Nature Climate Change. 2, 510–513 (2012) published online 15 April 2012; corrected after print 7 September 2012.

In the version of this Letter originally published, the unit on the right-hand y axis of Figure 2a and Supplementary Figures S3b, S5b, S7b, S9b and S11b should have read gC m−3. This error has now been corrected in the HTML and PDF versions of the Letter.