How big a deal is this indirect cloud effect? Huge, actually. In just 5 years it was responsible for a 2% decrease in low clouds (the kind that reflect incoming solar radiation by day) which, in turn, equates to an increase in surface warming of 1.2 Wm-2 from incident radiation — equivalent to some 85% of the IPCC’s estimate for the effect of all carbon dioxide increase since the Industrial Revolution.