Evolution of heat wave occurrence over the Paris basin (France) in the 21st century

We analysed the temporal evolution of heat wave frequency in the Paris region in a changing future climate (1960-1989,2020-2049,2070-2099). Firstly, a method for extracting heat waves from observed or simulated time series of daily minimum and maximum temperatures was proposed, based on heat-impact considerations. It was evaluated over the period 1951-2009 using observations from 2 meteorological stations of the Meteo-France operational network. The past heat waves that have affected the Paris region were correctly identified in terms of both dates and durations. Eight heat waves were extracted from the data for this period of time (i.e. a mean frequency of about 1 in 7 yr). The method was then applied to a large sample of climate projections, including 1 regional climate model (RCM) following 3 emission scenarios (A2, A1B, B1) and 9 combinations of RCMs driven by different global circulation models following the A1B emission scenario only (from the European ENSEMBLES project). The historical runs generated 1 to 5 heat waves, depending on the model, with a median value of 3 events (i.e. a frequency of 1 in 10 yr). All models from the ENSEMBLES database simulated a systematic increase in the frequency of heat wave occurrences with time: 1 heat wave every 2 yr on average over 2020-2049, rising to at least 1, and up to 2, heat waves per year on average over 2070-2099. They also became much longer, with mean durations varying between 6 and 12 d over 2070-2099, and exceptional durations reaching 5 to 9 wk. A comparison of the heat waves simulated with the 3 different emission scenarios highlights the major impact of the scenario on the number of heat wave days (and the duration of heat waves) at the end of the 21st century.