With only days left, tensions are rising as countries race to resolve outstanding differences and forge an agreement that - hopefully - will set the planet on a path to avoiding the worst consequences of climate change.

The goal is an agreement that would set the world on a path to limit warming to below 2 degrees Celsius, or perhaps even 1.5 degrees Celsius, above pre-industrial levels. But at a news conference here at the Le Bourget conference centre, scientists pointed out a factor that could make hitting these targets quite a lot harder.

It's called permafrost.

As the planet warms, this frozen northern soil is going to continue to thaw - and as it thaws, it's going to release carbon dioxide and methane into the air. A lot of it, it turns out. Potentially enough to really throw off the carbon budgets that have been calculated in order to determine the maximum emissions that we can release and still have a good chance of keeping warming to 2 C or below it.

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In particular, Susan Natali of the Woods Hole Research Centre explained that with a very high level of warming, permafrost emissions this century could be quite large indeed.
Natali used numbers from the 2013 report of the United Nations' Intergovernmental Panel on Climate Change, which found that humans can only emit about 275 more gigatons, or billion tons, of carbon (about 1000 gigatons of carbon dioxide, which has a greater molecular weight) to have a greater than 66 per cent chance of limiting warming to 2 C.

But out of that limited budget, she said, permafrost emissions could take up some 150 of those gigatons (or about 550 gigatons of carbon dioxide).

"That's on par with current US rates of emission," Natali said, which are about 1.4 gigatons of carbon per year. "So we're talking about another emitting region that's currently not included in our emissions scenarios."

Fortunately, even though they're not considered to be strong enough, the current national pledges to limit global warming appear to have taken the world off a truly high emissions path. These pledges, or "intended nationally determined contributions," could potentially limit warming to 2.7 C, according to the United Nations.

But in an interview, Natali and her Woods Hole colleague and fellow permafrost expert Max Holmes explained that even for lower warming scenarios like this, permafrost could emit 50 gigatons of carbon (or about 180 gigatons of carbon dioxide) in this century.

This is because under lower warming scenarios, only about 30 per cent, rather than about 70 per cent, of surface layer permafrost is expected to thaw.

Another 50 gigatons out of a 275 gigaton carbon budget - or, another 180 gigatons out of a 1000 gigaton carbon dioxide budget - would significantly constrain how much the world could emit and still have a strong chance of keeping warming below 2 C.

And Natali and Holmes also noted that permafrost emissions don't end at 2100 - they are expected to continue after that and even get worse. "Most of the release will happen after 2100," said Natali.

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That's a big problem because the global carbon budget is fixed, and after it is exceeded there can be zero further emissions. Because carbon dioxide lasts so long in the atmosphere, you don't get to start with a fresh budget in the next century. So permafrost emissions beyond 2100 would also have to be taken into account, and would restrict the budget even further.

Permafrost is a potential carbon bomb because over thousands of years, dead plant life has been slowly swallowed up by the soil but has not decomposed. Plants pull carbon out of the atmosphere as they grow, but release it again when they die and decompose. As permafrost warms and thaws, microbes will have more ability to break down the plant life it contains, which is what will trigger a steady stream of emissions.

"It's just like you put celery in your freezer and then you turn your freezer into a refrigerator, and it starts to rot," says Woods Hole's Max Holmes.