http://www.nature.com/climate/2009/0904/full/climate.2009.24.html Feature
Nature Reports Climate Change
Published online: 5 March 2009 | doi:10.1038/climate.2009.24A sleeping giant?
As the planet warms, vast stores of methane — a potent greenhouse gas — could be released from frozen deposits on land and under the ocean. Amanda Leigh Mascarelli reports on the race to understand a ticking time bomb.
In 2007, scientists scouting the icy waters of the Arctic Ocean began to notice some troubling signs. In about half of their seawater chemistry samples, the concentration of dissolved methane was two to ten times higher than in samples taken during previous years from the same locations. Then, last summer, they observed large rings of gas — sometimes as wide as 30 centimetres in diameter — trapped in ice, as well as methane plumes bubbling to the surface over hundreds of square kilometres of the shallow waters along the Siberian Shelf.
The team, from Russia and other nations, presented their results at the American Geophysical Union's Fall Meeting in December, where scientists cautiously voiced their concerns that large quantities of methane are becoming destabilized as the planet — and the ocean — heat up. Researchers have long speculated that warming could unleash vast stores of the greenhouse gas from where it lies frozen beneath the sea floor and locked up in Arctic soils. If those deposits were to melt, it would almost certainly trigger abrupt climate change. Methane heats the atmosphere with an efficiency 25 times that of carbon dioxide, and its release could put in motion a positive feedback loop in which warming releases methane, causing further warming, which liberates even more of the gas. Whether that's already happening is anyone's guess. Scientists are quick to point out that the Arctic methane plumes could be anomalous or simply part of a longer-term trend. Natalia Shakhova, a biogeochemist at the University of Alaska, Fairbanks, and one of the leaders of the Siberian Shelf study, says, "Two years is nothing in geologic time scales." James Kennett, an Earth scientist at the University of California, Santa Barbara, agrees and says it is very possible that the Arctic methane releases "just simply weren't observed before."
But the findings are part of a growing trend in which scientists are turning their attention to a threat conceivably worse than carbon dioxide. Though human activity has boosted atmospheric concentrations of methane by 150 per cent since the Industrial Revolution — mostly through agriculture and farming, the creation of landfills, biomass burning and fossil fuel use — that's nothing compared with the quantities that could be released from frozen deposits in the ground. "These deposits rival fossil fuels in terms of their size. It's like having a whole additional supply of coal, oil and natural gas out there that we can't control," says James White, a geochemist at the University of Colorado, Boulder.
Some of these deposits take the form of gas trapped within icy lattices of hydrogen bonds, known as hydrates or clathrates, that are tucked into pores of sediment below the sea floor. In shallow marine sediments such as those along the Siberian Shelf, hydrates are covered by a layer of frozen soil, called permafrost, that further protects them from melting. Both here and on land, permafrost stores vast quantities of carbon that could be converted to methane. Rich in organic material from dead plant and animal matter, thawed-out permafrost becomes alive with methane-producing microbes, which release the gas to the atmosphere. Some have compared it to unplugging a giant freezer: Warming temperatures could free up ancient carbon that's been safely tucked away for many thousands of years. "We've been putting carbon in this bank for 10,000 years," says White. "It's so cold that it doesn't decay away. But as the climate warms up, you start to take it out of the bank."
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