Trends in Heat Stress During Heat Waves Across Europe (1979-2023)

With heat waves becoming more frequent and severe under global warming, heat stress becomes a public health risk. The Simplified Wet Bulb Globe Temperature (sWBGT) is a widely used heat stress index that combines temperature and relative humidity effects. Using observations and reanalysis datasets, we identify heatwave days and analyze sWBGT variations and trends from 1979 to 2023 across Northern Europe (NEU), Western and Central Europe (WCE), and the Mediterranean (MED). Our results show positive sWBGT trends during heatwave days over most of Europe, except France, the Balkans, and western Russia. Decomposition into temperature-driven (sWBGT(const RH)) and humidity-driven (sWBGT(const T)) components reveals regional differences in contributions to sWBGT trends. In NEU, both temperature and relative humidity contribute to the total increase. In WCE, the trend is largely explained by sWBGT(const RH), with secondary role for sWBGT(const T). In MED, increases are mainly temperature-driven, with a weak relative humidity contribution that varies in sign across datasets. Comparisons of ERA5 with station records reveal biases in ERA5 relative humidity in cities with urban heat island effects. Station data (Oslo, Hamburg, Madrid, Vienna, Warsaw) show increases in days exceeding sWBGT “alert” (26.7–29.3) and “caution” (29.4–31.0) levels. Madrid shows the largest rise in alert-level days, from 19 (1979–2000) to 95 (2001–2023).These changes highlight a growing burden of heat stress, not only in vulnerable southern regions but also expanding into temperate zones. Our findings provide scientific evidence for understanding heat stress variability and support development of effective adaptation strategies.