Carbon budgets for 1.5 and 2 °C targets lowered by natural wetland and permafrost feedbacks

Abstract

Global methane emissions from natural wetlands and carbon release from permafrost thaw have a positive feedback on climate, yet are not represented in most state-of-the-art climate models. Furthermore, a fraction of the thawed permafrost carbon is released as methane, enhancing the combined feedback strength. We present simulations with an inverted intermediate complexity climate model, which follows prescribed global warming pathways to stabilization at 1.5 or 2.0 °C above pre-industrial levels by the year 2100, and which incorporates a state-of-the-art global land surface model with updated descriptions of wetland and permafrost carbon release. We demonstrate that the climate feedbacks from those two processes are substantial. Specifically, permissible anthropogenic fossil fuel CO2 emission budgets are reduced by 9–15% (25–38 GtC) for stabilization at 1.5 °C, and 6–10% (33–52 GtC) for 2.0 °C stabilization. In our simulations these feedback processes respond more quickly at temperatures below 1.5 °C, and the differences between the 1.5 and 2 °C targets are disproportionately small. This key finding holds for transient emission pathways to 2100 and does not account for longer-term implications of these feedback processes. We conclude that natural feedback processes from wetlands and permafrost must be considered in assessments of transient emission pathways to limit global warming.

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Fig. 1: Time series for the control model ensemble.
Fig. 2: Response of the permafrost soil column to warming through the twenty-first century.
Fig. 3: Summary results for the natural methane feedback experiment.

Change history

  • 15 October 2018

    In the version of this Article originally published, a parallelization coding problem, which meant that a subset of model grid cells were subjected to erroneous updating of atmospheric gas concentrations, resulted in incorrect calculation of atmospheric CO2 for these grid cells, and therefore underestimation of the carbon uptake by land through vegetation growth and eventual increases to soil carbon stocks. Having re-run the simulations using the corrected code, the authors found that the original estimates of the impact of the natural wetland methane feedback were overestimated. The permafrost and natural wetland methane feedback requires lower permissible emissions of 9–15% to achieve climate stabilization at 1.5 °C, compared with the original published estimate of 17–23%. The Article text, Table 1 and Fig. 3 have been updated online to reflect the revised numerical estimates. The Supplementary Information file has also been amended, with Supplementary Figs 6, 7, 8 and 9 replaced with revised versions produced using the corrected model output. As the strength of feedbacks remain significant, still require inclusion in climate policy and are nonlinear with global warming, the overall conclusions of the Article remain unchanged.

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Acknowledgements

This work was undertaken as part of the UK Natural Environment Research Council’s programme ‘Understanding the Pathways to and Impacts of a 1.5 °C Rise in Global Temperature’ through grants NE/P015050/1 CLIFFTOP (to E.C.-P., G.H. and S.E.C.), NE/P014909/1, MOC1.5 (to W.J.C., C.P.W., C.H., P.M.C. and S.S.) and NE/P014941/1 CLUES (to A.B.H., P.M.C. and T.P.). The authors also acknowledge support for E.J.B. and N.G. through the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101), E.J.B. through CRESCENDO (EU project 641816), A.B.H. through an EPSRC Fellowship ‘Negative Emissions and the Food–Energy–Water Nexus’ (EP/N030141/1), and C.H. through CEH National Capability Funding. The authors also acknowledge the wetland extent data products provided by B. Zhang of Auburn University and B. Poulter of the NASA Goddard Space Flight Center.

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G.H., E.J.B., S.E.C. and E.C.-P. conceived and developed the project. E.C.-P. and C.H. led the development of the inverse IMOGEN model system. E.J.B. and S.E.C. contributed code and expertise on permafrost and soil carbon modelling. N.G., S.E.C. and E.C.-P. contributed code and expertise on the JULES wetlands methane scheme. A.B.H. and T.P. contributed land-use change data, W.J.C. and C.P.W. ozone ancillary data and S.S. contributed expertise on the ozone damage effects, respectively. E.C.-P., C.H., G.H., E.J.B., S.E.C., W.J.C., C.P.W., P.M.C., A.B.H. and T.P. contributed to the design of the IMOGEN model runs. All authors contributed to the interpretation of the results and to the writing of the paper.

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Correspondence to Edward Comyn-Platt.

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Comyn-Platt, E., Hayman, G., Huntingford, C. et al. Carbon budgets for 1.5 and 2 °C targets lowered by natural wetland and permafrost feedbacks. Nature Geosci 11, 568–573 (2018). https://doi.org/10.1038/s41561-018-0174-9

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