Intrade gives 2011 a 34% chance to be the warmest year on record, so 10% seems low.
But that's global annual temperature, not US summer temperature. The closest thing I could find to US summer temperature with a 5 minute search is the NASA GISS dataset for the average northern hemisphere land-surface temperature in June-August. The record summer high for the northern hemisphere was broken in 2010, 2005, 1998, 1995, 1990, 1988, 1987, and 1983, which also suggests that the probability of a record-breaking US summer is around 30% rather than 10%.
A little more searching turned up this NOAA/USHCN data set, which shows that the hottest summer (June-Aug) in recorded US history (contiguous 48 states, since 1895) is still 1936, so maybe 10% is closer to the truth. The 10 hottest US summers on record are 1936 (74.64 F), 2006 (74.36 F), 1934 (74.18 F), 2010 (73.96 F), 2002 (73.96 F), 1988, 2007, 2003, 1933, and 2001.
To make this needlessly precise, I fit a little model to the data and estimated that there's a 7% chance of breaking the 1936 record and a 12% chance of topping the 2006 temperature. For the...
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!