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Differences between carbon budget estimates unravelled

Abstract

Several methods exist to estimate the cumulative carbon emissions that would keep global warming to below a given temperature limit. Here we review estimates reported by the IPCC and the recent literature, and discuss the reasons underlying their differences. The most scientifically robust number — the carbon budget for CO2-induced warming only — is also the least relevant for real-world policy. Including all greenhouse gases and using methods based on scenarios that avoid instead of exceed a given temperature limit results in lower carbon budgets. For a >66% chance of limiting warming below the internationally agreed temperature limit of 2 °C relative to pre-industrial levels, the most appropriate carbon budget estimate is 590–1,240 GtCO2 from 2015 onwards. Variations within this range depend on the probability of staying below 2 °C and on end-of-century non-CO2 warming. Current CO2 emissions are about 40 GtCO2 yr−1, and global CO2 emissions thus have to be reduced urgently to keep within a 2 °C-compatible budget.

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Figure 1: Proportionality of global-mean temperature increase to cumulative emissions of CO2.
Figure 2: An illustration of the methods for computing TEBs versus TABs.
Figure 3: Non-CO2 forcing and cumulative CO2 emissions.

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Acknowledgements

We acknowledge the work by IAM modellers that contributed to the IPCC AR5 Scenario Database and the climate modelling teams contributing to CMIP5. We thank IIASA for hosting the IPCC AR5 Scenario Database, and M. Meinshausen for detailed comments and feedback on the manuscript.

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Contributions

All authors contributed to parts of the underlying research during the writing process of the IPCC AR5. J.R. coordinated the conception and the writing of the paper. J.R. carried out the research with significant contributions from M.S., and developed the TEB and TAB conceptual framework. J.R. produced the figures and wrote the first draft of the manuscript. All authors contributed to interpreting and discussing the results, and writing the paper.

Corresponding author

Correspondence to Joeri Rogelj.

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The authors declare no competing financial interests.

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Rogelj, J., Schaeffer, M., Friedlingstein, P. et al. Differences between carbon budget estimates unravelled. Nature Clim Change 6, 245–252 (2016). https://doi.org/10.1038/nclimate2868

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