Main findings:

The difference between the years in the 1960s and 2008 (a proxy for 2011) for extreme heat events was pronounced, with the return period of a particular extreme heat event being more than an order of magnitude shorter for 2008 than for any of the 3 years from the 1960s. As an example, 100-yr return period MAMJJA and JJA heat events under 1964 conditions had only 5- and 6-yr return periods, respectively, under 2008 conditions.

Full report:

EEE 2011 = Explaining Extreme Events of 2011 from a Climate Perspective