Essentially a form of ice infused with methane, clathrates may seem an obscure topic for discussion. They exist only under extreme conditions: such as underneath oceanic sediment. What makes them significant is the sheer volume of methane they contain. While it is unclear what degree of warming would be required to induce methane release from clathrates, there is a very real possibility that such release could be self-reinforcing. Given the global warming potential of methane and the volume of the gas in oceanic clathrates, such a self-sustaining release could induce abrupt and massive climatic change.
As a greenhouse gas, methane is potent. Averaged across a 100 year span, one tonne of methane produces as much warming as 25 tonnes of carbon dioxide. Even worse, when atmospheric methane breaks down, it generally oxidizes into carbon dioxide and water. Taking into account secondary effects, the warming potential of a tonne of methane is about equal to 72 tonnes of CO2 (according to the Fourth assessment report of the IPCC). This is one reason people are so concerned about the climatic effects of meat production, as well as the reason for which methane capture projects are one of the more credible kinds of carbon offset.
Recent estimates hold that ocean clathrates contain 500-2500 gigatonnes of carbon dioxide equivalent: akin to 100-500 years worth of sustainable emissions. About 400 Gt of carbon dioxide equivalent is in the Arctic permafrost. If a substantial proportion of this methane were to be released, it would take the world into completely unknown climatic territory. As such, it is highly likely that the adaptive capacity of both humanity and existing ecosystems would be overwhelmed, perhaps to a degree akin to the Permian-Triassic extinction event. This is truly the nightmare scenario for climate change, though its probability cannot be accurately assessed in relation to any combination of human behaviours and natural variations.
The existence of such exceedingly dire possibilities affects economic calculations about climate change. While it may not be sensible to spend 20% of global GDP to avoid an outcome with a 0.1% chance of occurring, a strong argument can be made that heavy expenditure is justified in the face of catastrophic risk. It is not as though we have another planet to fall back on if this one gets rendered unfit for human habitation.
[Update: 4 February 2009] Here is a post on the danger of self-amplifying, runaway climate change: Is runaway climate change possible? Hansen’s take.
[Update: 19 February 2010] See also: The threat from methane in the North.