THE world is on the cusp of a "tipping point" into dangerous climate change, according to new data gathered by scientists measuring methane leaking from the Arctic permafrost and a report presented to the United Nations on Tuesday.
"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.
"Much more difficult" is an understatement. Estimates are that thawing permafrost could add 40% to global emissions - which means we have to cut our emissions by a lot more if we want to keep climate change under control. Though one of the problems is that none of the models used by the IPCC account for permafrost carbon release, and so systematically underestimate the effect on the climate. If we rely on those models when setting emissions reduction targets (and we have to - they're our best estimates), we are likely to set them too high.
But the fundamental problem is that the climate may already have tipped. This is happening now, and it is irreversible on human timescales. A two degree average rise was simply too high.
(The full UNEP report Policy Implications of Warming Permafrost can be read here)