Instrumental observations1,2 and reconstructions3,4 of global and hemispheric temperature evolution reveal a pronounced warming during the past ∼150 years. One expression of this warming is the observed increase in the occurrence of heatwaves5,6. Conceptually this increase is understood as a shift of the statistical distribution towards warmer temperatures, while changes in the width of the distribution are often considered small7. Here we show that this framework fails to explain the record-breaking central European summer temperatures in 2003, although it is consistent with observations from previous years. We find that an event like that of summer 2003 is statistically extremely unlikely, even when the observed warming is taken into account. We propose that a regime with an increased variability of temperatures (in addition to increases in mean temperature) may be able to account for summer 2003. To test this proposal, we simulate possible future European climate with a regional climate model in a scenario with increased atmospheric greenhouse-gas concentrations, and find that temperature variability increases by up to 100%, with maximum changes in central and eastern Europe.
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We thank the climatology and data teams of MeteoSwiss (Zürich, Switzerland) for providing access to their long-term homogenized temperature series, the Hadley Centre (Exeter, UK) for providing access to their climate change simulations, and the PRUDENCE team for access to climate simulations. We also thank our colleagues for comments on the manuscript. The computations were performed on the computing facilities of ETH and the Swiss Center for Scientific Computing (CSCS). This research was supported by the Fifth Framework Programme of the European Union (project PRUDENCE), by the Swiss Ministry for Education and Research, and by the Swiss National Science Foundation (NCCR Climate).
The authors declare that they have no competing financial interests.
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