Assessing Summer Humid Heat in Europe: Trends, Extremes, and Drivers

Humid heat has increased in intensity, frequency and duration across the world, including in mid-latitude regions not acclimated to it. While Europe is no stranger to deadly and economically damaging extreme heat events, many infamous European heat waves during the 21st century have been relatively dry, resulting in limited research on humid heat in the region. Using wet bulb globe temperature (WBGT), this study uses high-resolution reanalysis data to investigate trends in the intensity, frequency, and duration of humid heat throughout Europe during climatological summers, 1950–2023. Results show that average and extreme (90th percentile) WBGT intensity exhibit statistically significant increases throughout Europe. Extreme humid heat intensity is increasing faster than average humid heat intensity across much of northern Europe and the Mediterranean coast, especially at night. Daytime humid heat trends are largest in western and northern Europe, while nighttime trends exceed daytime trends in eastern and southern Europe. Furthermore, there are statistically significant and large frequency and duration trends in extreme humid heat, especially at night near the Mediterranean, where extreme events are occurring approximately three extra nights per decade and extreme event duration is more than 1 day longer each decade. Unlike the Southeast United States, where dewpoint (humidity) increases are most responsible for WBGT increases, temperature increases are the dominant driver of positive WBGT trends across most of Europe. Results demonstrate that while humid heat is worsening in many locations across the world, it can amplify for different reasons in different regions.