Nature Climate Change 4, 93–98 (2014); published online 29 January 2014; corrected after print 29 January 2014

In the version of this Perspective originally published, the final phrase of the abstract should have read 'are 0.2, which is larger than might be expected from some model simulations.' Additionally, the affiliation number for Norman Loeb was missing in the address list. These errors have been corrected in the online versions of the Perspective.