Nature Geoscience 7, 716–721 (2014); published online 14 September 2014; corrected after print 24 September 2014.

In the version of this Letter originally published, in the main text, the number of different combinations given for each grid point in Fig. 3 were incorrect and should have read “28 (77)”. Additionally, the second sentence describing the parameters in the final equation should have read “Significance at the 5% level is thus assigned for n = 28 combinations of E and P data sets if and for n = 77 combinations of E and Ep data sets also if .” Furthermore, in the key for Fig. 4a, the values should have read 1.5 (−1.5). These errors have no influence on the results of the study, and have now been corrected in the online versions of the Letter.