Understanding ice loss and glacial lake expansion in the Bhutanese Himalaya

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"Glacial lake area grew on average by 20.8% from 1990 - 2010. Proglacial lake development is a particularly important aggravating factor in mass loss; this variable has not been included in current ice loss projections and may result in increasing outburst flood hazard in 'at risk regions'. Proglacial lakes in the Himalayas can lead to the rapid acceleration of frontal ice loss through calving and sub-marine melt. In Bhutan, observations of the glacial response to proglacial lake development have been limited, and the long-term implications for lake driven mass loss in the region are poorly constrained and have been shown to be highly variable. Calving loss was shown to account for 10 - 25% of total mass loss for a lake terminating glacier in Canada, with an increase of 49% during exceptional summers.The study will utilise the Open Global Glacier Model (OGGM), an open source glacier model based in Python that can simulate historical and future mass-balance of glaciers of any size. The model has been shown to be effective in the Himalaya and can also incorporate frontal ablation parameters to allow the significance of lake development and calving to be understood. Yang et al. (2022) used the OGGM to model historical glacier retreat in Bhutan from four glacial stages under a suite of different historical climate scenarios calibrating the models temperature sensitivity and the residual bias for non-climatic parameters to achieve model accuracy, however this study did not test calving parameters. Debris cover must be considered in this study; 64% of glacier area in Bhutan is classified as 'debris covered', furthermore, many of these glaciers are low gradient, increasing the likelihood of proglacial lake development following retreat. It is important to incorporate debris cover into our mass balance models, to further understand how climate change will affect the glaciers in the Lunana region and/or wider region. Studies have also found that quantifying the effect of debris cover on the temperature sensitivity of ice has been one of the biggest challenges for accurately modelling glacial retreat; this parameter should therefore be calibrated through field observations. The effect of debris on calving processes has also seen little research and may be important in understanding the relationship between thinning rates and the transition to buoyant calving in High Mountain Asia."