The relevance of the past unsustainable increase of glacier runoff for large-scale basins in different climatological settings

Depending on the seasonality of temperature and precipitation, mountain glaciers seasonally store and release large amounts of freshwater. Therefore, glaciers have a strong influence on water availability in many regions of the world. In an ongoing global climate change, glaciers have an additional impact on water availability, as the net amount of stored ice changes in an unsustainable way. This results in glaciers not only altering the seasonal runoff, but also adding a net input into the drainage system.
To better understand the interplay between seasonal and long-term storage changes, we suggest to split the monthly seasonal mass balance into a sustainable fraction, which is derived by balancing solid precipitation by ablation proportional to positive temperatures, and an unsustainable fraction, which causes long-term glacier mass change.

Similarly, we consider the effect of glacier area changes, allowing us to separate seasonal runoff into components attributable to (unsustainable) area change, (unsustainable) mass change, or the (sustainable) seasonal runoff from the glacier.

By applying the concept to a reconstruction of global glacier change, we illustrate how the glacier input into river basins in different climatological settings has been affected by the glacier mass loss during the 20th century.