Glaciers remain stable when the snow that falls on their surface compensates for the melting at lower elevations (see box*). In our warming world, this balance has been broken and glaciers are shrinking – with consequences for water resources and natural hazards. "To understand how glaciers will develop in the future, it is important to know how much snow falls on their surface," says Marin Kneib, glaciologist at WSL and ETH Zurich.

One factor that has been little studied to date is avalanches. Observations of individual Alpine glaciers have shown that up to 20 % of the snow falling on them comes from this source. Now, together with an international research team, Kneib has estimated the influence of avalanches on all 200,000 glaciers on Earth – and was surprised by the results. "I never thought that this effect would be so significant on a global scale," says Kneib.

Snow slides down to the glaciers’ surface

On average, 11 % of the snow on Alpine glaciers comes from avalanches, 19 % in the eastern Himalayas and as much as 22 % in New Zealand, which tops the list. On individual glaciers, more than 50 % of the snow can come from avalanches. In flatter mountain regions such as Iceland or Greenland, on the other hand, avalanches hardly make any difference. The results are published in the scientific journal Nature Communications.

Avalanches are a benefit for small glaciers: thanks to them, these glaciers could survive longer than scientists expected despite climate change. In the Alps, forecasts show that glaciers less than one square kilometer in size – such as the Glärnischfirn – would lose three times less ice than previously assumed, at least in the most favourable climate scenarios. The smaller the glaciers, the greater the influence of avalanches, as these avalanches mostly influence the glaciers’ margins. "The significance of avalanches on glaciers will therefore increase in the future, as glaciers retreat" says Kneib. But this is no salvation: "In the Alps, we will lose more than 80 per cent of the ice volume of the year 2000 by 2100 anyway."

Furthermore, avalanches do not always bring snow. They can also remove a lot of snow from glaciers if these are steep enough. In the tropical Andes, for example, 8 % of the snowfall slides off the ice in avalanches, snow that is then missing from the glacier. Avalanches also remove snow from steep ice flanks at high altitudes. As the climate warms, the ice in these areas is more likely to disappear than previously anticipated, which in turn threatens to destabilise the underlying rock.

The study aims to inspire a new generation of refined glacier models. "This is only an initial estimate of a process that has been little studied to date" says Kneib. However, to refine the models and thus the forecasts for individual glaciers and catchment areas, more measurement data on avalanches from on-site observations and remote sensing is needed.

Box: *How glaciers evolve

Glaciers in the mountains generally receive mass when snow falls in their upper reaches. The snow solidifies into ice and flows downhill with the glacier's flow to lower elevations, where it undergoes more melting. A glacier is therefore the result of a balance between snow accumulation and ice melt. In steep mountain settings, avalanches can bring large masses of snow onto the glaciers regardless of the altitude and, in some cases, protect them from melting even at low altitudes.