A study published in AGU Advances examined how the timing of extreme heat events has shifted over the past several decades across different regions of the world. Researchers compared climate data from the 1980s baseline period against information from 2015-24, utilizing two separate reanalysis datasets to ensure reliability. The analysis measured both dry and humid heat using established meteorological standards, including wet-bulb globe temperature, which factors in temperature, humidity, wind speed and solar radiation.
The research revealed that during the 1980s, extreme heat was tightly concentrated within defined seasonal periods. Approximately 93 percent of global land area experienced more than 80 percent of extreme dry-heat days during their traditional heat seasons. The study also found that dry-heat and humid-heat seasons typically differ from one another, often offset by approximately one month, particularly in monsoon-influenced regions.
Over the four-and-a-half-decade period examined, the boundaries of extreme heat seasons have become increasingly blurred. Extreme heat events are now extending significantly into shoulder seasons—the two-month periods before and after the historically defined heat seasons—affecting more than half of global land areas. However, this extension occurs unevenly depending on location. Western Europe, southern Africa and northwestern India show greater concentrations of extreme heat in months preceding traditional heat seasons, while much of the United States, eastern China, northern Africa and eastern Europe experience increased extreme heat in months following historical peak heat periods.
Researchers explored whether uniform warming across the calendar year could explain these patterns. While annual average temperature increases account for shifts within traditional heat seasons, they cannot fully explain the uneven distribution of extreme heat spreading into shoulder seasons across different regions. The findings indicate that the mechanisms driving heat season timing shifts are more complex than simple overall warming effects.
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