Extreme heat events are no longer confined to traditional summer months, according to a study published in AGU Advances by researchers at NASA’s Goddard Institute for Space Studies, the Columbia Climate School, and New York University. The research shows that extreme temperatures are increasingly occurring in the spring and fall, expanding the duration of heat seasons across large portions of the globe and creating new adaptation challenges for infrastructure and human health.
How Extreme Heat Seasons Are Expanding Globally
According to climatologist Catherine Ivanovich, lead author of the study, extreme heat has become more frequent in the months before and after warm seasons. The research team analyzed global temperature data from 1980 through 2024 to track changes in extreme heat timing, defining “extreme” as days falling within the hottest 5% of daily temperatures across the six inhabited continents.
The analysis compared data from the 1980s baseline decade to the period between 2015 and 2024. Results indicate that extreme heat seasons expanded significantly across just over half the world’s land area for dry heat, measured via standard thermometer readings, and just under half for humid heat, which factors in humidity and solar radiation through wet bulb globe temperatures. These expansions are lopsided, pushing into spring in some territories and fall in others.
Did you know? In Phoenix, the median date for dry heat extremes shifted 10 days later in the year during the 2015–2024 period compared to the 1980s baseline, while humid heat extremes moved 5.5 days earlier. The city recorded 183 extreme heat days in the 1980s baseline decade, jumping to 338 in the decade ending in 2024.
Regional Differences in Spring and Fall Heat Shifts
The timing of seasonal heat expansion varies sharply by geographic region. In the western United States, eastern China, northern Africa, and eastern Europe, extreme heat events accelerated most rapidly in the two months following historical heat seasons. Conversely, western Europe, southern Africa, and northwestern India experienced earlier extreme heat spikes during the two months preceding traditional warm periods.
Co-authored by Benjamin Cook of NASA GISS and Sonali Shukla McDermid of New York University, the study utilized two independent datasets—one compiled by NASA and another by the European Centre for Medium-Range Weather Forecasts—to confirm the recurring global patterns.
Why Out-of-Season Heat Poses Unique Public Health Risks
The timing of extreme temperatures dictates how severely they impact human health and local ecosystems. According to researchers, heat stress is harder on the human body when it arrives before populations have acclimated to seasonal shifts or after months of sustained high temperatures. Humid heat poses specific physiological dangers because moisture in the air limits the body’s natural cooling mechanism through sweating.
Furthermore, out-of-season temperature spikes frequently overlap with other seasonal environmental hazards. Ivanovich noted that overlapping events—such as extreme heat intersecting with peak wildfire seasons in the western United States or peak hurricane seasons in the Southeast—create compounding dangers that amplify community vulnerability.
Frequently Asked Questions
How do researchers define an extreme heat event?
The study defines extreme heat events as individual days falling within the hottest 5% of daily temperatures recorded between 1980 and 2024.
What is the difference between dry heat and humid heat measurements?
Dry heat is calculated using standard air thermometer readings, whereas humid heat uses wet bulb globe temperatures, combining air temperature, humidity, and solar radiation to measure heat stress on the human body.
Why is out-of-season heat harder to prepare for?
Traditional municipal cooling centers, public alerts, and community response frameworks are typically structured around a standard summer calendar, leaving populations less prepared when extreme temperatures arrive early in spring or linger late into autumn.
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