Persistent river heatwaves are emerging worldwide under climate change.

Rivers and the organisms living within them are highly vulnerable to hot thermal extremes. However, very little is known about river heatwaves, consecutive episodes of anomalously high temperature in rivers, and how they may evolve under climate change. Here we show that river heatwaves will become more intense and more persistent globally by the end of the 21st century, with some tropical rivers reaching a persistent year-round heatwave state in the early 21st century. Under the high-greenhouse-gas-emission scenario (Representative Concentration Pathway 8.5), the average intensity of river heatwaves will increase by ~4.2-fold, and the average duration by ~95-fold, relative to the baseline period (1976-2005). Nearly half of the world's rivers are expected to experience a year-round heatwave state by the 2090 s. Global population exposure to river heatwaves will reach 16.8 billion person-weeks annually, with a disproportionately heavier burden on vulnerable low-income regions, such as the Congo River basin. Emerging persistent river heatwaves may push river ecosystems and aquatic organisms to their resilience limits, causing irreversible changes and widespread impacts.