Impact of the debris cover on High Mountain Asia glacier mass balances : a multi-scale approach

Abstract : High Mountain Asia (HMA) hosts the largest glacierized area outside the polar regions. Approximately 15 % of the ~100 000 km² of HMA glaciers is covered by a debris layer of various thickness. The influence of this debris on the HMA glacier response to climate change remains debated. In principle, the presence of a thick layer of debris reduces the melt of the ice beneath it, due to the insulating effect. However, other processes such as ablation of bare ice cliff faces, subaqueous melt of supraglacial ponds and internal ablation due to englacial hydrology could substantially contribute to enhance the debris-covered glacier mass losses. The aim of this PhD work is to assess the impact of the debris on glacier mass balance in HMA. Up to now, the influence of the debris cover has been assessed through glacier front position changes or on a restricted sample of glaciers, and no large scale study of the influence of the debris cover on the glacier-wide mass balance is available.As a starting point, we derived glacier mass changes for the period 2000-2016 for the entire HMA, with an unprecedented resolution, using time series of digital elevation models (DEMs) derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) optical satellite imagery. We calculated a total mass loss of -16.3 ± 3.5 Gt yr-1 (-0.18 ± 0.04 m w.e. yr-1) with contrasted rates of regionally-averaged mass changes ranging from -0.62 ± 0.23 m w.e. yr-1 for the eastern Nyainqentanglha to +0.14 ± 0.08 m w.e. yr-1 for the western Kunlun Shan.At the scale of HMA, the pattern of glacier mass changes is not related to the presence of debris, but is linked with the climatology. Consequently, we studied the influence of the debris-cover on mass balance within climatically homogeneous regions. Based on the mass balances of individual glaciers larger than 2 km² (more than 6 500 glaciers, which represent 54% of the total glacierized area), we found that debris-covered glaciers have significantly more negative mass balances for four regions out of twelve, a significantly less negative mass balance for one region and non-significantly different mass balances for the remaining seven regions. The debris-cover is generally a less significant predictor of the mass balance than the slope of the glacier tongue or the glacier mean elevation. The influence of the debris is not completely clear and complicated to untangle from the effect of the other morphological parameters, because heavily debris-covered tongues are situated at lower elevations than debris-free tongues, where ablation is higher.However, such a statistical analysis of the influence of the debris-cover on the glacier-wide mass balance variability is not very informative in terms of glaciological processes. In order to better constrain the contribution of the different ablation processes on debris-covered tongues, work at a finer scale is required. For the debris-covered tongue of Changri Nup Glacier, Everest region, Nepal, we quantified the contribution of ice cliffs to the ablation budget. Using a combination of very high resolution DEMs derived from Pléiades images and an unmanned aerial vehicle, we found that ice cliffs contributed to ~23 ± 5 % of the total net ablation of the tongue, over two contrasted years, although they occupy only 7 to 8 % of its area. Ice cliffs are large contributors to the ablation of a debris-covered tongue, but they cannot alone explain the so-called debris cover anomaly, i.e. the fact that debris free and debris covered tongues have similar thinning rates. This anomaly is probably due to smaller emergence velocity over debris-covered tongues than over debris-free tongues, resulting in similar thinning rates, despite less negative surface mass balance rates. We advocate for more measurements of ice thickness of debris-covered tongues in order to better understand their dynamics.
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Fanny Brun. Impact of the debris cover on High Mountain Asia glacier mass balances : a multi-scale approach. Glaciology. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAU024⟩. ⟨tel-01927944⟩

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