Path-dependent reductions in CO2 emission budgets caused by permafrost carbon release

• Published in: Nature Geoscience
• Main Authors: Gasser, T., Kechiar, M., Ciais, P., Burke, E.J., Kleinen, T., Zhu, D., Huang, Y., Ekici, A., Obersteiner, M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: NPG 2018
• Subjects: permafrost
• Online Access: https://pure.iiasa.ac.at/id/eprint/15453/; https://pure.iiasa.ac.at/id/eprint/15453/1/permafrost%26budgets_v11_main_final.pdf; https://pure.iiasa.ac.at/id/eprint/15453/7/s41561-018-0273-7.pdf; https://doi.org/10.1038/s41561-018-0227-0

Emission budgets are defined as the cumulative amount of anthropogenic CO2 emission compatible with a global temperature change target. The simplicity of the concept has made it attractive to policy-makers, yet it relies on a linear approximation of the global carbon-climate system’s response to anthropogenic CO2 emissions. Here, we investigate how emission budgets are impacted by inclusion of CO2 and CH4 emissions caused by permafrost thaw, a non-linear and tipping process of the Earth system. We use the compact Earth system model OSCAR v2.2.1 in which parameterization of permafrost thaw, soil organic matter decomposition, and CO2 and CH4 emission was introduced, based on four complex land surface models that specifically represent high-latitude processes. We find that permafrost carbon release makes emission budgets path-dependent (i.e. budgets also depend on the pathway followed to reach the target). The median remaining budget for the 2°C target is reduced by 8% [1–25%] if the target is avoided and net negative emissions prove feasible, by 13% [2–34%] if they do not prove feasible, by 16% [3–44%] if the target is overshot by 0.5°C, and by 25% [5–63%] if it is by 1°C. (Uncertainties are the minimum-to-maximum range across permafrost models and scenarios.) For the 1.5°C target, reductions in the median remaining budget range from ~10% to more than 100%. We conclude that the world is closer to exceeding the budget for the long-term target of the Paris climate agreement than previously thought.