Earlier and faster heatwave onset on land under a warming climate

Heatwaves threaten ecosystems and societies. Previous studies have examined their frequency, intensity and duration, whereas long-term changes in heatwave timing—including onset, ending and season length—remain poorly understood, limiting our ability to anticipate heat-related risks. Here we analyse global changes in these metrics and onset-speed type from 1979 to 2023. Globally, heatwave onset advanced by 3.29 ± 0.12 days per decade, ending was delayed by 5.41 ± 0.12 days per decade and the heatwave season lengthened by 8.71 ± 0.17 days per decade. Trends towards earlier onset, later ending and longer heatwave seasons covered 72.0%, 79.6% and 92.1% of global land, respectively, with statistically significant trends across 13.4%, 20.3% and 34.4%. A recent shift towards faster heatwave onset was observed. Drylands experienced more pronounced timing shifts than humid regions. These findings highlight the need to incorporate heatwave timing into early-warning systems, risk assessments and proactive adaptation under climate change.