Cascading downstream impacts of climate change in the world’s water towers

Mountains, often called the world’s "water towers" due to their important role in global hydrology and water resources including supply for human uses and ecological processes, are interconnected with lowlands in a system that encompasses both natural resources and society. Climate change in mountain regions affects the amount, timing and quality of mountain runoff, with important consequences downstream. While mountain streamflow and associated climate change impacts always travel downstream, these impacts can cascade not only spatially and temporally but also causally across a wide range of social-ecological systems. Additionally, upstream-downstream teleconnections can have important impacts that shape upstream water tower systems, for example, through infrastructure development based on priorities for downstream users.

We synthesize key water cycle changes in mountain regions worldwide and examine their consequences downstream, such as shifts in surface and groundwater availability, disaster risks, water quality, human water use, sediment transport, aquatic ecosystems, and sea-level rise. We link these dynamics to social processes, including culture, economy, and well-being in local and transboundary contexts. Furthermore, we highlight feedback mechanisms where downstream activities shape upstream water dynamics, including infrastructure development (e.g., hydropower), land and water use (roads, mining, tourism), and conservation (glacier protection, low-impact recreation). Our analysis underscores the importance of an integrated framework for advancing the understanding of interconnected mountain-lowland systems to inform sustainable water management and policy development in rapidly changing mountain regions and beyond.