"A slow motion time bomb"

Arctic thaw could prompt huge release of carbon dioxide : Nature News

It's all about what happens when permafrost stops being being permafrost:

Scientists have long debated how the global climate might be affected by thawing of the Arctic's permanently frozen soils, known as permafrost. When permafrost melts, microbes decompose organic matter in the soil, producing greenhouse gases. But when plants have access to warmer, deeper soils, they grow faster and take in carbon dioxide....

The study by Schuur and his colleagues, published today in Nature¹, shows that after 15 years of thaw, plants initially grow faster and take in more carbon than is released by the melting tundra, so the ecosystem is an overall carbon sink. But after a few decades, the balance shifts and the ecosystem becomes a source of carbon.

"The plants are growing faster, but after a few decades the rate of carbon loss from the soils is so high the plants can't keep up," says Schuur.

It's estimated that permafrost soils store about twice as much carbon than is currently present in the atmosphere², and the stores of carbon are unlikely to run out any time soon. "It's a slow-motion time bomb," says Schuur.

The end result:

Extrapolations of the experimental findings to the whole Arctic region suggest that CO₂ emissions from future permafrost thawing could be roughly a billion tonnes per year — of the same order of magnitude as emissions from current deforestation of the tropics. Burning of fossil fuels releases about 8.5 billion tonnes of CO₂ a year.

Note that the experiment also does not look at the release of methane, a much more powerful greenhouse gas, from ex-permafrost.