Scientists say vast swaths of Arctic tundra could become massive sources of carbon dioxide despite increased plant growth spurred by warmer temperatures in the North.
"It's going to happen as a slow-motion time bomb," said Ted Schuur, an ecologist at the University of Florida, and the lead author of a paper that appeared Wednesday in the journal Nature.
Researchers have long known that permafrost contains huge amounts of frozen carbon trapped in organic material. It's believes the amount of carbon stored in permafrost around the world is almost double the amount currently in the atmosphere.
But scientists have been unsure of the fate of that carbon as global warming slowly melts the permafrost.
Some have held it would be released into the atmosphere and amplify the effects of climate change. Others suggest it would be absorbed by heavier vegetation .
Schuur's research in a well-studied area near Denali National Park in Alaska used radio-carbon dating to find out how much old carbon released from permafrost was being absorbed by plants. His findings suggest the plants would win -- for a little while.
"At first, the first couple decades, these areas were net sinks of carbon," he said. "It seemed good.
"But then, over more decades, we found that plants were still growing faster but they couldn't offset even more old carbon coming out. Finally, that rate overwhelms anything that the plants can do."
Schuur said the switch would come sometime between 15 and 50 years after the permafrost thawed.
Even if a stretch of previously frozen tundra sprouted and supported an entire boreal forest, it would only capture and store about five per cent of the carbon being released by microbes feasting on the organic material in the newly thawed permafrost.
"You can't grow enough forest to balance it out."
Even more sobering, said Schuur, is that carbon release from permafrost is something that can't be controlled once it starts.
"It's good that there's an offset. But it is a little bit of false security because what you're doing is going down this path of destabilizing and what we're showing is that (carbon emission) just doesn't stop.
"It's going to keep emitting carbon over hundreds of years."
That slow trickling of carbon will be difficult, if not impossible, to reverse, said Schuur. Immense sections of Canada, Russia, the United States and Scandinavia could turn into climate feedback machines.
There are 13 million square kilometres of permafrost around the world. Thawing along the southern edges has already begun.
Ultimately, Schuur said, global permafrost could add as much as one billion tonnes of carbon per year to the atmosphere.
That's about as much carbon as is now being added by deforestation in the tropics, and about 15 per cent of the entire amount of carbon generated around the globe by burning fossil fuels.
