Extreme Heat Seasons Expand Into Spring and Autumn
A new study finds extreme heat events are spreading into shoulder seasons globally, with over half of land areas seeing earlier or later heatwaves.
Extreme heat events are increasingly occurring outside the traditional summer months, according to new research. The study finds that in more than half of the world's land area, dangerous heat is creeping into spring and autumn.
Researchers analyzed climate data from 1980 to 2024 to measure shifts in the timing of extreme heat. They defined local heat seasons as the three consecutive months when extreme heat happened most often in the 1980s. The study then examined how heat is spreading into the two-month periods before and after these historical seasons.
Defining Local Heat Seasons
The study moved beyond standard meteorological definitions of summer. Instead, it established a flexible baseline for each location based on historical heat patterns from the 1980s. This allowed the team to track how the concentration of annual extreme heat days has changed over the last 45 years.
The analysis used data from the MERRA2 and ERA5 reanalysis datasets. It considered both dry heat, measured by dry-bulb temperature, and humid heat, measured by wet-bulb globe temperature. The wet-bulb measure combines temperature, humidity, wind speed, and solar radiation.
Uneven Expansion Across Globe
The research reveals an uneven global pattern in the expansion of extreme heat. In the 1980s, extreme heat was tightly confined, with 93% of land area experiencing over 80% of its extreme dry-heat days within its traditional heat season.
That pattern is now blurring. The maps below illustrate the shift. In western Europe, southern Africa, and north-western India, more extreme heat is occurring in the months before the historical heat season. Conversely, in much of the United States, eastern China, northern Africa, and eastern Europe, extreme heat is increasing in frequency in the months after the traditional season.
The study also found that extreme dry- and humid-heat seasons often differ, typically offset by one month. This is pronounced in monsoon-influenced regions like north-western Mexico and central India, where the dry-heat season precedes the humid-heat season.
Drivers Beyond Simple Warming
Researchers tested whether a uniform annual temperature increase could explain the shifts. They created a synthetic timeseries by warming the 1980s baseline data by the average local temperature change observed between 1980-1989 and 2015-2024.
This experiment showed that annual average warming alone cannot explain the uneven creep into shoulder seasons. The study's authors state there must be other factors at play. Potential drivers include long-term changes in seasonal precipitation and soil moisture, land-use changes like agricultural intensification, and natural climate fluctuations such as the Pacific Decadal Oscillation.
Implications for Hazard Management
The expansion of extreme heat into shoulder seasons has direct implications for public safety. Heat early warning systems and cooling centres may now be needed outside traditional summer months. The researchers call for more study on the overlap of extreme heat with other seasonal hazards.
In the western United States, heat stretching later into autumn could increase overlap with the wildfire season. In the eastern United States, a similar extension could increase overlap with the Atlantic hurricane season. Understanding these intersections is key for climate adaptation, as compound hazards are far more dangerous than isolated events.
