Scientists have long debated how the global climate might be affected by thawing of the Arctic’s permanently frozen soils, known as permafrost. As permafrost melts, bacteria break down the organic matter in the soil, releasing greenhouse gases. But at the same time, plants flourish with access to warmer, deeper soils, taking in carbon dioxide. The overall affect on the climate was assumed to be the balance between the gassing and greening.

A new study in this week’s Nature [subscription], suggests that initially, after 15 years of thaw, plants grow faster and take in more carbon than is released by the melting tundra, making the ecosystem 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 Edward Schuur from the University of Florida in Gainesville, who led the research. When Schuur extrapolated the findings to the entire Arctic region, they suggested a release of around a billion tonnes of carbon every 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.

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

Read the full story over on Nature News.

Anjali Nayar is an intern reporter with Nature News.