Will Oremus has reported at Slate that more people nowadays are believing in global warming, because more people have experienced extreme weather recently.
What accounts for the rebound? It isn’t the economy, which has thawed only a little. And it doesn’t seem to be science: The percentage of respondents to the Yale survey who believe “most scientists think global warming is happening” is stuck at 35 percent, still way down from 48 percent four years ago. . . . No, our resurgent belief in global warming seems to be a function of the weather. A separate Yale survey this spring found that 82 percent of Americans had personally experienced extreme weather or natural disasters in the past year.
Pat Robertson changed his mind about global warming, too, because he reported a few years back that his back yard was noticeably hotter. (Note: Robertson more recently said he's not a "disciple of global warming" because there are no SUV's on Mars, so there's that… if you hold your views on weak evidence, it's easy for other weird thoughts to influence you.) And, do you remember how the warming denialists went crazy when D.C. had that big snowstorm?
And so we see the problem with anecdotal evidence: it is certainly relevant, but it is not systematic, often not representative, regularly selective, and too often framed by how the question was asked or by the intensity of the event reported.