Parsing complex terrain controls on mountain glacier response to climate forcing

Glaciers are a key indicator of changing climate in the high mountain landscape. Glacier variations across a mountain range are ultimately driven by regional climate forcing. However, changes also reflect local, topographically driven processes such as snow avalanching, snow wind-drifting, and radiation shading as well as the initial glacier conditions such as hypsometry and ice thickness. Here we assess the role of these various terrain influences on change to Little Ice Age (LIA) glaciers in Glacier National Park, U.S.A. With available data for LIA and modern glacier areas, we estimate glacier volumes using simple ice flow assumptions and topographically driven processes using terrain proxies. At the LIA glacial maxima there were 82 glaciers larger than 0.1 km² ranging from 0.11 to 4.97 km². Over the course of the 20th century, every single LIA glacier decreased in area and 60% (49 glaciers) diminished to below the 0.1 km² threshold. Glaciers with large initial area (>1.5 km²) at the end of LIA persisted. Within the intermediate size class (0.5 km² < area < 1.5 km²), LIA glacier persistence is poorly explained by initial glacier volume, ice thickness, or elevation. Instead, wind exposure is an important explanatory factor. Our analysis demonstrates the complex response of cirque glaciers to post-LIA climate change in this region: individual glaciers have not necessarily undergone equivalent and synchronous change. Nevertheless, that all glaciers in this mountain range experienced retreat demonstrates that local processes mediated adjustments of some glaciers but completely decoupled none from the regional climate forcing.