Interannual Dynamics of Ice Cliff Populations on Debris‐Covered Glaciers From Remote Sensing Observations and Stochastic Modeling. Issue 10 (13th October 2021) - Kneib, M.
Abstract: Ice cliffs are common on debris‐covered glaciers and have relatively high melt rates due to their direct exposure to incoming radiation. Previous studies have shown that their number and relative area can change considerably from year to year, but this variability has not been explored, in part because available cliff observations are irregular. Here, we systematically mapped and tracked ice cliffs across four debris‐covered glaciers in High Mountain Asia for every late ablation season from 2009 to 2019 using high‐resolution multi‐spectral satellite imagery. We then quantified the processes occurring at the feature scale to train a stochastic birth‐death model to represent the cliff population dynamics. Our results show that while the cliff relative area can change by up to 20% from year to year, the natural long‐term variability is constrained, thus defining a glacier‐specific cliff carrying capacity. In a subsequent step, the inclusion of external drivers related to climate, glacier dynamics, and hydrology highlights the influence of these variables on the cliff population dynamics, which is usually not a direct one due to the complexity and interdependence of the processes taking place at the glacier surface. In some extreme cases (here, a glacier surge), these external drivers may lead to a reorganization of the cliffs at the glacier surface and a change in the natural variability. These results have implications for the melt of debris‐covered glaciers, in addition to showing the high rate of changes at their surface and highlighting some of the links between cliff population and glacier state. Plain Language Summary: A large portion of High Mountain Asia's glaciers is covered by a layer of rock debris that reduces their melt. This melt reduction is compensated to some extent by the presence of ice cliffs that cover up to 12% of these glaciers and melt several times faster than the surrounding debris‐covered ice. This enhanced contribution to melt needs to be taken into account in glacier melt models as it has implication for downstream water resources. However, due to the difficulty to map ice cliffs from satellite images, we have little knowledge about their actual distribution and most importantly, on how quickly their population and therefore relative area changes in time at the scale of one glacier, let alone what drives this variability. Here, we systematically mapped ice cliffs on a yearly basis over a period of 10 years for four Asian glaciers to disentangle these questions. We found that the cliff area of a glacier could commonly change by 20% in 1 year, and in some cases up to 80%. We also found that a number of climatic and glaciological variables influenced to some extent this variability but none stood out clearly due to the complexity of the debris‐covered glacier system. Key Points: The proportion of ice cliffs on debris‐covered glaciers can change by up to 20% from 1 year to the next There is no direct influence of external drivers on cliff variability due to the complexity and interdependence of glacier surface processes The long‐term variability of the cliff population is constrained unless it is modified by an extreme climatic and/or glacier change