Background
Much has been made of the supposed uniqueness of the summer-of-2003 European heat wave, its implied connection to CO2-induced global warming, and the proposal that it is evidence of a climatic regime shift to one of greater variability that supports the more frequent occurrence of more extreme warm events (Schar et al., 2004; Stott et al., 2004; Trigo et al., 2005).

What was done
In a study designed to test these claims, Chase et al. utilized NCEP global reanalysis data for the period 1979-2003 to calculate extreme tropospheric temperature events over the region 22°N to 80°N throughout the June-July-August period (and globally using annual averages), after which they compared the results with the corresponding particulars of the European heatwave of 2003 in terms of "standard deviations exceeded and correlations between regional extremes and temperatures at larger spatial scales."

What was learned
Quoting the researchers who conducted the work, (1) "extreme warm anomalies equally, or more, unusual than the 2003 heat wave occur regularly," (2) "extreme cold anomalies also occur regularly and can exceed the magnitude of the 2003 warm anomaly," (3) "warmer than average years have more regional heat waves and colder than average years have more cold waves," (4) "natural variability in the form of El Niņo and volcanism appears of much greater importance than any general warming trend in causing extreme regional temperature anomalies," and (5) "regression analyses do not provide strong support for the idea that regional heat or cold waves are significantly increasing or decreasing with time during the period considered here."

What it means
Chase et al. conclude by saying their analysis "does not support the contention that similar anomalies as seen in summer 2003 are unlikely to recur without invoking a non-stationary statistical regime with a higher average temperature and increased variability." In other words, the 2003 European summer heat wave implies nothing at all about CO2-induced global warming. It was merely a rare, but not-unprecedented, weather event, of which there have been several other examples (both hot and cold, and some stronger) over the past quarter-century.

References
Schar, C., Vidale, P.L., Luthi, D., Frei, C., Haberil, C., Liniger, M.A. and Appenzeller, C. 2004. The role of increasing temperature variability in European summer heatwaves. Nature 427: 332-336.

Stott, P.A., Stone, D.A. and Allen, M.R. 2006. Human contribution to the European heatwave of 2003. Nature 432: 610-614.

Trigo, R.M., Garcia-Herrera, R., Diaz, J., Trigo, I.F. and Valente, M.A. 2005. How exceptional was the early August 2003 heatwave in France? Geophysical Research Letters 32: 10.1029/2005GL022410.