Climate warming drives thermal shocks and accelerated freshwater habitat fragmentation

Climate change is exposing freshwater fish to warmer temperatures, causing widespread range contraction and population declines. However, how climate-driven disruptions will affect freshwater fish persistence under future thermal extremes remains poorly understood. Here, using weekly water temperature data (1976–2099), we assess the vulnerability of 9,809 freshwater species to short- and long-term thermal extremes and show that gradual warming can trigger abrupt thermal exposure. Tropical regions experience a high proportion of species at risk, whereas non-tropical regions face severe thermal extremes. We introduce the concept of thermal fragmentation exposure to understand how heterogeneous warming fragments freshwater habitat, thereby amplifying local extinction risks. Around 63% of riverine fish will experience more persistent thermal barriers under a high-emissions scenario, with thermal fragmentation accelerating over time. These findings underscore the urgent need for targeted conservation strategies to safeguard freshwater fish biodiversity.