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.

Agua Sustentable Irpavi La Paz Bolivia

Centre for Quaternary Research Department of Geography Royal Holloway University of London Egham UK

Czech Academy of Sciences Global Change Research Institute Brno Czech Republic

Department of Geography Universidad de Concepción Concepción Chile

Department of Geography University of British Columbia Vancouver British Columbia Canada

Department of Geology University of Dayton Dayton OH USA

Faculty of Geosciences Department of Physical Geography Utrecht University Utrecht The Netherlands

FutureWater Wageningen The Netherlands

Indian Institute of Science Divecha Center for Climate Change Bangalore India

Institute of Tibetan Plateau Research Chinese Academy of Sciences Beijing China

International Centre for Integrated Mountain Development Kathmandu Nepal

International Institute for Applied Systems Analysis Laxenburg Austria

Johns Hopkins University Department of Environmental Health and Engineering Baltimore MD USA

Joint Institute for Regional Earth System Science and Engineering University of California Los Angeles CA USA

National Geographic Society Washington DC USA

Planetary Science Institute Tucson AZ USA

School of Geography and Sustainable Development University of St Andrews St Andrews UK

State Key Laboratory of Earth Surface Processes and Resource Ecology Beijing Normal University Beijing China

Swiss Federal Research Institute WSL Birmensdorf Switzerland

Universidad Mayor de San Andrés Institute for Physics Research La Paz Bolivia

University of Maine Climate Change Institute Orono ME USA

University of Maryland Department of Atmospheric and Oceanic Science College Park MD USA

University of Utah Department of Geography Salt Lake City UT USA

University of Zurich Department of Geography Zurich Switzerland

Wageningen University and Research Water and Food Group Wageningen The Netherlands

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