Doubled length of western European summer heat waves since 1880

[1] We analyzed a new data set of 54 high-quality homogenized daily maximum temperature series from western Europe (Austria, Belgium, Croatia, Czech Republic, Denmark, Finland, France, Germany, Ireland, Netherlands, Portugal, Spain, Sweden, Switzerland, United Kingdom) to define more accurately the change in extreme warm Daily Summer Maximum Temperature (DSMT). Results from the daily temperature homogeneity analysis suggest that many instrumental measurements in the late 19th and early 20th centuries were warm- biased. Correcting for these biases, over the period 1880 to 2005 the length of summer heat waves over western Europe has doubled and the frequency of hot days has almost tripled. The DSMT Probability Density Function (PDF) shows significant changes in the mean (+ 1.6 +/- 0.4 degrees C) and variance (+ 6 +/- 2%). These conclusions help further the evidence that western Europe's climate has become more extreme than previously thought and that the hypothesized increase in variance of future summer temperature has indeed been a reality over the last 126 years.