Paris climate targets could be exceeded sooner than expected as key emissions from melting permafrost are included in forecasts for the first time.
The new study explained that the rise in global temperatures means previously year-round frozen soil is melting faster than thought.
Permafrost is soil frozen for at least two years and as a result stores large amounts of carbon and other nutrients from organic matter.
It represents a large carbon reservoir, scientists at the International Institute for Applied Systems Analysis (IIASA) in Austria said.
As it melts it releases the built up CO2 but these emissions are rarely considered in projections of potential future global warming – even though it eventually becomes irreversible.
The new study has for the first time comprehensively accounted for permafrost carbon release when estimating emission budgets for climate targets.
The results show that the world might be closer to exceeding the budget for the long-term target of the Paris climate agreement than previously thought.
Emissions budgets represent the upper limit of CO2 emissions associated with keeping below a specific global average temperature.
Until now, the efforts of policymakers to remain below dangerous levels of warming have been almost solely focused on the linear rise in CO2 emissions due to human activity and the global temperature.
Dr Thomas Gasser, a researcher with the IIASA Ecosystems Services and Management Programme said: “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.
“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.”
The study also shows that the effect can become even more significant for overshooting trajectories, which means first exceeding the targeted level, and then going back down to the target.
The Paris Agreement explicitly acknowledges an overshooting trajectory, peaking first at ‘well-below’ 2°C and then pursuing efforts to get back to 1.5°C.
But during the overshooting period rising temperatures will lead to further permafrost carbon thaw.
Dr Gasser said: “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.”
The study was published in Nature Geoscience
