Scientists could be seriously underestimating the amount of snow stored atop mountains — an important source of water for people around the world.

Mountain snowpack is notoriously hard to calculate because satellites can’t measure snow depth. Melissa Wrzesien at the University of North Carolina in Chapel Hill and her colleagues studied four global data sets that quantify mountain snowpacks. The four data sets agree reasonably well with each other — but they do not agree with high-resolution simulations of snow across several North American mountain ranges, such as the Cascades.

The simulations suggest that 40–66% more snow is frozen in those mountains than scientists had realized. If that pattern holds true globally, then the data sets could be missing as much as 1,500 cubic kilometres of mountain snowpack.

If melted, that snow would supply an amount of water equivalent to 4% of that carried by the world’s rivers.