Study finds extreme heat spreading into spring and autumn across more than half of global land

The study, published in AGU Advances, finds that extreme heat events are spreading into the shoulder seasons on either side of the hottest part of the year in more than half of global land areas. The authors say it is the first study to measure whether the timing of extreme heat during the calendar year is changing worldwide.

What the study measured

The authors used climate records from 1980 to 2024. They did not rely on calendar seasons. They defined a local heat season as the three consecutive months when extreme heat happened most often in the 1980s. They then examined the two months before and after that span, which they call shoulder seasons.

They compared 1980-89 with 2015-24, the opposite ends of the data, to register a larger change and account for cumulative effects of climate change. They ran the analysis on MERRA2 reanalysis data and repeated it on ERA5 to check the result. They picked the 1980s as a baseline because it was the start of the common period between the two datasets.

In the 1980s, about 93% of the world's land area had more than 80% of its extreme dry-heat days inside its traditional dry-heat season. The study says that concentration is breaking down. In the 1980s, this was even true in the tropics, where seasonal temperature swings are smaller.

Where the season is shifting

The shift is uneven. Western Europe, southern Africa and northwestern India are seeing more extreme heat before their historical heat seasons. Much of the US, eastern China, northern Africa and eastern Europe are seeing more of it afterward.

Dry heat and humid heat do not necessarily peak together. The study measures both, using dry-bulb temperature for dry heat and wet-bulb globe temperature for humid heat. The latter was created by the US military in the 1950s and combines temperature, humidity, wind speed and solar radiation. Dry heat is more threatening to plants and ecosystems, while humid heat is tougher on humans. The study also found extreme dry- and humid-heat seasons are often offset by one month, especially in places influenced by monsoon systems such as northwestern Mexico and central India.

Why out-of-season heat is riskier

Heatwaves in spring or the first heatwave of summer are riskier because bodies are not yet accustomed to the heat and cooling may not be available. Autumn heatwaves can place further strain on bodies and on local infrastructure already under stress after a long summer.

The WHO says heat stress is a leading cause of weather-related deaths. Heat-related mortality for people over 65 rose by about 85% between 2000-2004 and 2017-2021, according to the WHO. Studies estimate about 489,000 heat-related deaths a year between 2000 and 2019. That burden gives planners a reason to treat out-of-season heat as a public health risk, not only a summer inconvenience.

What remains unclear

The study indicates that year-round warming alone does not fully explain the split. Seasonal precipitation and soil moisture shifts, land-use changes such as agricultural intensification or greater irrigation, and natural swings in regional climate may also affect when extreme heat arrives.

What planners are told

Yahoo News reported that heat planning needs to cover more than the traditional summer calendar. Cooling centres and heat early warning systems may need to stay in place during spring and autumn, and more region-specific studies are needed to understand the uneven changes.