Sorting and Ranking Complex Terrain Controls on the Persistence of Cirque Glaciers

Abstract:

Initial ice volume and glacier hypsometry predictably control mountain glacier response to climate forcing. Large glaciers situated at high elevations likely linger on the mountain landscape even as climate warms. Yet rugged mountain terrain in the high alpine promotes variable snow accumulation and melt conditions. This can complicate glacier response, because snow avalanching, snow wind-drifting and radiation shading create refugial settings that sustain ice and disrupt the predicted demise of small cirque glaciers. To parse these complex terrain controls, we analyzed glacier area change since the Little Ice Age glacial maximum in Glacier National Park, USA. We estimated initial ice thickness and glacier volume using simple ice flow assumptions, assessed topographically driven processes using terrain proxies, then sorted and ranked these various factors using multivariate analysis. We found that 100% of the 82 glaciers present at the Little Ice Age have since decreased in area, and 60% (49) diminished to less than 0.1 km². Initial glacier volume and elevation did not explain the persistence of intermediate sized glaciers (0.5 km² < area < 1.5 km²). Instead, median wind exposure emerges as an important explanatory variable. Overall, glacier area, median glacier elevation, and median wind exposure are characteristically different for glaciers that persisted, whereas solar shading and snow avalanche factors are less reliable descriptors. This evidence suggests that predicting cirque glacier fate requires prioritizing accurate representation of starting glacier volume, glacier hypsometry, and glacier exposure to predominant storm tracks.