Perkins-Kirkpatrick, SarahPalmer, LucindaKing, AndrewZiehn, Tilo2025-12-192025-12-19WOS:001616043200001ORCID:/0000-0001-9443-4915/work/197987617https://hdl.handle.net/1885/733796729While historical and future increases in heatwave frequency, duration and intensity are well documented, no studies have yet examined heatwave changes after anthropogenic greenhouse gas emissions reach net zero. We address this by examining heatwave projections from millennial-scale simulations run with the Australian Earth System Model, ACCESS-ESM1-5. Each simulation branches off the SSP5-8.5 scenario at 5 year intervals between 2030-2060, from which point anthropogenic carbon dioxide emissions are set to net zero. Heatwaves are systematically hotter, longer and more frequent the longer net zero is delayed and reach their highest values when net zero is delayed until 2060. Moreover, most regional trends show no decline over the entire 1000 years of each simulation, indicating that heatwaves do not start to revert to preindustrial conditions. Some regions even display significantly increasing millennial-scale trends when net zero occurs by 2050 or later. Furthermore, the longer net zero is delayed, the more occurrences of historically rare and extreme heatwave events. This is problematic for low-latitude countries which are also generally more vulnerable, where historically record-breaking events occur once a year or more when net zero is delayed until after the middle of the 21st Century. Should the global effort to permanently reach net zero occur before 2040, future heatwaves will be less severe than in a 2 degrees C warmer world, however if net zero is not reached until 2060, heatwaves will be systematically more severe than this upper threshold of the Paris agreement. Our research critically challenges the general belief that conditions after net zero will begin to improve for near future generations. While our results are concerning, they provide a novel length of foresight, such that effective and permanent adaptation measures can be planned and implemented while the world is still on the imperative path to permanent net zero.S.E.P.-K. and A.D. King are funded by Australian Research Council Grant Number CE230100012. L.P. is funded by Australian Research Council Grant Number CE170100023.17en© 2025 The Author(s)Climate changeHeatwavesNet zero202510.1088/2752-5295/ae0ea4105021920834