A new global study published in Nature Climate Change reveals that heat waves on land are beginning earlier, lasting longer, and ending later than they did in the past. Scientists from the South China Botanical Garden (SCBG) at the Chinese Academy of Sciences (CAS) analyzed climate data from 1979 to 2023, focusing on how the timing and duration of heat waves have changed over time. The research found that the first heat wave of each season has started about 3.29 days earlier per decade, while the last heat wave has ended 5.41 days later per decade. This shift has extended the overall heat wave season by roughly 8.71 days per decade. Over the 45-year study period, the first heat wave of the year advanced by about two weeks, the last heat wave occurred about 24 days later, and the total heat wave season lengthened by approximately 39 days. The study found that 72% of global land areas showed a trend of earlier heat wave onset, 79.6% showed later end dates, and 92.1% experienced longer heat wave seasons. These changes were particularly pronounced in dryland regions. Scientists categorized the first heat wave of each season as either rapid-, moderate-, or slow-onset based on how quickly the temperature peak occurred during the event. From 2001 to 2023, the proportion of rapid-onset heat waves increased significantly, suggesting a recent shift toward faster-developing heat waves. The study highlights that heat wave risks are not only determined by how hot or long they are, but also by when they occur and how quickly they develop. Longer heat wave seasons are increasing the exposure of crops to extreme heat. Between 2001 and 2023, the proportion of crop areas exposed to heat waves during critical growth stages—such as flowering and silking—increased by 1.6% to 13.3% for several major crops compared to the period from 1979 to 2000. Heat wave exposure also began earlier and lasted longer during various stages of crop development. Xu Wenfang, the study's lead author and one of its corresponding authors, emphasized that the findings highlight the importance of considering heat wave timing and development speed in climate risk assessments. The research provides valuable scientific evidence for improving monitoring and early-warning systems for extreme heat. In addition to temperature intensity and duration, the timing of heat wave onset and end, as well as how quickly the first heat wave of the season reaches its peak temperature, should be factored into heat monitoring and risk assessments. This information could support better adaptation strategies in public health, agriculture, ecosystem management, and energy supply.