Observed Exceptional Heat–Humidity Combinations Increasing Faster than Previously Reported, and Implications for Projections

Significant increases in joint heat-humidity extremes are a robust feature of climate projections. The wet-bulb temperature threshold of 35°C is considered particularly important, corresponding to the level at which it is impossible for the human body to maintain homeostasis, but in the literature to date it has never been reported in observational data. Our analysis of more than 7500 global weather stations from HadISD reveals that instances of 35°C have already been briefly but reliably recorded in some cities of the coastal Middle East. Additionally, South Asia and coastal areas throughout the subtropics regularly experience values perilously close to this threshold.

We develop and employ a generalized-extreme-value model to estimate that the 30-year return value of global-maximum wet-bulb temperature at ERA-Interim resolution will exceed 35°C when global-mean temperature has risen between 1.5°C and 2.0°C above the pre-industrial, which at current rates of warming will occur by mid-century, rather than occurring only in the late 21^st century and under high-emissions scenarios, as had been expected from previous work. The impending occurrence of such severe heat and humidity over large populated regions represents a situation never before experienced.

We also present observational evidence that these wet-bulb temperature extremes have increased significantly over the 1979-2017 period, and are fostered by locally high SSTs as well as modulated by regional dynamics such as monsoons and large-scale climate modes of variability such as ENSO, leading to concentrated and simultaneous exposure risks. Overall, our results show that the wet-bulb temperature ‘safety margin’ between currently reported values and 35°C is both smaller than previously understood and rapidly shrinking, presenting a serious challenge to human survival in the hottest and most humid places on Earth.