Heatwaves over land worldwide are beginning earlier, ending later, and occurring over longer seasons, while the first heatwave 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 frequent, intense, and persistent heatwaves are 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 heatwave onset and end dates, heatwave season length, and the onset speed of the first heatwave of each season.
The researchers found that, on average, the first heatwave began 3.29 days earlier per decade, while the last heatwave ended 5.41 days later per decade, extending the heatwave season by 8.71 days per decade. Over the 45-year study period, the first heatwave advanced by about two weeks, the last heatwave occurred about 24 days later in the year, and the heatwave season lengthened by about 39 days.
Across global land areas, 72 percent showed a trend toward earlier heatwave onset, 79.6 percent toward later heatwave end dates, and 92.1 percent toward longer heatwave seasons, with generally greater changes observed in drylands.
The researchers also classified the first heatwave 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 heatwaves within heatwave-affected land areas increased significantly, indicating a recent shift toward faster heatwave onset. These findings suggest that heatwave 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 heatwave seasons are expanding crop exposure to extreme heat. From 2001 to 2023, the proportion of crop areas exposed to heatwaves during critical reproductive stages, such as flowering and silking, increased by between 1.6 percent and 13.3 percent for several major crops compared with 1979–2000. Heatwave exposure also extended into earlier and later stages of crop development.
Implications for risk assessment
"In the past, we have mostly focused on whether heatwaves are becoming more frequent, stronger, and longer. Our study further shows that their timing and onset speed are also changing. If heatwaves 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 heatwave changes by highlighting heatwave timing and development speed as important aspects of climate risk.
The findings provide scientific evidence for improving extreme-heat monitoring and early-warning systems. Alongside temperature intensity and duration, the timing of heatwave onset and end, as well as how rapidly the first heatwave of the season reaches its temperature peak, could also be considered in heat monitoring and risk assessment.
Such information could help improve heatwave risk assessment and support adaptation strategies across public health, agriculture, ecosystem management, and energy supply, the researchers added.
