A new study published in AGU Advances examined changes in when extreme heat events occur globally over the past 45 years. Researchers at New York University analyzed climate data from 1980 to 2024, comparing the 1980s baseline decade to the period between 2015 and 2024 to assess shifts in the timing of temperature extremes.
The analysis focused on “local heat seasons,” defined as the three consecutive months when extreme heat events occurred most frequently during the 1980s, along with the two-month periods before and after these seasons, termed “shoulder seasons.” Rather than relying on standard meteorological definitions of summer, the study measured how extreme heat is encroaching into spring and autumn months. Researchers examined both dry heat, measured by dry-bulb temperature, and humid heat, assessed using wet-bulb globe temperature—a metric developed by the U.S. military that accounts for temperature, humidity, wind speed and solar radiation.
Findings showed that during the 1980s, extreme heat was tightly confined to specific seasons, with approximately 93 percent of global land area experiencing over 80 percent of extreme dry-heat days during traditional heat seasons. However, the study found that in more than half of global land areas, extreme heat is now spreading unevenly into shoulder seasons. Some regions, including western Europe, southern Africa and northwestern India, show more extreme heat occurring in months preceding historical heat seasons. Other areas, such as much of the United States, eastern China, northern Africa and eastern Europe, experience increased extreme heat frequency in months following traditional heat seasons.
Researchers investigated whether simple year-round warming could explain these shifts but found the explanation more complex. While uniform warming across the calendar year accounts for changes within traditional heat seasons, it does not explain the uneven pattern of heat expansion into shoulder seasons across different regions. The research underscores that heat extremes occurring outside peak summer months pose particular risks, as populations may lack adequate heat-adaptation infrastructure or physiological acclimatization.
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