A hill of permafrost "slumps" due to thawing in the Mackenzie River Delta, in the Northwest Territories, Canada. (Rick Bowmer/Associated Press)

The Arctic will switch from being a carbon sink to a carbon source by the end of this century as the permafrost thaws and emits greenhouse gases, a new study suggests.

The UN's Intergovernmental Panel on Climate Change had predicted that land-based ecosystems in the Far North would store more carbon from the atmosphere as the climate gets warmer.

That's because more plants are expected to flourish in the North, taking in more carbon as they grow and as their growing season gets longer. Storing more carbon this way would turn the Arctic into a carbon "sink" and help mitigate climate change, as carbon dioxide is considered one of the main heat-trapping greenhouse gases.

But the UN panel based its prediction on models that didn't account for the effect of thawing permafrost.

That permanently frozen soil layer contains billions of tonnes of plant and animal matter that has remained trapped there for up to tens of thousands of years. The thawing of the permafrost could allow that material to decompose and release its carbon back into the atmosphere.

A new study, to be published this week in Proceedings of the National Academy of Sciences, used mathematical models to predict that the Arctic will release 62 billion tonnes (plus or minus seven billion tonnes) of carbon over the 21st century, roughly 620 megatonnes a year, equivalent of 2.2 billion tonnes of CO2 per year.

That is three times the amount of carbon dioxide emitted by Canada in 2009, reported by Environment Canada to be 690 megatonnes.

The amount of carbon dioxide expected to be released from permafrost "is just a fraction of the amount of carbon that we emit as a species per year, but it’s important," said Charles Koven, project scientist at Lawrence Berkeley Lab in Berkeley, Calif., and lead author of the study.

According to the Global Carbon Project, humans emitted 8.4 billion tonnes of carbon or 30.8 billion tonnes of CO2 in 2009.

Most of the nine billion tonnes of CO2 emitted globally each year by the burning of fossil fuels currently gets absorbed by either the land or the ocean, he said. "The big question is whether that’s going to continue.”

If the Arctic stops taking in carbon emissions, more carbon will likely end up in the oceans — and the atmosphere.

The increased carbon dioxide in the atmosphere is expected to increase plant growth and boost emissions of methane, another greenhouse gas. That is because there will be more plant material for wetland bacteria to decompose into methane, the paper said. However, this is expected to be partially offset by the drying of some wetlands because of warmer temperatures.

Because tropical ecosystems are also expected to become a carbon source as the climate gets warmer, the findings suggest that climate change will leave "only the mid-latitudes as potential climate regulators."

Problem could worsen after 2100

The paper added: "We note as well that significant permafrost stocks exist and a steep loss continues at 2100, so that beyond the time horizon considered here there is still a potential for enormous carbon losses from high-latitude soils to continue."

There are an estimated 1,500 to 2,000 billion tonnes of carbon stored in Arctic permafrost.

The researchers predict that mean soil temperatures at high latitudes will increase by eight degrees Celsius by 2100 — far more than temperatures closer to the equator — and about 30 per cent of Arctic permafrost area will be lost.

About two-thirds of the predicted carbon loss due to Arctic warming is expected to come from permafrost and the other third from soils that freeze in the winter, then thaw in the summer.

"In some places, you can expect a complete loss of permafrost near the surface with climate warming," Koven said, adding that in others, the surface layer that melts in summer will simply thicken at the expense of the permafrost below.

Among Koven's collaborators for the study was Agriculture Canada researcher Charles Tarnocai, who provided "crucial" maps of soil carbon for the study, Koven said.