New study reveals extreme heat seasons are expanding symmetrically

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.

That timing matters. Extreme heat is harder on the body when it comes before people have acclimated to the season, or if it comes after people have already had to endure months of it. It's also harder for communities to prepare for, since cooling centers and heat alerts are built around a summer calendar.

In some places, we have a larger expansion of extreme heat during the spring, before the traditional heat season starts. In other places, there's a much faster expansion of the heat season into fall," says Catherine Ivanovich, lead author of the new study, which appears in AGU Advances. Ivanovich is a climatologist at NASA Goddard Institute for Space Studies (GISS), which is affiliated with the Columbia Climate School.

Research on seasonal warming has tended to concentrate on average temperatures and shifts in the timing of seasons. Summer weather conditions in mid-latitude regions, for example, have lengthened by roughly six days per decade since 1990. Little if any research has addressed the timing of individual extreme heat events, which may not move in step with the seasonal average.

Ivanovich and her colleagues, fellow GISS climate scientist Benjamin Cook and New York University's Sonali Shukla McDermid, expected that the rising global temperatures associated with climate change would make it uniformly easier to cross dangerous heat thresholds throughout the year. That's not what they found.

The researchers counted extreme heat events on the six inhabited continents between 1980 and 1989, defining "extreme" as days in the hottest 5% of daily temperatures. They did this twice: once using standard thermometer readings, a measure of dry heat, and once using wet bulb globe temperature, a measure of humid heat that combines humidity, solar radiation and air temperature to quantify heat stress as the human body experiences it."

The authors then compared those 1980s baseline figures to the most recent decade in the record, 2015 through 2024. The 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.

In the western United States, 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.

The city of Phoenix, for example, recorded 183 extreme heat days by temperature in the baseline decade of the 1980s, and 338 in the decade ending in 2024. None of the 1980s events fell after the heat season had ended; between 2015 and 2024, 6 percent did. The median date of the city's dry heat extremes moved 10 days later in the year, while its humid heat extremes moved 5.5 days earlier.

After 113 consecutive days above 100°F in 2024, from late September into mid-October, the city went on to tie or break daily temperature records 21 days in a row. Maricopa County, where Phoenix is located, had 608 heat-related deaths in 2024, with 46 percent of them in July, the hottest month of the year; in 2023, July accounted for 64 percent of that year's 645 deaths. And this past March (after the study period had ended), the city had nine days that topped 100°F, something that had happened only once before in March over the entire historical record.

The researchers ran their analysis with two datasets-one compiled by NASA, the other by the European Centre for Medium-Range Weather Forecasts-to be sure the pattern held up. It largely did.

There are very clear asymmetries in how extreme heat seasons are expanding in different parts of the world. Extreme heat is starting to become something different in a lot of these regions."

Catherine Ivanovich, lead author of the new study

Ivanovich says the findings should be taken as a compelling first line of evidence. Extreme heat is rare by definition, and extreme heat outside its season is rarer still, making changes hard to pin down statistically. A next step in the research is to repeat the comparison using climate simulations, which can provide many more theoretical versions of reality (and thus a larger sample of extreme events) under the same climatic conditions. If the models agree with what the observations show, that will be more evidence that the observed changes are indeed a new pattern.

When the researchers tested whether ordinary warming alone could produce the pattern, it accounted for changes in the heart of the heat season but not for the lopsidedness at its edges. Climate models should help determine the respective contributions of human-induced climate change and natural variability. "We can't confirm what share of the signal is due to climate change using observations alone," says Ivanovich, but "it's certainly the primary component of the story." 

Whatever the causes, the implications are many. 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.

Changes in the seasonal timing of extreme heat also make it more likely to intersect with other seasonal hazards: peak wildfire season in the western United States, peak hurricane season in the Southeast. When multiple hazards happen at once or in rapid succession, "they are much more dangerous and impactful than if these events happened in isolation," says Ivanovich.