The hottest nights of the year are warming faster than the hottest days—by 0.32°C per decade, compared with 0.27°C per decade. That is the key finding of a global study on heat stress covering the period since 1950, published in Nature Climate Change.
The difference may seem small, but nighttime heat is particularly dangerous because it prevents the human body from recovering. As a result, deaths from cardiovascular and respiratory diseases increase. Heat, overall, is already the leading weather-related cause of death worldwide.
The researchers used the Universal Thermal Climate Index (UTCI), a measure of “feels-like” temperature that accounts for humidity, wind, and radiant heat. Because the index is nonlinear, it rises faster than standard air temperature: under hot conditions, the human body experiences disproportionately greater stress than a thermometer alone would suggest. In some regions, the number of heat-stress days has increased by as many as 50 days per year, while the hot season has become significantly longer.
Dangerous heat has expanded into regions where it was previously uncommon, extending its geographic footprint across every continent. At the same time, compound heat events—when an extremely hot day is followed by an exceptionally hot night—have become more frequent, more intense, and longer-lasting.
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