Nature Geoscience 6, 65–70 (2013); published online 25 November 2012; corrected after print 27 June 2014.

In the version of this Article originally published, the flive values in Fig. 4c should have been listed in the reverse order, with '0.0–0.1' corresponding to the black curve and '0.9–1.0' to the red curve, as shown below. This error has now been corrected in all online versions of the Article.