Emission budgets and pathways consistent with limiting warming to 1.5 °C

Abstract

The Paris Agreement has opened debate on whether limiting warming to 1.5 °C is compatible with current emission pledges and warming of about 0.9 °C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 warming to less than 0.6 °C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers. We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0 °C above the mid-nineteenth century. If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5 °C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5 °C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.

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Figure 1: Warming as a function of cumulative CO2 emissions in the CMIP5 ensemble.
Figure 2: Emissions, forcing and temperature response associated with various mitigation scenarios.
Figure 3: Temperature trajectories and associated emissions consistent with 1.5 °C warming in 2100 for a range of climate responses under an adaptive mitigation regime.

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Acknowledgements

The authors would like to thank E. Hawkins, R. Pierrehumbert, G. Peters, R. Knutti, D. van Vuuren, K. Riahi and E. Kriegler for useful discussions and/or comments on an earlier draft. R.J.M. and P.F. acknowledge support from the Natural Environment Research Council project NE/P014844/1; R.J.M. and M.R.A. from the Oxford Martin Net Zero Carbon Investment Initiative (co-I Cameron Hepburn); M.R.A. from the Oxford Martin Programme on Resource Stewardship; and J.S.F. and R.B.S. from the Research Council of Norway through projects 235548 and 261728. H.D.M. acknowledges support from the Natural Sciences and Engineering Research Council of Canada. D.J.F. acknowledges support from the Deep South National Science Challenge and an internal grant from Victoria University of Wellington.

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R.J.M. conducted the analysis and produced Figs 2 and 3. J.R. conducted the CMIP5 analysis and produced Fig. 1. H.D.M. conducted the integrations with the UVic ESCM. R.J.M. produced an initial draft of the manuscript along with J.S.F., M.G., P.F. and M.R.A. All authors contributed to the experimental design, interpretation and revisions of the manuscript.

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Correspondence to Richard J. Millar.

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Millar, R., Fuglestvedt, J., Friedlingstein, P. et al. Emission budgets and pathways consistent with limiting warming to 1.5 °C. Nature Geosci 10, 741–747 (2017). https://doi.org/10.1038/ngeo3031

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