Since when are 10% of summers historically hot?
Since climate change began pushing up average temperatures. See for example: http://www.google.com/hostednews/afp/article/ALeqM5jbK6a-zNlRk3Az-Upzue83KHF5Bw
It's hard to get a good sense of precisely what the probability is, given that I'm not a climate scientist, but 10% sounds about right - perhaps even a little low.
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!