Global policymakers have failed to account for a key factor in setting carbon dioxide (CO2) emissions budgets to rein in the climate crisis—melting permafrost—a new study finds, and warns that the world could exceed the global warming limits laid out in the Paris climate accord sooner than expected.

"The scientific answer to 'How soon are we likely to exceed our Paris target?' is somewhere between 10 years ago and the next 20 years. Definitely not later than that," lead author Thomas Gasser, a researcher with the International Institute for Applied Systems Analysis (IIASA) Ecosystems Services and Management Program, told The Independent.

The IIASA team of researchers, whose study was published Monday in the journal Nature Geoscience, says theirs is the first to account for the role of so-called "tipping" point of thawing permafrost in warming projections. Taking those dynamics into account, the study estimates how much carbon budgets may need to be slashed in order to meet the Paris targets.

The area of active permafrost is already expanding, releasing with it not only carbon but the potent greenhouse gas methane. This melting also casts light on the problem of viewing as linear the relationship between global temperature rise and cumulative CO2, the researchers say.

"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," said 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," he added.

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The Paris agreement commits nations to keeping "a global temperature rise this century well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius."

The researchers say this "overshooting"—hitting a 2-degree rise first and then attempting to bring it down—is problematic.

"Overshooting is a risky strategy and getting back to lower levels after an overshoot will be extremely difficult. 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. Policymakers should understand that there is no elementary proportionality between cumulative CO2 emissions due to human activity and global temperature, as previously believed, and that overshooting may have serious consequences," Gasser stated.

To achieve the 1.5 degree Celsius target, "reductions in the median remaining budget range from ~10% to more than 100%," the researchers warned.

"We should have changed course a while ago," Gasser told The Independent, "and we should now significantly increase our efforts to do so."