Samuel Bartusek , Kai Kornhuber , Mingfang Ting

Heat conditions in North America in summer 2021 exceeded prior heatwaves by margins many would have considered impossible under current climate conditions. Associated severe impacts highlight the need for understanding the heatwave’s physical drivers and relations to climate change, to improve the projection and prediction of future extreme heat risks. Here, we find that slow- and fast-moving components of the atmospheric circulation interacted, along with regional soil moisture deficiency, to trigger a 5-sigma heat event. Its severity was amplified ~40% by nonlinear interactions between its drivers, likely driven by land–atmosphere feedbacks catalyzed by long-term regional warming and soil drying. Since the 1950s, global warming has transformed the event’s peak daily regional temperature anomaly from virtually impossible to a presently-estimated ~200-yearly occurrence. Its likelihood is projected to increase rapidly with further global warming, possibly becoming a 10-yearly occurrence in a climate 2°C warmer than preindustrial, which may be reached by 2050.