The global heatwave season lengthened by about 39 days between 1979 and 2023, as the first heatwaves began earlier in the year and the last ones ended later, according to a global study published on 23 September in Nature Climate Change. This means the window of exposure to extreme heat has expanded by about five weeks compared with the start of the study period.
Earlier heatwave onset and later season end
The researchers analysed 44 years of land temperature data and found that the first heatwave of the season advanced by an average of 3.29 days per decade. In contrast, the end of the last heatwave was delayed by 5.41 days per decade, producing an overall increase of 8.71 days per decade in the length of the season.
Wenfang Xu, the study's lead author and a researcher at the South China Botanical Garden of the Chinese Academy of Sciences, said assessing heatwave risks requires attention to their timing, as well as their number, duration and intensity. Extreme heat arriving earlier can catch populations, natural systems and infrastructure before they have had enough time to adapt and prepare, he explained.
Study data and the definition of a heatwave
The analysis was based primarily on daily global ERA5 data, after which the researchers tested the findings using two independent datasets. The comparison was used to ensure that the observed trends in heatwave timing and season length were not linked to a single source of temperature data.
The team defined a heatwave as a period of at least three consecutive days during which the maximum temperature exceeds a high threshold calculated against the usual conditions for each day of the year. Under this definition, the researchers tracked the date of the first heatwave of the season, the timing of the last heatwave's end, and changes in the interval between them.
The heat season is lengthening across world regions
Dry regions, including the Middle East and North Africa, were more affected by changes in heatwave onset than humid regions. Heatwave onset advanced in arid areas by an average of 6.02 days per decade, more than twice the rate recorded in humid regions, which was about 2.47 days per decade.
The researchers observed simultaneous indications of earlier heatwave onset and a later season end in the Mediterranean region, North Africa, and parts of Europe and East Asia. The combination of the two trends extended exposure to extreme heat on both sides of summer, rather than limiting the risk to its usual months.
Faster heatwaves and impacts on agriculture and services
The study also examined how quickly the first heatwave reached its peak, classifying heatwaves according to their onset speed as fast, moderate or slow. Since 2001, the team recorded a clear increase in the proportion of heatwaves that reach peak temperatures rapidly, reducing the time available for populations, health systems and electricity networks to prepare and respond.
The effects of a longer season are not limited to human exposure to heat, as its extension increases the likelihood that heatwaves will coincide with sensitive stages of crop growth. The study found that the proportion of agricultural land exposed to heat during important reproductive stages, including flowering and grain formation, increased by between 1.6% and 13.3% during 2001–2023 compared with 1979–2000.
Crops were also exposed to heat during earlier and later stages of their growth cycle. The research team said this finding calls for a change in how early warning systems address the heat season, because beginning traditional preparations at the start of summer may be too late in regions that are now experiencing extreme temperatures earlier.
Heatwaves continuing into autumn could keep demand for cooling high for longer, placing additional pressure on electricity networks, water resources and health services. Ecosystems could also face extended periods of heat stress and drought as the season lengthens and some heatwaves reach their peaks more rapidly.
The researchers noted several methodological limitations, as the basic definition of a heatwave focused on maximum daytime temperatures. Definitions that account for night-time heat, humidity, wind speed or apparent temperature could provide a different picture of the scale of risks faced by people.
The team used defined seasonal windows to track heatwaves, but tested several alternative windows and found that the overall trends remained similar. The study was funded by the National Natural Science Foundation of China, the National Key Research and Development Programme of China, the Regional Innovation and Development Fund, research projects in Guangdong and Guangzhou, and a Chinese national laboratory for severe weather science. The researchers declared no competing interests.
