Study: Dangerous hot days are spreading beyond summer

Sept. 15, 2026, 6:05 a.m. ET

Sick of sultry Septembers? This story isn’t good news for folks who hate the heat.

Around the world, extreme heat is sneaking out of the summer and spreading both earlier into spring and later into fall, a new study suggests.

“We found that over more than half of global land areas, extreme heat seasons are expanding significantly,” Casey (Catherine) Ivanovich, a climate scientist at Columbia University and lead author of the new study, told USA TODAY.

Time matters

The timing of extreme heat matters: It’s harder on the body when it comes before folks have gotten used to the season, or if it comes after people have already had to endure months of it, according to a statement from the Columbia Climate School, which is affiliated with the NASA Goddard Institute for Space Studies.

“In some places, we have a larger expansion of extreme heat during the spring, before the traditional heat season starts,” she said. “In other places, there’s a much faster expansion of the heat season into fall.”

This makes it harder for communities to prepare for, since cooling centers and heat alerts are often organized around a summer calendar.

The study was published Sept. 10 in the journal AGU Advances, a publication of the American Geophysical Union.

What did the study find about extreme heat in the United States?

“In much of the U.S., we’re seeing a larger expansion of the extreme heat season into the fall than the spring,” Ivanovich told USA TODAY in an email. “This can unfortunately increase the overlap of extreme heat with other seasonal hazards, such as the wildfire season in the West and the Atlantic hurricane season in the East.”

When multiple hazards such as these happen at once or in rapid succession, “they are much more dangerous and impactful than if these events happened in isolation,” she added.

What about worldwide?

The study found that in eastern China, northern Africa and eastern Europe, extreme heat events became more common more rapidly in the two months following their historical heat seasons. The opposite held in western Europe, southern Africa and northwestern India, where extreme heat came predominantly in the two months before.

Why would heat waves in the spring and fall be more dangerous than in the summer?

According to Ivanovich, “heat waves in the spring can catch us off guard before our bodies have acclimated to the heat and before we’re ready to utilize cooling strategies like installing air conditioning.”

“In the fall, heat waves can be more dangerous if we’ve already experienced repeated extreme heat events that leave our bodies stressed.”

What’s new about the study?

“While recent papers have explored changing average temperatures and how the start and end of the summer season are changing, our study is the first to our knowledge to explicitly measure the change in the timing of extreme heat,” Ivanovich told USA TODAY.

In the study, “we also differentiate between dry and humid heat extremes, as these types of climate hazards have distinct impacts.” For example, dry heat is harder on crops and ecosystems; humid heat is more dangerous to people, because it limits the body’s ability to cool itself by sweating. Adapting to one is not the same as adapting to the other.

1980s vs. today

To do the study, scientists compared extreme heat events in the 1980s to those more recently (2015 to 2024).

Their analysis found that extreme heat seasons had expanded significantly across just over half the world’s land area for dry heat, and just under half for humid heat. And they had expanded lopsidedly, pushing further into spring in some regions and fall in others, according to the Columbia Climate School.