Extreme heat is creeping into spring and autumn across half the world

Extreme heat is no longer just a summer problem. High-temperature spikes are spreading into spring and autumn across about half the globe, and they are doing so in a surprisingly lopsided way.

Researchers from Columbia University have found that while some parts of the world are baking much earlier in the year, others are staying dangerously hot long after summer should have ended.

In Western Europe, southern Africa, and northwestern India, the worst temperature spikes now arrive predominantly in the two months before the traditional summer season. But in the western US, eastern China, northern Africa, and eastern Europe, the shift is happening in the opposite direction. There, extreme heat has become more common in the two months following the historical summer.

Why off-season heat is more dangerous

This seasonal drift poses a unique threat to human health. When extreme heat arrives in early spring, people have not yet had time to acclimatise to rising temperatures. When it lingers deep into autumn, communities are already physically worn down by months of summer heat.

It also catches local governments and services off guard. Municipal heat plans, water management strategies, and cooling centres are almost always run on a rigid summer calendar. If a heatwave hits in April or October, the infrastructure to protect vulnerable people is often not yet online.

Shifting the baseline

To map these changes, scientists looked at temperature data across the six inhabited continents, comparing the decade between 1980 and 1989 with the period from 2015 to 2024.

They tracked both dry heat and humid heat stress, defining "extreme" days as those falling into the hottest five per cent of historical daily temperatures.

While previous research has shown that average summer conditions in mid-latitude regions have lengthened by roughly six days per decade since 1990, this study focused on individual extreme events. The authors found that these acute spikes do not always move in step with gradual seasonal averages.

Catherine Ivanovich, a climatologist at the NASA Goddard Institute for Space Studies and lead author of the study, said the findings suggest extreme heat is "starting to become something different" in many regions.

Because out-of-season extreme heat is still statistically rare, the researchers described their findings as a compelling first line of evidence, calling for further study to understand the exact atmospheric drivers behind this lopsided expansion.