Study: Extreme Heat Is Spreading Into Spring and Autumn Across More Than Half the World's Land

A study published in AGU Advances finds that extreme heat — already one of the deadliest climate hazards — is increasingly occurring outside the traditional summer months. Researchers report that hazardous heat is appearing more often during spring and autumn across more than half of the world’s land area, with implications for human health, infrastructure and ecosystems.

How the Study Was Done

The team analyzed climate records from 1980 to 2024. Instead of using fixed calendar seasons, they defined a local heat season for each location as the three consecutive months in the 1980s that showed the highest frequency of extreme heat. They then compared the two months before and after that three-month window and evaluated changes between the decades 1980–89 and 2015–24.

Key Findings

In the 1980s roughly 93% of the planet’s land experienced more than 80% of its extreme dry-heat days within the historical dry-heat season. The researchers found this concentrated pattern is breaking down: extreme heat is becoming less confined to a single core season and is spilling into the shoulder seasons on many continents.

The geographic pattern of change is uneven. Western Europe, southern Africa and northwestern India are experiencing more extreme heat earlier (in spring), while much of the United States, eastern China, northern Africa and eastern Europe are seeing increases in extreme heat later in the year (in autumn).

Different Types of Heat, Different Risks

The study also found that dry heat and humid heat do not necessarily peak at the same time. Dry heat tends to be more damaging to plants and ecosystems, whereas humid heat poses greater risks to human health because of reduced evaporative cooling (sweating).

Why The Seasonality Is Shifting

Year-round warming alone does not fully explain these shifts. The authors point to several likely contributors: seasonal changes in precipitation and soil moisture, land-use changes (including intensified agriculture or expanded irrigation), and natural variability in regional climate systems. These factors can alter when and where extreme temperatures emerge.

Implications for Preparedness

Heat arriving outside the expected season can catch communities and infrastructure unprepared. Spring heat waves may strike before people and systems have acclimated or before cooling centers and other protective measures are operational. Late-season heat layered on prolonged summer exposure can further stress health services, power grids and ecosystems. The study recommends that heat-preparedness planning extend beyond the traditional summer period, keeping early-warning systems, cooling resources and outreach active into spring and autumn where appropriate.

On-the-Ground Examples

• Western U.S. cities experienced a history-making October heat wave that pushed temperatures into summer ranges.
• Intensifying summer heat globally is increasing the likelihood of record extremes in many regions.
• Several European countries have seen unprecedented, tenfold increases in some extreme-heat patterns.
• In many urban areas, heat and air pollution are combining to create heightened health risks.
• Europe recorded a surge in heat-related deaths during one of its worst summers on record.

The authors call for more region-specific studies to identify local drivers and to guide targeted adaptation measures. Reporting by Carbon Brief summarizes the study's methods and regional patterns.