Paris climate targets could be exceeded sooner than expected

September 18, 2018 – A new study has, for the first time, comprehensively considered the release of carbon in permafrost when estimating emission budgets for climate targets. The results show that the world could be closer than expected to budget overruns for the long-term goal of the Paris Climate Agreement.

Emissions budgets represent the upper limit of total carbon dioxide (CO_{2 persons}) the emissions associated with staying below a specific global average temperature. The simplicity of the concept has made it an attractive tool for policymakers, who must stay below dangerous levels of warming, although it is highly dependent on the assumption of a linear relationship between overall temperature rise. and cumulative CO emissions._{2 persons} emissions due to human activity. In their study, researchers studied the impact of the phenomenon of nonlinear CO feedback on current emission balances._{2 persons} and methane emissions from thawing permafrost.

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Permafrost is a soil that has been frozen all year for at least two years. Because of the long periods when it remains frozen, soil stores large amounts of carbon and other nutrients from organic matter and is therefore an important reservoir of carbon, which is rarely taken into account in the projections of potential future global warming. The upper layer of permafrost (the active layer) periodically thaws in summer, but in recent years, the active layer of permafrost has progressively developed due to the increase in temperatures. This means that more permafrost thaws and releases the carbon previously trapped in the atmosphere.

"The release of carbon from permafrost from previously frozen organic material is caused by global warming and will certainly reduce the CO budget._{2 persons} 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 can therefore be considered as a "rocking" element of the Earth's carbon-climate system that tests the linear approximation of the Earth's framework. budget, "says Thomas Gasser, a researcher with the IIASA's Ecosystem Services and Management Program and senior author of the study published in Geoscience of nature.

This is the first time that such a switchover process is properly reflected in the program budgets and, according to the researchers, shows that the world is about to exceed the long-term goal budget. of the Paris Agreement.

Disturbingly, the study also shows that the effect can become even more important for overtaking trajectories. Exceeding means going beyond the target level and then back down to the target. The Paris Agreement explicitly recognizes a path of overshoot, culminating first at "well below 2 ° C", then continuing its efforts to return to 1.5 ° C. During the overtaking period, however, the rising temperatures will cause a permafrost carbon thaw, which will lead to an increase in released carbon that will have to be removed from the atmosphere for the global temperature to decrease.

"Exceeding is a risky strategy and going back to lower levels after going over will be extremely difficult. However, as we are officially on a path of overtaking, we must be prepared for the possibility that we will never return to safer levels of warming. Policy makers need to understand that there is no elementary proportionality between_{2 persons} emissions from human activity and global temperature, as previously thought, and that going beyond that could have serious consequences, "says Gasser.

The researchers hope that their work will have an impact on the scientific community by demonstrating that emission budgets are not as simple a tool as they think and that they will also help inform policy makers when designing future ones. climate mitigation strategies.

Reference

Gasser T, Kechiar M, P Ciais, EJ Burke, Kleinen T, Zhu D, Huang Y, Ekici A, Obersteiner M (2018). Reductions in CO2 emission balances due to the spread of carbon in permafrost. Geoscience of nature DOI 10.1038 / s41561-018-0227-0 [pure.iiasa.ac.at/id/eprint/15453]