A study published on 17 September in Nature Geosciences suggests the emission budget for long-term targets set out by the Paris climate agreement may be exceeded much sooner than previously thought (1). For the first time, researchers have comprehensively accounted for the carbon dioxide (CO2) and methane (CO4) emissions caused by permafrost thaw, a non-linear tipping process of the Earth system. Permafrost carbon release was shown to strongly deviate from the linear approximation of the emission budget framework.
Emission budgets or ‘carbon budgets’ define the upper limit of total carbon dioxide emissions required to stay below a specific global average temperature. Current budgets assume a linear relationship between global temperature rise and cumulative CO2 emissions due to human activity, which is currently based on the cumulative amount of anthropogenic CO2 emission. This simplified concept is easier for policy-makers to interpret but the linear approximation of the global carbon-climate system’s response to anthropogenic CO2 emissions may be inaccurate. The study assessed how emission budgets may be affected by including permafrost thaw, a non-linear process. But what exactly is permafrost and how does it affect climate change?
Permafrost is defined as soil remains at or below 0°C for at least two consecutive years and occurs in regions where the summer temperatures are not warm enough to thaw deeper layers of soil―ranging from a few metres to many hundreds of metres―such as mountainous areas, mostly in Alaska, Canada and Siberia, that account for approximately a quarter of land surface on the Earth. Although seldom considered in projections of potential future global warming, permafrost acts an enormous reservoir for large volumes of CO2 and CH4 that are released into the atmosphere as they thaw. This creates a positive feedback loop since the release of greenhouse gases further increases global warming leading to more permafrost melting. Moreover, the process is not readily reversible.
The study led by Thomas Gasser, a researcher from the International Institute for Applied Systems Analysis, a non-profit research organisation based near Vienna, Austria, is the first to account for a tipping process like permafrost thawing in emission budgets. According to the authors, “the world is closer to exceeding the budget for the long-term target of the Paris Climate Agreement than previously thought.” The researchers also showed that owing to this positive feedback loop, the effects of permafrost melting may be even greater for overshooting trajectories, first exceeding a target and then returning to the target level. Additional carbon capture would be required to counteract the extra emission from the thawed permafrost that occurs during the overshoot period.
Gasser told the UK Independent, “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.” He adds, “We should have changed course a while ago, and we should now significantly increase our efforts to do so.”
(1) Gasser, T. et al. Path-dependent reductions in CO2 emission budgets caused by permafrost carbon release. Nature Geoscience (2018) DOI: 10.1038/s41561-018-0227-0