Snow and ice in the high mountains of Asia feed rivers supplying water to over a billion people, but airborne soot and dust darken the snow and speed up melting. We built a 17-year record of these conditions by blending satellite observations with a complex model that simulates weather, air pollution, land, and snow as one connected system. The record agrees well with independent measurements and traces soot on the glaciers to its regional sources, helping assess risks to Asia's water supply.