Changes of western European heat wave characteristics projected by the CMIP5 ensemble

We investigate heat waves defined as periods of at least 3 consecutive days of extremely high daily maximum temperature affecting at least 30 % of western Europe. This definition has been chosen to select heat waves that might impact western European electricity supply. Even though not all such heat waves threaten it, the definition allows to identify a sufficient number of events, the strongest being potentially harmful. The heat waves are characterised by their duration, spatial extent, intensity and severity. The heat wave characteristics are calculated for historical and future climate based on results of climate model simulations conducted during the 5th Phase of the Coupled Model Intercomparison Project (CMIP5). The uncertainty of future anthropogenic forcing is taken into account by analysing results for the Representative Concentration Pathway scenarios RCP2.6, RCP4.5 and RCP8.5. The historical simulations are evaluated against the EOBS gridded station data. The CMIP5 ensemble median captures well the observed mean heat wave characteristics. However, no model simulates a heat wave as severe as observed during August 2003. Under future climate conditions, the heat waves become more frequent and have higher mean duration, extent and intensity. The ensemble spread is larger than the scenario uncertainty. The shift of the temperature distribution is more important for the increase of the cumulative heat wave severity than the broadening of the temperature distribution. However, the broadening leads to an amplification of the cumulative heat wave severity by a factor of 1.7 for RCP4.5 and 1.5 for RCP8.5.