Why is Arctic warming so threatening to humanity? Fred Pearce explains in an article in New Scientist:

[P]ositive feedback from the release of methane from melting permafrost could lead to runaway warming. The danger is that if too much methane is released, the world will get hotter no matter how drastically we slash our greenhouse gas emissions. Recent studies suggest that emissions from melting permafrost could be far greater than once thought. And, although it is too early to be sure, some suspect this scenario is already starting to unfold: after remaining static for the past decade, methane levels have begun to rise again, and the source could be Arctic permafrost.

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Most of this is the result of positive feedbacks (see illustration) from lost ocean ice, says David Lawrence of the National Center for Atmospheric Research in Boulder, Colorado. His modeling studies show that during periods of rapid sea-ice loss, warming extends some 1500 kilometers inland from the ice itself. “If sea-ice continues to contract rapidly over the next several years, Arctic land warming and permafrost thaw are likely to accelerate,” he says.

Changes in wind patterns may accelerate the warming even further. “Loss of summer sea ice means more heat is absorbed in the ocean, which is given back to the atmosphere in early winter, which changes the wind patterns, which favors additional sea ice loss,” says James Overland, an oceanographer at the Pacific Marine Environmental Laboratory in Seattle. “The potential big deal is that we now may be having a positive feedback between atmospheric wind patterns and continued loss of sea ice.

The rapid warming in the Arctic means that a global temperature rise of 3 °C, likely this century, could translate into a 10 °C warming in the far north. Permafrost hundreds of meters deep will be at risk of thawing out.

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This is where things go global. The Arctic is not just a reflective mirror that is cracking up. It is also a massive store of carbon and methane, locked into the frozen soils and buried in icy structures beneath the ocean bed.

A quarter of the land surface of the northern hemisphere contains permafrost, permanently frozen soil, water and rock. In places, deep permafrost that formed during the last ice age, when the sea level was much lower, extends far out under the ocean, beneath the seabed. Large areas of permafrost are already starting to melt, resulting in rapid erosion, buckled highways and pipelines, collapsing buildings and “drunken” forests.

The real worry, though, is that permafrost contains organic carbon in the form of long-dead plants and animals. Some of it, including the odd mammoth, has remained frozen for tens of thousands of years. When the permafrost melts, much of this carbon is likely to be released into the atmosphere.

No one knows for sure how much carbon is locked away in permafrost, but it seems there is much more than we thought. An international study headed by Edward Schuur of the University of Florida last year doubled previous estimates of the carbon content of permafrost to about 1600 billion tonnes – roughly a third of all the carbon in the world’s soils and twice as much as is in the atmosphere.

Since existing models do not include feedback effects such as the heat generated by decomposition, the permafrost could melt within 100 years, far faster than generally thought.

And then there are methane hydrates, a form of ice containing trapped methane. The permafrost is not the only source of methane in the Arctic. Shallow Arctic Ocean sediments are believed to contain huge amounts of methane hydrates which are vulnerable to the warming of surface waters.

While shrinking sea ice has gotten the world’s people’s attention, what is really disturbing was a sudden jump in methane levels in 2007.The carbon stored in the far north has the potential to raise global temperatures by 10 °C or more. If global warming leads to the release of more greenhouse gases, these releases will cause yet more warming and still more carbon will escape to the atmosphere in a runaway greenhouse effect.