Influence of the debris layer on the behavior of the Shallap and Llaca glaciers, Cordillera Blanca, Ancash, Peru

The retreat of tropical glaciers is a visible effect of climate change, which is evident in Peru. These glaciers are sensitive to global and local temperature increases, causing accelerated glacier mass loss. From 1962 to 2020, there has been a decrease in glacier cover at the national level, with a loss of approximately 1,280.95 km². This phenomenon significantly affects local ecosystems and water availability, putting at risk the supply of water for human consumption, agriculture and other essential activities, particularly during dry seasons.

The research was carried out on the Shallap and Llaca glaciers, located in the Cordillera Blanca in Ancash, Peru. The Shallap glacier is characterized by two clearly differentiated zones: one covered by debris and the other with a clean surface. While the Llaca glacier has a glacial tongue completely covered by debris which terminates in a proglacial lake. This distinction in the surface characteristics of both glaciers allows for detailed comparisons, providing insights into the role of supraglacial debris in modifying glacier melt rates, processes which have been rarely studied in the Peruvian Andes.

Repeat Unmanned Aerial Vehicle (UAV) imagery conducted in 2019 and 2024 allowed the calculation of mass and area changes of both glaciers, and the assessment of morphological changes between the time periods. Imagery was co-registered to reduce the planimetric errors in the Digital Elevation Models (DEMs).

Much lower mass losses were found over Shallap glacier in the debris-covered compared to clean ice zones, with the debris-covered zone retreating by a maximum of 36 meters, with an area loss of 8,257 m² (equivalent to -14,202 m³ of ice). On the other hand, the debris-free zone retreated by a maximum of 165 meters, with an area loss of 24,439 m², representing a volume of -61,160 m³. However, the Llaca glacier showed a maximum retreat of 122 meters, with an area loss of 21,144 m² (equivalent to -397,813 m³ of ice). This difference in volume loss compared to Shallap glacier, could be due to the presence of the proglacial lagoon, which is still in contact with Llaca glacier and that would be contributing to a greater glacial melting.

Another factor that modulates glacial behavior is the supraglacial debris layer, according to the results obtained from the loss of glacier volume considering the movement recorded between June and November 2024, it was observed that the debris layers of 1.5 cm and 5.5 cm were associated with ice losses of 3.6 m and 2.5 m, respectively. In contrast, the 33 cm and 50 cm thick debris layers showed considerably smaller ice losses of only 0.4 m and 0.2 m. It can be seen that the thicker the debris layer, the lower the ice thickness loss.