Heimdal Glacier in southern Greenland
Heimdal Glacier in southern Greenland. Credit: NASA/John Sonntag

On average, most of Earth’s mountain glaciers are continuing to melt. Scientists know this by calculating the glaciers’ “average mass balance”: taking accumulated snowfall on those glaciers and subtracting ice losses due to melting, meltwater runoff (drainage away from glaciers) and evaporation (when a liquid turns into a gas). Their calculations show decades of more ice losses than gains.