The mass balance - decisive for glacier advance and retreat

If a lot of snow falls over several years due to cool weather and the snow melts away late in spring, the glacier tongue receives more ice than can be melted away in summer and autumn: the mass balance is positive and the glacier advances. Conversely, in warmer years with little snow, the glacier loses more ice than can be added - the mass balance is negative and the glacier recedes, as is the case today.

Glacier advance or retreat only occurs with a delay after a change in mass. Small glaciers, such as the Lower Grindelwald Glacier, react to changing climate conditions after only a few years. Larger glaciers, on the other hand, such as the Great Aletsch Glacier, react sluggishly. Their reaction time to longer-term climate changes has been calculated at 20 to 30 years.