Concrete actions to curtail greenhouse gas emissions have so far been limited on a global scale1, and therefore the ultimate magnitude of climate change in the absence of further mitigation is an important consideration for climate policy2. Estimates of fossil fuel reserves and resources are highly uncertain, and the amount used under a business-as-usual scenario would depend on prevailing economic and technological conditions. In the absence of global mitigation actions, five trillion tonnes of carbon (5 EgC), corresponding to the lower end of the range of estimates of the total fossil fuel resource3, is often cited as an estimate of total cumulative emissions4,5,6. An approximately linear relationship between global warming and cumulative CO2 emissions is known to hold up to 2 EgC emissions on decadal to centennial timescales7,8,9,10,11; however, in some simple climate models the predicted warming at higher cumulative emissions is less than that predicted by such a linear relationship8. Here, using simulations12 from four comprehensive Earth system models13, we demonstrate that CO2-attributable warming continues to increase approximately linearly up to 5 EgC emissions. These models simulate, in response to 5 EgC of CO2 emissions, global mean warming of 6.4–9.5 °C, mean Arctic warming of 14.7–19.5 °C, and mean regional precipitation increases by more than a factor of four. These results indicate that the unregulated exploitation of the fossil fuel resource could ultimately result in considerably more profound climate changes than previously suggested.
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We thank M. Berkley for assistance in accessing data, and G. Boer and N. Swart for providing comments on the initial version of the manuscript. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We acknowledge EMIC AR5 contributors for providing data for EMIC inter-comparison.
The authors declare no competing financial interests.
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Tokarska, K., Gillett, N., Weaver, A. et al. The climate response to five trillion tonnes of carbon. Nature Clim Change 6, 851–855 (2016). https://doi.org/10.1038/nclimate3036
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