Thick debris paradoxically controls the ‘anomalous’ thinning of debris-covered glaciers in High Mountain Asia

 

Thick debris cover, greater than about 5 cm, insulates ice and reduces melt rates. Despite this melt-suppressing effect, glaciers often thin rapidly under thick debris cover. In High Mountain Asia, the European Alps, and Alaska many debris-covered and debris-free glacier tongues are thinning at similar rates (e.g., Kääb et al., 2012). This apparent paradox is known as the ‘debris-cover anomaly’ (Pellicciotti et al., 2015). Two mechanisms have been proposed to explain this behavior, which are not mutually exclusive. First, glacier thinning under thick debris is enhanced by melt hotspots (lakes, ice cliffs, and streams) within otherwise continuous debris cover. Second, the decline in ice flow from upglacier leads to thinning under thick debris (e.g., Vincent et al., 2016).

We propose a new mechanism to explain why thinning amplifies under thick debris. It appears that debris cover—through its affect on the melt pattern—controls glacier geometry (i.e., patterns of ice thickness and surface slope). A characteristic debris-perturbed driving stress pattern results which in turn controls where dynamical thinning amplifies, often in the upper reaches of debris-covered tongues. Our explanation is supported with data from a suite of glaciers in the Himalaya and with simulations from a numerical debris-covered glacier model responding to climate change (Anderson and Anderson, 2016).

In all numerical simulations, the zone of maximum glacier thinning initially occurs upglacier from the debris cover. This zone of maximum thinning then propagates downglacier into the debris-covered portion. We explain how this zone of maximum thinning can be spatially pinned and amplified at different locations relative to the terminus depending on debris thickness, bed slope, glacier size, and glacier topology. This seemingly paradoxical mechanism in which debris itself controls thinning under thick debris is further supported by an analysis of published thinning data from glaciers across High Mountain Asia.