A study published in AGU Advances suggests that extreme heat — already among the deadliest climate dangers — is increasingly occurring outside of summer months.
Here's what to know
The researchers found that in more than half of the world's land areas, hazardous heat is showing up more often in spring and autumn, with possible consequences for human health, infrastructure, and ecosystems.
Using climate records from 1980 to 2024, the study determined that extreme heat events are becoming more frequent in the "shoulder seasons" on either side of the hottest part of the year, Carbon Brief reported.
Instead of relying on standard calendar seasons, the researchers identified a "local heat season" for each place as the three consecutive months in the 1980s when extreme heat occurred most often. They then examined the two months before and after that span. When they compared 1980-89 with 2015-24, they found that heat extremes had become less concentrated within that single core season.
In the 1980s, roughly 93% of the world's land area experienced more than 80% of its extreme dry-heat days within its traditional dry-heat season. However, researchers say that pattern is starting to break down.
The shift is not happening the same way everywhere. Western Europe, southern Africa, and northwestern India are seeing more extreme heat ahead of their historical heat seasons, while much of the United States, eastern China, northern Africa, and eastern Europe are seeing more of it afterward.
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The researchers also found that dry heat and humid heat do not necessarily reach their highs at the same time. Carbon Brief reported that dry heat is more threatening to plants and ecosystems, while humid heat is tougher on humans.
More background
Heat arriving outside the usual season can take people by surprise. When heat waves strike in spring or early summer, they can arrive before people have acclimated to hotter weather and before cooling centers and other protective steps are fully available. Heat later in the year on top of prolonged exposure over the summer can tax both people and local infrastructure, making another period of extreme heat more difficult to handle.
The study indicates that year-round warming alone does not fully explain why some places are seeing heat expand more into spring while others are seeing more of it in autumn. Seasonal precipitation and soil moisture shifts, land-use changes including agricultural intensification or greater irrigation, and natural swings in regional climate may also be affecting when extreme heat shows up.
What can be done?
Heat planning needs to cover more than the traditional summer calendar for climate adapting strategies. For example, cooling centers and heat early warning systems may need to stay in place during spring and autumn, too.
More region-specific studies will need to be conducted to gain more insight on the uneven changes.
Where can I learn more?
These stories show how that shift beyond the usual summer window is playing out on the ground, from autumn heat in the western U.S. to sharper extremes across Europe. They also highlight the ripple effects on health, air quality, and everyday life when dangerous heat arrives outside the months many communities expect it.
• Across western U.S. cities, a history-making heat wave pushed October temperatures into summer territory.
• Across much of the world, driving devastating summer heat is making extreme temperatures more likely.
• Across multiple European nations, extreme heat has increased tenfold in patterns scientists call unprecedented.
• In many cities, heat and air pollution are combining into a dangerous double whammy.
• Across Europe, heat-related deaths surged during one of the region's worst summers.
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