Concentrated and Intensifying Humid Heat Extremes in the IPCC AR6 Regions

Extreme humid heat events have seen rapid increases globally in recent decades, but regional changes and higher-order temporal characteristics, such as interannual and intra-annual clustering, have not been widely explored. Using ERA5 reanalysis data from 1979 to 2019, we find increasing trends of varying magnitudes in extreme wet-bulb temperatures at the Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC AR6) regional scale. In many locations, interannual variations in extremes show a strong relationship with the El Nino-Southern Oscillation. The temporal proximity of precipitation events to humid heat days in arid regions suggests that local moisture effects may lead to clustering. Knowledge of these spatial and temporal patterns aids in understanding how potential heat stress is increasing, as well as facilitates the development of regionally specific adaptation and mitigation strategies for combating the associated societal impacts. Plain Language Summary Extreme humid heat, or the combination of high temperature and humidity, poses a more severe threat to human health than does dry heat alone. Though extremes are particularly dangerous, even moderate levels of humid heat can lead to a variety of health and socioeconomic effects. Motivated by the growing demand for regional, decision-relevant climate information, we calculate historical changes in the intensity of humid heat extremes in the regions used in the Intergovernmental Panel on Climate Change Sixth Assessment Report. Humid heat extremes have intensified in most regions, though some areas have experienced greater increases than others. The timing of extremes also affects their impacts, and thus we additionally analyze how humid heat extremes are distributed, both within the year and across all years. In many locations across the world, extreme humid heat is more common during strong El Nino episodes. In some typically dry regions, extremes tend to occur near each other within a given year and around the same time as rainfall events. Our results help advance the understanding of potential heat stress and the development of regionally specific strategies for combating its impacts.