Correction to: Nature Geoscience https://doi.org/10.1038/s41561-018-0159-8, published online 9 July 2018.

In the version of this Article originally published, the units of the x and y axes in Fig. 3a were incorrectly given as ‘mg km–2 yr–1’; the correct units are ‘Mg km–2 yr–1’. These errors have now been corrected in the online versions.