This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Heat waves over land worldwide are beginning earlier, ending later and occurring over longer seasons, while the first heat wave of the season has recently shifted toward faster onset, according to a new study published in Nature Climate Change. The study, led by researchers from the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS), shows that global warming is changing not only how frequently, intensely and persistently heat waves occur but also when they occur and how rapidly they develop.
In the new study, the research team analyzed global climate data from 1979 to 2023, examining changes in seasonal heat wave onset and end dates, heat wave season length and the onset speed of the first heat wave of each season. The researchers found that, on average, the first heat wave began 3.29 days earlier per decade, while the last heat wave ended 5.41 days later per decade, extending the heat wave season by 8.71 days per decade. Over the 45-year study period, the first heat wave advanced by about two weeks, the last heat wave occurred about 24 days later in the year and the heat wave season lengthened by about 39 days.
Across global land areas, 72% showed a trend toward earlier heat wave onset, 79.6% toward later heat wave end dates and 92.1% toward longer heat wave seasons, with generally greater changes observed in drylands. The researchers also classified the first heat wave of each season as rapid-, moderate- or slow-onset based on the relative timing of the temperature peak within each event. From 2001 to 2023, the proportion of rapid-onset first heat waves within heat wave-affected land areas increased significantly, indicating a recent shift toward faster heat wave onset.
These findings suggest that heat wave risks depend not only on how hot events become and how long they last but also on when they arrive and how quickly they develop. The study also found that longer heat wave seasons are expanding crop exposure to extreme heat. From 2001 to 2023, the proportion of crop areas exposed to heat waves during critical reproductive stages, such as flowering and silking, increased by between 1.6% and 13.3% for several major crops compared with 1979–2000.
Heat wave exposure also extended into earlier and later stages of crop development. "In the past, we have mostly focused on whether heat waves are becoming more frequent, stronger and longer. Our study further shows that their timing and onset speed are also changing.
If heat waves arrive earlier and develop more rapidly, the time available for societies and ecosystems to prepare and adapt could become even shorter," said Xu Wenfang of SCBG, the study's first author and one of its corresponding authors. According to the researchers, the study provides new insights into global heat wave changes by highlighting heat wave timing and development speed as important aspects of climate risk. The findings provide scientific evidence for improving extreme-heat monitoring and early-warning systems.
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