Modelling the complex response of debris covered glaciers on variations in climate and debris input

Debris-covered glaciers are found in most glaciated areas in the world and often represent an important water resource for downstream areas. The dynamics of coupled debris and glacier interactions are not fully understood, which is why there has been an increasing effort in recent years to use numerical modelling to gain a better understanding thereof.

As process understanding is quite limited, implementations of the debris-glacier system vary widely. Here we model the glacier in the along-flow dimension and set the focus on debris transport and tracking debris within and on the ice. We examine feasible implementations of involved processes and their coupled effects on glacier dynamics in a transient climate and that allows to vary the location and rate of debris input into the system.

We perform a sensitivity analysis on this model and conduct experiments of increasing complexity, varying both climate forcing and debris deposition and on both simple synthetic and realistic bedrock topographies. Our modelling corroborates the earlier finding from earlier simpler models (without internal debris tracking) of strongly delayed retreat that only sets in after stagnation of the tongue. Our results show beyond that after retreat following warming, debris covered glaciers show a long-term re-advance effect, even when the absolute debris entrainment rate stays the same. We explain this by the increase of the debris-ice ratio in the debris deposition zone. Interestingly, results also show that – when accompanied with a permanent, regular supply of debris input– single large deposition events can have a sustainable growth effect on the glacier, even after the debris from that event has exited the system. In experiments with below century scale fluctuations in climate and/or debris input the glacier length does not really respond. We conclude that this insensitivity and the response of debris covered glacier in general is not only influenced by debris insulation on the tongue, but is also affected by the long time-scale (centuries) involved in transporting the debris through the glacier system.