From Wired Science:
No, you cannot blame individual events like [the tornado in Brooklyn] on climate change, as it was possible for them to occur even without the human-made changes to the atmosphere. However, it is fair to ask whether the human changes have altered the likelihood of such events. There the answer seems to be yes. Storms driven largely by latent heat, and that includes thunderstorms, are expected to become stronger as the air becomes warmer and contains more moisture. Global warming does cause just such a tendency.
–James Hansen, scientist at NASA