Heat Stress Threats in Europe: A Comprehensive Analysis of sWBGTVariations and Trends (1979 -2023)

Heat stress has lately been acknowledged as a significant threat to public health, with heat waves becoming more frequent and severe due to global warming. The Simplified Wet Bulb Globe Temperature (sWBGT) is a effective indicator for heat stress, combining both temperature and relative humidity. Using observations and reanalysis datasets, we identify annual heatwave days (HWD) and analyze sWBGT variations and trends during HWD. We focus on three European regions: Northern Europe (NEU), Western and Central Europe (WCE), and the Mediterranean (MED). We observed an increasing trend in sWBGT over most of Europe , with the exception of areas around the Black Sea, parts of eastern and western WCE, and the western MED. Importantly, the  contribution of temperature and humidity on heat stress vary by regions. In NEU, positive trends in both temperature and relative humidity contribute to increased heat stress, with temperature showing a more significant rising trend (0.4°C/decade). In WCE, while the overall trend in sWBGT is positive, changes in relative humidity are minimal (0.007% /decade), with temperature trends being the primary driver. In MED, a positive trend in sWBGT of 0.3 /decade is a residual of a  negative trend in relative humidity and a positive temperature trend. Comparing ERA5 dataset with meteorological station data revealed biases in the ERA5 data in Mediterranean cities with pronounced urban heat island effects. Analysis of sWBGT threat levels showed that NEU and WCE regions currently remain at safe levels. In contrast, most MED regions are at alert levels, with some areas escalating to caution levels. Our research provides comprehensive insights into heat stress variations across European regions over recent decades. This work can provide scientific evidence to help policymakers develop effective adaptation to address potential future heat stress threats.