Scientists from around the world have assessed the planet's 78 mountain glacier-based water systems. For the first time, they ranked them in order of their importance to adjacent lowland communities while assessing their vulnerability to future environmental and socioeconomic changes. These systems, known as mountain water towers, store and transport water via glaciers, snow packs, lakes and streams, thereby supplying invaluable water resources to 1.9 billion people globally—roughly a quarter of the world's population.