Thirty percent of the Earth's surface is land. Twenty percent of the land is permafrost. There is over a trillion tons of carbon frozen and buried in the land permafrost. More than half the land covered by the topmost layer of permafrost will probably thaw by 2050.
Permafrost ice contains a lot of methane (CH4) from past decomposition. CH4 is 70 times stronger than CO2 over 20 years. Decomposition will speed up when the permafrost thaws. A very large release of CH4 when the ice melts, followed by large chronic emission of CO2 and CH4.
A frozen peat bog in western Siberia the size of France and Germany put together contains about 500 billion tons of carbon. Western Siberia has warmed faster than almost anywhere else on the Earth, with an increase in average temperature of about 3C in the last 40 years.
Even more Siberian permafrost is under the ocean, an area six times the size of Germany containing about 540 billion tons of carbon. That submarine permafrost is perilously close to thawing. Three to 12 kilometers from the coast the sea sediment is just below freezing. The permafrost has grown porous, there is a loss of rigor in the frozen sea floor, and the surrounding seawater is highly oversaturated with solute methane.
"...Researchers were investigating "alarming" reports in the last few days of the release of methane from long frozen Arctic waters, possibly from the warming of the sea…" -"Arctic sea ice drops to 2nd lowest level on record," AP, 27 Aug '08
The CH4 level in the air never reached 750 parts per billion (ppb) the last million years, but is now 1,780 ppb. Currently there is about 3.5 billion tons of CH4 in the air; 10 billion more would be like doubling the CO2 level for 20 years. Since the future amount of CH4 (or CO2) entering the air from melting permafrost isn't known, it is not included at all in current climate models. The UN warned this year natural CH4 emissions are a major climate wild card.
"If the Siberian (submarine) permafrost-seal thaws completely and all the stored gas escapes, the methane content of the planet's atmosphere would increase twelve fold. The result would be catastrophic global warming." -"A Storehouse of Greenhouse Gases Is Opening in Siberia," Spiegel, 17 April '08
To summarize, while usually people think emissions from melting permafrost could be a contributor to future global warming, they fail to realize that it is the permafrost below the surface of the ocean that presents the biggest most immediate threat. Particularly, the submarine permafrost off the coast of Siberia is very close to whole scale melting, and the consequences of that happening are dire indeed.