Abstract

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 the 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 parameterizations of permafrost thaw, soil organic matter decomposition and CO2 and CH4 emission were introduced based on four complex land surface models that specifically represent high-latitude processes. We found that permafrost carbon release makes emission budgets path dependent (that is, budgets also depend on the pathway followed to reach the target). The median remaining budget for the 2 °C target reduces 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 overshot by 1 °C. (Uncertainties are the minimum-to-maximum range across the 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.

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Data availability

RCP scenarios are available at http://www.pik-potsdam.de/~mmalte/rcps/. The data that support the findings of this study are available from the corresponding author upon request.

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Acknowledgements

We thank A. H. MacDougall for sharing data and O. Boucher for data used in Supplementary Fig. 6. This work is part of the European Research Council Synergy project ‘Imbalance-P’ (grant no. ERC-2013-SyG-610028). Simulations with OSCAR were carried out on the IPSL Prodiguer-Ciclad facility, which is supported by CNRS, UPMC and Labex L-IPSL, and funded by the ANR (grant no. ANR-10-LABX-0018) and the European FP7 IS-ENES2 project (grant no. 312979). E.J.B. was supported by PAGE21 (EU project no. GA282700), CRESCENDO (EU project no. 641816) and the Joint UK DECC/Defra Met Office Hadley Centre Climate Programme (GA01101). A.E. was also supported by PAGE21.

Author information

Affiliations

  1. International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria

    • T. Gasser
    • , M. Kechiar
    •  & M. Obersteiner
  2. École Polytechnique, Palaiseau, France

    • M. Kechiar
  3. Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, Université Paris-Saclay, CEA – CNRS – UVSQ, Gif-sur-Yvette, France

    • P. Ciais
    • , D. Zhu
    •  & Y. Huang
  4. Met Office Hadley Centre, Exeter, UK

    • E. J. Burke
  5. Max Planck Institut für Meteorologie, Hamburg, Germany

    • T. Kleinen
  6. Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland

    • A. Ekici
  7. Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

    • A. Ekici

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Contributions

T.G. designed the study. T.G. developed the permafrost emulator with inputs from P.C. and M.K. T.K. provided JSBACH data. Y.H., D.Z. and P.C. provided ORCHIDEE data. E.J.B. and A.E. provided JULES data. T.G. and M.K. set up the simulations with OSCAR, processed the outputs and created the figures. T.G., M.K., P.C. and M.O. discussed the preliminary results. T.G. wrote the manuscript with contributions from all the authors.

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The authors declare no competing interests.

Corresponding author

Correspondence to T. Gasser.

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https://doi.org/10.1038/s41561-018-0227-0