Extreme heat events are not only becoming more frequent: in many regions, they are also reaching new months of the year.
To measure this change, researchers analyzed 45 years of global climate data, from 1980 to 2024. They first defined, for each region, a reference season corresponding to the three months when intense heat was most frequent between 1980 and 1989. This method avoids automatically considering June, July, and August to be the hot season everywhere on Earth.
Intensity of the heatwave observed in Western Europe on May 28, 2026, with the highest values shown in dark red.
Credit: European Union, Copernicus Emergency Management Service. Contains modified Copernicus data (2026).
The researchers then examined when days exceeding the threshold of the 5% hottest days in this reference period occurred. They distinguished dry heat from humid heat. The latter takes air humidity into account, among other factors, which reduces the effectiveness of sweating in cooling the body.
The result is clear. Between the 1980s and the recent period, the season of extreme dry heat expanded across 50% of the land studied. For intense humid heat, this proportion reached 48%. However, the extension does not occur in the same way everywhere.
In Western Europe, southern Africa, and northwestern India, extreme events are increasing more before the usual hot season. In the western United States, eastern China, northern Africa, and eastern Europe, they are becoming more frequent mainly after this period. The dangerous season can therefore begin earlier or end later depending on the region.
One might think that this shift simply results from the general rise in temperatures. If every day becomes hotter, the months around summer should logically exceed former records more often. The researchers tested this explanation by applying the observed average warming to the historical distribution of temperatures.
This generally does not suffice to reproduce the measured changes. The rise in average temperature does contribute to the phenomenon, but the distribution of intense heat throughout the year is also changing.
These out-of-season events pose a very concrete problem. In spring, people may be less accustomed to heat, and cooling systems are not always ready. In autumn, a prolonged hot period may add to the high temperatures already endured during summer. It may also coincide with other seasonal hazards, particularly wildfires.
The authors therefore believe that warning systems should take these new calendars into account rather than operate according to a fixed hot season. A next step will be to study more precisely the situations in which this unusual heat overlaps with periods of wildfires or cyclones.