Ranking European capitals by exposure to heat waves and cold waves

In warming Europe, we are witnessing a growth in urban population with aging trend, which will make the society more exposed and vulnerable to extreme weather events. In the period 1950-2015 the occurrence of extreme heat waves increased across European capitals. As an example, in 2010 Moscow was hit by the strongest heat wave of the present era, killing more than ten thousand people. The cold extremes will have decreasing tendency as global warming progresses, however due to higher variability of future climates, the cold wave hazard may remain locally important threat. Moreover, the heat and cold-related mortality will be enhanced with foreseen demographic evolution in European cities. Here we focus on larger metropolitan areas of European capitals (EU28 plus Moscow, Oslo and Zurich). By using an ensemble of eight EURO-CORDEX models under the RCP8.5 scenario, we detected heat waves and cold waves events by deployment of Heat Wave Magnitude Index daily and its cold wave counterpart. We introduce a ranking procedure based on ensemble predictions using the median of metropolitan grid cells for each capital, and population density as a proxy to quantify the future exposure. All the investigated European metropolitan areas will be more vulnerable to extreme heat in the coming decades. Based on the impact ranking, results reveal that cold waves will represent some threat in mid of the century but they will not be the major threat in any of European capitals, and that they are projected to completely vanish by the end of this century. On the contrary, in near, but even more so in distant future, extreme heat events in European capitals will be not exclusive to traditionally exposed areas such as the Mediterranean and the Iberian Peninsula. The ranking of European capitals based on their exposure to extreme heat is of paramount importance to decision makers in order to mitigate the heat related mortality, especially with the foreseen increase of global mean temperature. Furthermore, this simple comparative indicator helps communicating the global, complex and impersonal issue of climate change locally thus contributing to raise awareness and call for action.