The drought was brought about by the failure of two consecutive rainy seasons: the "short rains" in late 2010 and the "long rains" at the start of 2011. Climatologist Peter Stott of the Met Office Hadley Centre in Exeter, UK, and his colleagues ran climate models, with and without a human influence on climate, and compared the likelihood of the rains failing.
Humanity's activities had no effect on the short rains – they failed because of a strong La Niña in the Pacific. "That's natural," says Stott.
But climate change did affect the long rains, making them more likely to fail (Geophysical Research Letters, doi.org/kmv). The model could only reproduce the scale of the drought if it included greenhouse gas emissions.