Global heat stress intensification and its expanding footprint on the human population

Climate warming is driving more frequent and intense heat extremes. Yet, changes in heat stress, the leading cause of weather-related mortality, remain poorly quantified at the global scale. Here, using the Universal Thermal Climate Index, we assess heat stress globally since 1950, examining daytime extremes, nocturnal heat and compound daytime–nighttime events, revealing a pronounced, multidimensional intensification. Extreme ‘feels-like’ temperatures have become more frequent on every continent, and the spatial footprint of hazardous heat has expanded, exposing previously unaffected regions. Heat stress days and tropical nights have increased, with some regions experiencing up to 50 additional heat stress days annually and an extended heat stress season. The hottest nights of the year are warming faster (0.32 °C per decade) than the hottest days (0.27 °C), and compound events are more frequent, severe and prolonged. Population exposure to dangerous heat has increased markedly, driven by intensifying heat stress in addition to population growth.