I think of it like rolling dice. Say you've got a pair of 6-sided game die. Rolling a 12 represents a massive weather event (eg so-called "century storms"), and gobal warming replaced some of each cube's faces (say, changing the 4's or 5's to a 6) then for a given roll, you can't say for sure whether global warming's to blame -- but when 12's start showing up more than 1/12th of the time, you also can't say "that's absolutely not related".