You're making the "It's snowing there can't be global warming" argument in reverse.
One local drought can't be attributed to or be proof of global warming.
It's like smoking causing lung cancer; (the following numbers are made up) 5 in 100 non-smokers might get lung cancer but 20 in 100 smokers do. 5 of those smokers would have gotten lung cancer anyway and 15 wouldn't have but there's no way to know which individuals are in which cohort.
