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The report notes there is still considerable uncertainty about carbon emissions estimates given the relatively limited observational measurements. But it also warns that the Arctic region — which is warming at more than twice the rate of the rest of the world, may have already turned into the global warming accelerator long been feared.

The findings come just as U.N. climate negotiators meet in Madrid to address the need for more ambitious cuts in greenhouse gas emissions, and would mean that the world faces an even steeper challenge in meeting the targets outlined in the Paris Climate Agreement.

Schuur said that the carbon being emitted by the Arctic amounts to less than 10% of fossil fuel emissions each year. “So it’s a small addition to what humans are already producing,” he said.

Photo by AFP PHOTO / European Geosciences Union / Mario HOPPMANN

However, that number is likely to grow with time, as the Arctic continues to warm. “We’ve crossed the zero line,” Schuur said.

“We don’t think the Arctic is going to admit so much more emissions that it will make fossil fuel emissions irrelevant,” but any extra emissions complicate the already difficult task of slashing them to net zero by mid-century to limit global warming to no more than 1.5-degrees Celsius, he said.

Merritt Turetsky, an ecologist at the University of Guelph who was not involved in the Arctic Report Card, said three new discoveries support its conclusion.

New information on fall and winter carbon, as opposed to summer when plants are active in the far north, “shows much greater ecosystem losses of carbon to the atmosphere than we expected,” she said in an email. “So our biosphere in the North is leakier than we thought because soils are remaining warm and respiring both carbon dioxide and methane.” Methane is another powerful greenhouse gas.