Mountains are the water towers of the world, supplying a substantial part of both natural and anthropogenic water demands1,2. They are highly sensitive and prone to climate change3,4, yet their importance and vulnerability have not been quantified at the global scale. Here we present a global water tower index (WTI), which ranks all water towers in terms of their water-supplying role and the downstream dependence of ecosystems and society. For each water tower, we assess its vulnerability related to water stress, governance, hydropolitical tension and future climatic and socio-economic changes. We conclude that the most important (highest WTI) water towers are also among the most vulnerable, and that climatic and socio-economic changes will affect them profoundly. This could negatively impact 1.9 billion people living in (0.3 billion) or directly downstream of (1.6 billion) mountainous areas. Immediate action is required to safeguard the future of the world’s most important and vulnerable water towers.
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Acknowledgements This project was funded as part of the National Geographic Society and Rolex Partnership to Support a Perpetual Planet. We are grateful to the Strategic Priority Research Program of the Chinese Academy of Sciences for their support, to D. Farinotti for providing the data on glacier volume, and to N. Wanders for providing precipitation datasets used in the uncertainty analysis.
© 2019, The Author(s), under exclusive licence to Springer Nature Limited.