Limited meltwater potential in the Asian Water Tower to mitigate downstream urban scarcity.

The Asian Water Tower, a vital freshwater source for billions downstream, is rapidly transforming under climate change. Accelerated glacier and snowmelt are reshaping meltwater patterns, threatening urban water security amid rapid population growth. Using a high-resolution cryosphere-hydrology model, we quantify meltwater's role in mitigating urban water scarcity. Here we show that over the past three decades, meltwater availability has mitigated water scarcity for 22.9-28.7 million urban residents by supplementing water during dry seasons. However, future meltwater availability is projected to become increasingly uneven, rising during the monsoon when rainfall is abundant but declining substantially in the dry season. This seasonal redistribution substantially weakens the buffering capacity of meltwater during critical dry periods. As urban populations facing water scarcity are projected to increase by 31-53%, meltwater gains will remain minimal (4-10%), indicating an increasing supply-demand mismatch. Our findings highlight that despite projected increases in annual volume, the potential of meltwater to alleviate future urban water scarcity is increasingly constrained by seasonal misalignments, underscoring the urgent need to integrate meltwater variability into regional water resource planning.