10%: the United States has a historically hot summer.
This is only notable because it's the only factor that could get cap-and-trade legislation through a Republican House. (Whose bright idea was it to have the 2009 Copenhagen climate change summit in winter? The juxtaposed headlines with the winter storm may have set back the legislation indefinitely.)
Looks like I was underconfident in retrospect: according to the source that I agreed on when I made the prediction, 2011 was the second-warmest US summer on record.
(Even though it wasn't the warmest on record, the fact that it came in second is evidence that I probably should have estimated this at somewhere around the Intrade global temperature figure of 34%.)
As we did last year, use this thread to make predictions for the next year and next decade, with probabilities attached when practical.
Happy New Year, Less Wrong!