Extreme Heat Is Creeping Into Spring and Autumn, Global Study Finds

Extreme heat is one of the deadliest climate hazards, and its danger depends on when it strikes: a spring heatwave catches people off guard before bodies and cooling systems are ready, while an autumn heatwave strains bodies and infrastructure already worn down by summer. A study published in AGU Advances, by researchers led by Ivanovich, is the first to measure whether the timing of extreme heat is changing worldwide.

Rather than fixed calendar seasons, the study defines a "local heat season" as the three consecutive months when extreme heat happened most often in the 1980s at each location, and "shoulder seasons" as the two months before and after it. Using the MERRA2 reanalysis dataset, cross-checked against ERA5, the researchers tracked how the share of annual extreme-heat days in these periods shifted over 45 years, comparing the 1980s with 2015-2024. They measured dry-bulb temperature (dry heat, damaging to plants and ecosystems) and wet-bulb globe temperature (humid heat, more strenuous for people) — combining temperature, humidity, wind and sunlight, devised by the US military in the 1950s.

In the 1980s, extreme heat was tightly confined to a single season: about 93% of the world's land area saw more than 80% of its extreme dry-heat days fall within that season, even in the tropics, where temperatures vary less over the year. Dry-heat and humid-heat seasons were often offset by about a month, particularly in monsoon regions such as north-western Mexico and central India, where the dry-heat season precedes the humid-heat season.

By 2015-2024, that confinement had loosened: extreme heat is spreading across the calendar year in more than half of the world's land area, but unevenly. In western Europe, southern Africa and north-western India, more extreme heat now occurs in the months before the traditional heat season, creeping toward spring. In much of the US, eastern China, northern Africa and eastern Europe, the increase is instead in the months after, creeping toward autumn.