A new analysis in today’s Nature by Stott, Stone and Allen concludes that human-caused climate change doubles the odds of heat waves like the summer 2003 bender that killed at least 22,000 people:
Using a threshold for mean summer temperature that was exceeded in 2003, but in no other year since the start of the instrumental record in 1851, we estimate it is very likely (confidence level >90%) that human influence has at least doubled the risk of a heatwave exceeding this threshold magnitude.
Stott et al. make a very interesting point, which is worth quoting:
“It is an ill-posed question whether the 2003 heatwave was caused, in a simple deterministic sense, by a modification of the external influences on climate?for example, increasing concentrations of greenhouse gases in the atmosphere?because almost any such weather event might have occurred by chance in an unmodified climate.”
In other words, you can’t just say “global warming caused the heat wave.” All you can do is look at ranges of statistical probabilities, and determine where this fits in. It’s an important point to remember in understanding climate change science. I’m glad they said it so explicitly up front.
 Human contribution to the European heatwave of 2003, Peter A. Stott, D. A. Stone and M. R. Allen, Nature 432, 610 – 614 (02 December 2004); doi:10.1038/nature03089