A study examining global climate data from the past 45 years has found that extreme heat is no longer confined to traditional summer months but is creeping into spring and autumn in varying patterns across the world. Researchers at New York University analyzed temperature records from 1980 to 2024, comparing the 1980s baseline period with the 2015-24 decade to measure changes in when extreme heat occurs throughout the calendar year.
The research defines “local heat seasons” as the three consecutive months when extreme heat events occurred most frequently during the 1980s, rather than relying on conventional meteorological season definitions. Scientists then measured how extreme heat is expanding into the two-month periods before and after these peak seasons, which they termed “shoulder seasons.” The findings reveal that in the 1980s, approximately 93% of the world’s land area experienced more than 80% of extreme dry-heat days during its traditional heat season. However, this pattern has shifted significantly over recent decades.
The expansion into shoulder seasons varies geographically. In western Europe, southern Africa, and northwestern India, extreme heat is increasingly occurring during months before the historical heat seasons. Conversely, in much of the United States, eastern China, northern Africa, and eastern Europe, extreme heat events are becoming more frequent in the months following traditional heat seasons. Researchers note that heat events occurring outside peak summer months pose distinct risks, as people and infrastructure may lack adequate cooling resources and biological adaptation.
The study examined both dry and humid heat separately, using different measurement standards including wet-bulb globe temperature, which accounts for temperature, humidity, wind speed, and solar radiation. Researchers found that while uniform warming across the calendar year explains some shifts in extreme heat timing within traditional heat seasons, it does not account for the uneven creeping into shoulder seasons observed globally. This suggests that more complex climate mechanisms are driving the geographic variations in how extreme heat timing is changing worldwide.
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