Nature Clim. Change 5, 138–142 (2015); published online 26 January 2015; corrected after print 28 January 2015

In the version of this Letter originally published, the fourth author affiliation should have read '4Institute for Atmospheric and Climate Science, Department of Environmental Sciences, ETH Zurich, CH-8092 Zurich, Switzerland.' This error has been corrected in the online versions of the Letter.