Climate matters: Differences in trends and drivers of summer humid heat in Europe and the Southeast United States

Humid heat has increased in intensity, frequency, and duration across the world, including in mid-latitude regions not acclimated to it. Wet bulb globe temperature (WBGT) is one of the most representative humid heat metrics in terms of impacts to human health. Here, an established WBGT estimation formula and the high-resolution (9-km) ERA5-Land reanalysis dataset are used to examine summer humid heat trends and drivers across Europe and the Southeast United States. In Europe, both daytime and nighttime WBGT are increasing significantly across nearly the entire domain. Daytime and nighttime trends are of similar magnitude in many areas, with daytime trends largest in western and northern Europe and nighttime trends greatest in eastern and southern Europe. In addition, there are statistically significant and large frequency and duration trends in extreme (90^th percentile) WBGT, especially at night near the Mediterranean, where there are approximately three additional extreme nights per decade and extreme event duration is more than one day longer each decade. Unlike in Europe, nighttime WBGT trends in the Southeast United States are larger and more widely statistically significant than daytime trends across most of the region. Like in Europe, there are large increases in the frequency and duration of extremes, particularly at night. For example, regions near the Gulf and in Florida are experiencing nearly three additional extreme summer nights per decade and extreme events are approximately one night longer per decade. A quantification of the importance of WBGT components (temperature, dewpoint, wind speed, solar radiation) shows that dewpoint increases exceed temperature increases and are the primary driver of WBGT trends across the Southeast United States, a region characterized by its hot humid climate and proximity to warm water. However, in the milder and drier climate of Europe, temperature increases are the dominant driver of WBGT changes, with a strong correlation to positive trends in solar radiation. Overall, results suggest that the drivers of humid heat trends depend on regional climate characteristics, which may have broad applications to climate modelling of future humid heat.