Extreme heat days spread beyond summer worldwide

Extreme heat is turning up before summer and holding on into autumn across the world, a study published on 10 September 2026 finds. Researchers at Columbia Climate School examined all six inhabited continents and compared the 1980s with 2015 to 2024. Their results show that the hottest days are no longer following the same seasonal timetable everywhere.

That matters because heat can hit people before they have acclimatised to the season, or after months of punishing temperatures. Cooling centres and heat warnings are often planned around the usual summer calendar, making an early or late heat spell harder to prepare for.

The team counted a day as extreme when it fell within the hottest 5% of daily temperatures. They measured dry heat with standard thermometer readings and humid heat with wet-bulb globe temperature, which combines air temperature, humidity and sunshine to show the strain on the human body.

Spring arrives hotter in some regions

Dry-heat seasons expanded significantly across just over half of the world’s land area. Humid-heat seasons grew across slightly less than half. But the change was uneven rather than a simple lengthening at both ends of the season.

Extreme heat increasingly arrived in the two months after the old hot season in the western United States, eastern China, North Africa and eastern Europe. In western Europe, southern Africa and north-west India, it became more common in the two months before the traditional season.

Phoenix, in the United States, illustrates the shift. It recorded 183 extreme dry-heat days in the 1980s and 338 between 2015 and 2024. None came after its usual hot season in the earlier period; 6% did in the later one. The average date for its dry-heat extremes moved 10 days later in the year, while humid-heat extremes came 5.5 days earlier.

Heat risks can collide

The researchers checked the pattern against two datasets, one from NASA and one from the European Centre for Medium-Range Weather Forecasts. Both broadly pointed to the same uneven shift.

Catherine Ivanovich, the study’s lead author, says the findings are strong early evidence of a new pattern, but rare out-of-season extremes remain difficult to measure with certainty. Climate-model tests are next, to separate the influence of human-driven climate change from natural variation.

Dry heat is particularly damaging to crops and ecosystems. Humid heat poses a greater danger to people because it makes sweating less effective at controlling body temperature. A shifted heat season can also overlap with other hazards, including peak wildfire season in the western United States and hurricane season in the south-east, increasing the danger when crises arrive together.

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Source: Europa Press / Mallorca.com