Extreme heat events becoming more common beyond summer months, study finds

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Globally, extreme heat events have become more frequent in the months preceding summer and those following it, a new study has found.

Findings published in the journal AGU Advances show that expansion of extreme heat events is lopsided, pushing further into spring in some regions and into fall in others.

In western Europe, southern Africa, and northwestern India, extreme heat came predominantly in the two months before the historical summer season during 2015-2024, compared to the 1980s.

However, in western US, eastern China, northern Africa and eastern Europe, extreme heat events became more common more rapidly in the two months following their historical heat seasons, researchers from Columbia University, the US, found.

They said extreme heat is harder on the body when it comes before people have acclimatised to the season, or if it comes after people have already had to endure months of it.

They added it can be harder for communities to prepare for, since cooling centres and heat alerts are built around a summer calendar.

"We find that extreme dry and humid heat seasons have expanded over roughly half of the world and that this expansion has been asymmetric, with most regions experiencing greater increases in extreme heat frequency in the months after (western United States, eastern China, northern Africa, and eastern Europe) or before (western Europe, southern Africa, and northwestern India) the traditional extreme heat season," the authors wrote.

The researchers counted extreme heat events on the six inhabited continents between 1980 and 1989 -- "extreme" hot days being those counted in the hottest five per cent of daily temperatures. This was measured twice, once for dry heat and once for measuring heat stress due to the combined effect of heat, humidity and air temperature.

Baseline figures of the 1980s were then compared with those in the most recent decade on record.

Research on seasonal warming has tended to concentrate on average temperatures and shifts in the timing of seasons, the researchers said. Summer weather conditions in mid-latitude regions, for example, have lengthened by roughly six days per decade since 1990, they said.

Few studies have looked at the timing of individual extreme heat events, which may not move in step with the seasonal average, the team said.

"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," lead author Catherine Ivanovich, a climatologist at NASA Goddard Institute for Space Studies (GISS), which is affiliated with the Columbia Climate School at Columbia University, said.

Ivanovich said 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, the lead author said, adding that more research is needed as evidence.

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