Nature Climate Change 5, 519–527 (2015); published online 21 May 2015; corrected after print 17 March 2016

In the version of this Perspective originally published, the units in Fig. 3a and the corresponding main text should have read kgCO2 GJ−1, not tCO2 GJ−1. This error has been corrected in the online versions of this Perspective.