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Extreme heat is spilling from summer into spring and autumn, a study finds

· BehindLines newsroom

Days of extreme heat are no longer as strictly concentrated in summer: over more than half of the world's land, they now spill into spring or autumn. That is the conclusion of a study by Casey Ivanovich, Benjamin Cook and Sonali Shukla McDermid, published in the journal AGU Advances under the title "Extreme Dry and Humid Heat Seasons Are Changing Asymmetrically". The study is not new: Carbon Brief and the News-Medical site (relaying the Columbia Climate School) presented it on 10 September 2026. Only Mongabay's coverage, published on 2 November, is recent.

How the authors measured the phenomenon

The researchers set a local threshold at the 95th percentile of daily maxima in 1980–1989 and also apply it to 2015–2024, according to the study methods. The local heat season comprises the three months with the most extremes in 1980–1989, where those months are consecutive; its margins cover the two months before and after. They distinguished two types of heat: dry heat, measured by air temperature, and humid heat, measured by the wet-bulb globe temperature (WBGT), which also accounts for humidity, wind and solar radiation. According to Carbon Brief, they rely on NASA's MERRA-2 reanalysis for 1980-2024, checked against ERA5 from the European Centre for Medium-Range Weather Forecasts. They compare the 1980s with the period 2015-2024.

The starting point was clear-cut: in the 1980s, about 93% of land had more than 80% of its extreme dry-heat days in its usual hot season, including in the tropics, according to Carbon Brief.

What has changed

According to News-Medical, the extreme dry-heat season has lengthened over a little more than half of the land, and the humid-heat season over a little less than half. Mongabay sums it up: extreme-heat seasons have stretched toward spring and autumn. The change is uneven:

Phoenix (Arizona) serves as an example. According to the summary relayed by News-Medical, the total of extreme dry-heat days there rose from 183 over the 1980s decade to 338 over 2015-2024, and 6% of those days fall after the end of the hot season, compared with none in the 1980s. The median date of extreme dry heat has shifted about ten days later, that of humid heat 5.5 days earlier.

Why it cannot all be explained

Average warming does not explain everything. In a test, the authors shifted the 1980s data by the average local rise: this reproduces most of the changes in the core of the season, but not the uneven shifts at its edges, Carbon Brief reports. Casey Ivanovich suggests other leads there, to be examined region by region: seasonal rainfall, soil moisture, land use (irrigation, agricultural intensification) and natural climate variability. In the Columbia Climate School release, she calls climate change the "primary" component, without being able to put a number on its share from observations alone. The authors plan climate simulations. Because extremes are rare, the statistical signal is hard to isolate.

Why it matters

According to Carbon Brief and Mongabay, out-of-season heat creates practical problems: public pools and cooling centres that often close at the end of summer, and power grids equipped for summer air-conditioning demand, which can fail. It can also overlap with the wildfire season in the western United States or the Atlantic hurricane season. Mongabay recalls, citing the World Health Organization, about 489,000 heat-related deaths a year worldwide between 2000 and 2019; that figure does not come from the study. Benjamin Cook argues, according to Mongabay, that mitigation plans centred on summer overlook spring and autumn heat waves.

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