"The permafrost carbon feedback is irreversible on human time scales," says the report, Policy Implications of Warming Permafrost. "Overall, these observations indicate that large-scale thawing of permafrost may already have started."
While countries the size of Australia tally up their greenhouse emissions in hundreds of millions of tonnes, the Arctic's stores are measured in tens of billions.
Human-induced emissions now appear to have warmed the Arctic enough to unlock this vast carbon bank, with stark implications for international efforts to hold global warming to a safe level. Ancient forests locked under ice tens of thousands of years ago are beginning to melt and rot, releasing vast amounts of greenhouse gases into the air.
The report estimates the greenhouse gases leaking from the thawing Arctic will eventually add more to emissions than last year's combined carbon output of the US and Europe – a statistic which means present global plans to hold climate change to an average 2degree temperature rise this century are now likely to be much more difficult.