Dangerous heat days are expanding beyond summer, with regional spring-fall shifts
Extreme heat has become more common—both research and lived experience document this—but when it arrives is a different question. A new study finds that, rather than occurring only during warm seasons, extreme heat events have become more frequent in the months before and after, too.
Extreme heat is no longer limited to just the summer months, according to a new study. Researchers found that in some regions, the hottest days have started to occur more frequently in the spring and fall, before and after the traditional heat season. This shift in timing can make extreme heat events more dangerous and harder for communities to prepare for.
In places like Phoenix, the number of extreme heat days has risen significantly over the past few decades, with the city experiencing 338 extreme heat days in the decade ending in 2024, compared to just 183 in the 1980s. The study, which analyzed data from six inhabited continents, found that extreme heat seasons have expanded unevenly around the world, pushing into spring in some areas and fall in others.
The researchers used two different temperature measures to define an extreme heat event and compared current data with figures from the 1980s. They found that the lopsided expansion of extreme heat seasons is a clear pattern, likely driven by human-induced climate change. The findings highlight the need for communities to adapt to the changing timing of extreme heat, as it can intersect with other seasonal hazards like wildfires and hurricanes.
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