A 41-year bioclimatology of thermal stress in Europe

The bioclimatology of thermal stress over Europe between 1979 and 2019 was analysed using the Universal Thermal Climate Index (UTCI) derived from ERA5-HEAT reanalysis. The bioclimatology of different European regions was assessed using Koppen-Geiger climate classification. The annual number of hours with heat stress (UTCI > 32 degrees C) increased significantly during the study period for all the analysed Koppen-Geiger climate subclasses, showing also a clear increase towards southern Europe. The highest percentage of hours (20% of all hours) with cold stress (UTCI < -13 degrees C) occur over northern Europe. A significant increasing trend (>0.05 hr center dot year(-1)) in the number of hours with heat stress was observed for 23 out of 32 analysed European cities representative for the Koppen-Geiger climate subclasses. For these cities not only the number of hours with heat stress has increased but also the heat stress is more persistent, while the number of cases and the persistence of the periods with cold stress have decreased over the last four decades. The UTCI values showed a statistically significant increase between 0.6 and 3.2 degrees C for all the analysed cities over the study period reflecting the rising of global mean temperatures.