Mortality impacts of the most extreme heat events

Extreme heat threatens human life, evidenced by >260,000 heat-related fatalities collectively in the deadliest events since 2000. In this Review, we link physical climate science with heat mortality risk, including crossings of uncompensable thresholds (beyond which human core body temperature rises uncontrollably) and unsurvivable thresholds (lethal core temperature increase within 6 h). Uncompensable thresholds (wet-bulb temperatures ~19–32 °C) depend strongly on age and the combination of air temperature and relative humidity. These thresholds have been breached rarely for younger adults (~2.2% of land area over 1994–2023) but more widely for older adults (~21%). Unsurvivable thresholds (wet-bulb temperatures ~20–34 °C) were only exceeded for older adults (~1.8% of land area). Anthropogenic warming will lead to more frequent threshold crossings, including tripling of the uncompensable land area for young adults if warming reaches 2 °C above preindustrial levels. Interdisciplinary work must improve the understanding of the deadly potential of unprecedented heat and how it can be reduced. Ensuring reliable access for all to cool refugia is an urgent priority as the atmosphere threatens to increasingly overwhelm human physiology under climate warming. Extreme heat is increasing in magnitude and frequency, threatening human health. This Review assesses mortality risk associated with extreme heat, revealing that human thermal tolerances (that is, uncompensable thresholds) were crossed for ~2% and 21% of global land area for young adults and older adults, respectively, from 1994 to 2023.