Water frozen at the tops of mountains that helps sustain up to a quarter of the human population is under threat from climate change, population growth and lack of proper management, according to a new international study. The systems store and transport water through glaciers, snow packs, lakes and streams. They supply water to 1.9 billion people on Earth. A group of 32 international scientists — including one Canadian — published a paper in the journal Nature on Monday to highlight the importance and vulnerabilities of what they call Earth's 78 water towers.