Study: Extreme Heat Is Creeping Out of Summer — Spring and Fall Are Getting Hotter

Across much of the globe, extreme heat is no longer confined to the traditional summer months. A new study led by Casey (Catherine) Ivanovich of Columbia University finds that heat extremes are arriving earlier in spring and lingering later into fall, expanding the seasons of dangerous temperatures over large parts of Earth's land surface.

What the Study Found

The peer‑reviewed analysis, published Sept. 10 in AGU Advances, compares patterns of extreme heat from the 1980s with observations from 2015–2024. The authors report that the season of extreme dry heat has expanded across just over half of the world’s land area, while humid heat seasons have expanded across just under half. These shifts are uneven: some regions are gaining heat earlier in spring, others are seeing the heat season extend further into fall.

Why Timing Matters

“Heat waves in the spring can catch us off guard before our bodies have acclimated to the heat and before we're ready to utilize cooling strategies,” Ivanovich said. “In the fall, heat waves can be more dangerous if we've already experienced repeated extreme heat events that leave our bodies stressed.”

Changes in timing make preparedness harder. Cooling centers, public warnings and infrastructure planning are typically organized around a summer calendar; heat arriving outside that window can expose vulnerable people who are unprepared.

Regional Patterns And Hazards

The analysis highlights regional differences: eastern China, northern Africa and eastern Europe have seen extreme heat become more common in the two months after their historical heat seasons. By contrast, western Europe, southern Africa and northwestern India have seen increases primarily in the two months before the traditional season. In the United States, the study found larger expansions into fall than into spring.

Those shifts can increase overlap with other seasonal hazards — notably wildfire season in the West and the Atlantic hurricane season in the East — creating compound risks that are more damaging than single events occurring in isolation.

Dry Versus Humid Heat

The authors distinguish dry and humid heat because their impacts differ: dry heat tends to stress crops and ecosystems, while humid heat is more dangerous to human health because high humidity reduces the body’s ability to cool by sweating. Adapting to one kind of heat does not necessarily prepare communities for the other.

Causes And Next Steps

Using observations alone, the study cannot fully apportion how much of the timing change is due to human‑caused climate change versus other factors such as land‑use change or natural variability. However, Ivanovich and colleagues note that rising temperatures linked to climate change are likely the primary driver. The team calls for further research to quantify the relative contributions of climate change, land‑use shifts and natural variability.

Implications

Earlier springs and longer falls with extreme heat mean public health systems, emergency services and infrastructure planners must rethink seasonality. Improved monitoring, flexible heat‑response calendars and targeted communication can help communities better prepare for the extended threat of extreme heat.

Reporting credit: Doyle Rice/USA TODAY. Study published Sept. 10 in AGU Advances.