ABSTRACT
Earth’s hydrological cycle-the sun-powered movement of water between the sea, air, and land-is an irreplaceable asset that human actions are now disrupting in dangerous ways. Although vast amounts of water reside in oceans, glaciers, lakes, and deep aquifers, only a tiny share of Earth’s water-less than one one-hundredth of 1 percent-is fresh, renewed by the hydrological cycle, and delivered over land. That precious supply of precipitationsome 110,000 cubic kilometers per year-is what sustains most terrestrial life.1
