How glaciers work

Firn is the name of this site and the stage between snow and ice. Snow that survives a summer is compacted by the snow above it; the grains round off and fuse. After a year it is firn, dense and granular. After years to decades, as air is squeezed out, it becomes glacier ice.

A glacier has two parts. In the accumulation area, high up, more snow falls in winter than melts in summer. In the ablation area below, summer melt removes more than winter brings. The line between them, the equilibrium line, sits roughly where the snow line lies at the end of summer. Its altitude (ELA) is a good gauge of a glacier’s health.

Ice flows downhill under its own weight, carrying mass from the accumulation area to the tongue. If the glacier loses more at the bottom than flow delivers, the tongue retreats. A glacier with no accumulation area left, snow-free to the top at the end of summer, has no future in the current climate.

Length change

Since 1880, observers have measured the position of glacier tongues each autumn, today on around 100 glaciers. It is the longest and simplest record, but a tongue responds to climate with a delay of years to decades, longer for big glaciers.

Mass balance

On about 20 glaciers, teams drill stakes into the ice and measure snow depth and density in spring and melt in autumn. The mass balance is the net gain or loss over the whole glacier in a year, expressed as a layer of water (metres water equivalent, m w.e.). It responds to the weather of that very year.

Volume change

Comparing elevation models from old maps, aerial photographs and laser scans shows how much a glacier’s surface has dropped. All 1,400 glaciers are surveyed from aerial photographs in six-year cycles.

Inventories

Glacier inventories map every glacier at one time. Switzerland has inventories for 1850 (reconstructed from maps and moraines), 1931 (from terrestrial photographs), 1973, 2010 and 2016. This site uses 1850, 1931, 1973 and 2016; the 2010 inventory used different mapping rules and is not directly comparable.

Ice thickness

Helicopter-borne radar has measured the thickness of most larger Swiss glaciers since 2016, which is how the total ice volume is known: about 43 km³ at the end of 2026.

Glacier models take climate model output for a scenario and compute, year by year, how much snow falls on each glacier, how much melts and how the ice flows. The result is a range, not a forecast: each scenario is run with many climate models, and the spread between them is shown as a shaded band.

The maps on this site show past extents from real inventory outlines. Future extents are illustrative: the projected area of each glacier is removed from its lowest ice first. Real glaciers retreat mostly from the tongue too, but thinning, debris cover and shading change the details.

The four scenarios are explained on the future page.

Each scenario name has two parts. The first, SSP1 to SSP5, is a story about how society develops: population, economy, technology, cooperation. The second number, 2.6, 4.5, 7.0 or 8.5, is how much extra energy greenhouse gases trap in the climate system by 2100, in watts per square metre of the Earth’s surface (the “radiative forcing”). Higher number, more warming.