Climate change triples the frequency of atmospheric-riverine compound heatwaves in US and Europe rivers

Record-breaking heatwaves have become a hallmark of the warming climate, disrupting water, energy and food systems and straining public health. While a nascent body of research has begun to document riverine heatwaves, their co-occurrence with atmospheric events and the emerging threat remain largely unexplored. Here we analyze 796 river basins in the US and Europe to characterize atmospheric-riverine compound heatwaves (ARCH) events, where atmospheric and riverine heatwaves co-occur. Using observational data and deep learning model, and we find ARCH frequency has tripled over the past four decades, at an increasing rate of +0.4 events per decade (p< 0.001). This trend is driven by the rapid intensification of riverine heatwaves (RHWs), with increases in frequency (114%), duration (148%), and intensity (95%) between the 1981–1990 and 2010–2019 decades, far outpacing changes in their atmospheric counterparts (AHWs). The occurrence of ARCH events is primarily controlled by climatic (59.2%), topographic (22.4%), and hydrological (18.3%) factors, with amplified trends in high-elevation (>3000 m) mountain rivers (+128% per decade), highlighting the vulnerability of these critical water sources to global change. These compound events exert greater thermal and oxygen stress on aquatic ecosystems than isolated heatwaves. Projections under a high-emissions scenario (SSP5-85) indicate that by 2100, nearly all riverine heatwaves will coincide with atmospheric heatwaves. These findings highlight the escalating threat of compound heatwaves to freshwater ecosystems and the importance of incorporating ARCH dynamics into future water risk assessments.