Melting permafrost is closing in on a critical tipping point where it will release large volumes of methane and carbon dioxide into the atmosphere, driving atmospheric warming above the maximum 2.0°C target in the Paris Agreement much sooner than scientists hoped, according to a new study in the journal Nature Geoscience.

The International Institute for Applied Systems Analysis (IIASA), which conducted the analysis, says this is the first permafrost study that has ever factored that tipping point into a global carbon emissions budget. Past global warming models “have largely failed to factor the release of vast amounts of carbon held in this frozen rock and sediment into their climate projections,” The Independent reports.

“Policy-makers had made assumptions about climate change based on a ‘linear relationship’ between global temperature rise and CO2 emissions,” the UK-based paper adds. But the IIASA team “found that the release of huge amounts of carbon would render past emissions projections useless, as they fail to account for the exponential growth triggered by melting permafrost.”

“Permafrost carbon release from previously frozen organic matter is caused by global warming, and will certainly diminish the budget of CO2 we can emit while staying below a certain level of global warming,” explained lead author Thomas Gasser. “It is also an irreversible process over the course of a few centuries, and may therefore be considered a ‘tipping’ element of the Earth’s carbon-climate system that puts the linear approximation of the emission budget framework to the test.”

That means “we should have changed course a while ago, and we should now significantly increase our efforts to do so.”

While the pathway envisioned in the Paris Agreement is for emissions to exceed the eventual 1.5°C target for average global warming, peak at “well below” 2.0°C, then use negative emissions technology to bring warming back to the 1.5°C threshold, Gasser warned against a “lazy solution” that depends on “hypothetical technology, and a lot of wishful thinking”.

“Overshooting is a risky strategy, and getting back to lower levels after an overshoot will be extremely difficult,” he said. “However, since we are officially on an overshooting trajectory, we have to prepare ourselves for the possibility that we may never get back to safer levels of warming.”

Bob Ward, policy director at the London-based Grantham Research Institute on Climate Change, said the study “shows how dangerous it could be for the world to trigger the tipping point beyond which thawing permafrost releases large volumes of carbon dioxide and methane into the atmosphere.” He added that, “if this happens, it could become inevitable that we suffer dangerous climate change. That is why it is so vital that global emissions of carbon dioxide and other greenhouse gases from human activities are cut strongly and urgently.”