The importance of small glaciers for accurate projection of future runoff in High Mountain Asia

High Mountain Asia is characterized by a substantial glacier coverage with glaciers of varying sizes. These glaciers are crucial in the area's hydrological cycle since they feed large rivers such as the Indus, Ganges, and Brahmaputra rivers. However, ongoing climate change is having a significant negative impact on glacier mass and projections show strong further declines of glacier mass in the future. This is raising concerns about future water security. How big the impact of the evolution of small glaciers (< 2 km²) is towards changing water availability remains to be investigated.

Most studies focus on the regional evolution of glaciers as a whole, which means that small-scale glaciers are often overlooked due to larger glaciers dominating the signal in area and volume changes, despite the fact that small glaciers make up about 30% of the glacierized area in High Mountain Asia. To address this issue, we applied the Global Glacier Evolution Model (GloGEM) to simulate all ca. 100’000 glaciers of High Mountain Asia (Regions 13-15 of the Randolph Glacier Inventory v6.0) under various climate scenarios in the period of 1980-2100. We compared the spatio-temporal variability of the timing of peak water, as well as glacier volume change, between small and large glaciers for a set of approximately 30 catchments in the headwater of Indus, Ganges and Brahmaputra rivers.

We find that there is a larger difference between future scenarios for the timing of peak water for smaller glacier, with it ranging from 2030-2060 and then runoff declining rapidly. Meanwhile, peak water for larger glaciers is likely to occur between 2070-2080 according to an intermediate emission scenario, with glacier runoff decreasing gradually thereafter. As for the ice volume change, smaller glaciers are expected to reach volumes close to zero near the year 2080, while larger glaciers are expected to reach this point only after 2100. The quicker response of small glaciers compared to large glaciers emphasize the need for a particular focus on small glaciers to better understand their responses to climate change and make accurate projections about local and regional scale near future water availability.