Nature 498, 355–358 (2013); doi:10.1038/nature12278

In this Letter, the affiliation for Matthew T. Woodhouse was incorrectly listed as CSIRO, Australia (affiliation number 2), whereas this should have been set as his ‘present’ address. The address for M.T.W. while the work in this Letter was being carried out was: Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK (affiliation number 1). This has been corrected in the HTML and PDF versions of the manuscript.