Nature Geoscience 9, 683–686 (2016); published online 22 August 2016; corrected after print 6 September 2016.

The original version of this Letter contained a typographical error in the final sentence of the first paragraph in the main text. This sentence should have read 'The ratio, described by its δ13CO2 value7, is strongly affected by the exchange of carbon between the biosphere and the atmosphere because photosynthesis preferentially takes up 12C, resulting in a low δ13C in biosphere-derived carbon, at a mean value around −25‰ (ref. 8).' This has been corrected in all the online versions of this Letter.