This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 harder on the body when it comes before people have acclimated to the season or 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 midlatitude 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 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 with 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. They had expanded lopsidedly, pushing further into spring in some regions and fall in others.
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