Uncertainty assessment of modeling the impact of debris cover on global glacier mass change: challenges and solutions

It is essential to test the potential impact of processes missing in the global-scale models that are used to project on glacier mass balance in the glacier models, such as the modification of the mass balance through debris-cover. In this study, we evaluate the impact of a parameterization of debris cover on glacier mass change projections using the Open Global Glacier Model (OGGM). The assessment of uncertainties about the potential impact of debris cover on global scale modeling is complicated by the scarcity of suitable data on debris-covered glaciers for validation (e.g., mass balance measurements for individual elevation bins on debris-covered glaciers). To calibrate and validate the mass balance module, we rely on glacier-wide geodetic volume changes. Debris cover can enhance ice melting if less than a few centimeters thick, or decrease ice melting through insulation of the underlying ice by a thick layer of debris. Ice cliffs, supraglacial ponds and streams associated with debris cover may increase the absorption of heat and increase ice melting. In OGGM, the effects of debris cover are parameterized by a simple modification of the mass balance module, through introducing an elevation-dependent temperature sensitivity parameter (“degree-day factor”) and including a debris-related melt correction factor. While debris cover plays only a minor role on glacier mass change on the global scale, this becomes important on regional and individual glacier scales. Our results show the effect of debris cover could improve model performance on the mass balance gradient, not the overall mass balance. To validate our results on the mass balance gradient, we rely on the geodetic mass balance for each elevation band.