Global hotspots and trends of ecological water scarcity in river basins

Healthy freshwater ecosystems are crucial for sustainable development, yet they face growing threats from unsustainable water use. However, current water scarcity metrics focus almost exclusively on human demand, overlooking whether sufficient water remains to sustain ecosystems. We address this gap by presenting the first evaluation of ecological water scarcity (EWS) across 1,182 river basins, introducing a novel indicator that recognizes ecosystems as legitimate water users to those of humans. Our findings reveal that in the 2000s, 412 basins (35% globally; 17% annual, 18% seasonal) experienced EWS, with affected areas expanding by 10 million km² since the 1950s. Human intervention intensified EWS in 13% of basins and caused 53% of seasonal and 60% of annual EWS cases. By the 2050s, affected basins range from 392 under RCP2.6–SSP1 to 416 under RCP8.5–SSP5, with persistent hotspots in regions of intensive water use. These findings show that climate mitigation can reduce future EWS risk, but environmental flow protection, water-use efficiency, water recycling, ecologically informed infrastructure operation and ecosystem restoration are also needed to safeguard freshwater ecosystems.