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How much CO2 at 1.5 °C and 2 °C?

The atmospheric concentration of CO2 at the time of passing 1.5 °C or 2 °C is unknown due to uncertainties in climate sensitivity and the concentrations of other GHGs. Impacts studies must account for a wide range of concentrations to avoid either over- or underestimating changes in crop yields and land and marine biodiversity.

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Acknowledgements

This work was supported by the European Union Seventh Framework Programme FP7/2007-2013 under grant agreement no 603864 (HELIX: High-End cLimate Impacts and eXtremes; www.helixclimate.eu) and the BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). We gratefully acknowledge M. Richardson and colleagues12 for making their data freely available, and B. Booth, L. Gohar, G. Jones, M. Webb and P. Stott for advice.

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Correspondence to Richard A. Betts.

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Further reading

Fig. 1: PDFs and multimodel ensemble projections of CO2 and CO2e concentrations when GWLs of 1.5 °C and 2 °C are reached.
Fig. 2: ERF versus actual CO2, with and without other forcings.