who presented the results at a meeting of the American Geophysical Union in San Francisco this week.

Not everyone sees hydrates as a threat. Oil companies, for instance, have long talked of harvesting them as an energy resource. India and Japan have initiated hydrate exploration programmes, says Timothy Collett of the US Geological Survey in Denver, Colorado. And on Alaska's North Slope, the oil company BP will be drilling a test well in February to explore the feasibility of mining hydrates for energy.

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