Debris cover effect on the evolution of glaciation in the Northern Caucasus

A common disadvantage of almost all global glacier models is that they ignore the explicit description of the debris cover on the heat exchange of the glacier surfaces with the atmosphere. Debris cover plays a key role in the regulation of melt processes. A debris cover more than a few centimeters reduces melting, since it isolates the underlying ice. In this way, debris covered areas are thought to be less exposed to rising temperatures, thereby reducing glacier retreat and mass loss.        

In the foothills of the North Caucasus, an important agricultural region, the problem of expected changes in mountain glaciation is particularly acute, since fluctuations in the flow regime of local rivers depend on the evolution of glaciers: the contribution of glacial runoff to total discharge is very significant.

Here, we present the assessment of debris cover influence on the glacier evolution of the Northern Caucasus on a regional scale (Terek and Kuban river basins). The aim is to determine how much the characteristics of mountain glaciation (its mass balance, area, volume, position of the glacier fronts) of the Northern Caucasus depend on the debris cover evolution. In order to accomplish this goal, we use the GloGEMflow model and a newly created debris cover dynamic module, which is calibrated using newly mapped debris cover outlines. The debris thickness evolution is simulated with a steady deposit model adapted from Verhaegen et al. (2020) and Anderson & Anderson (2016), where debris input onto the glacier is generated from a fixed point on the flow line.

The results reveal that the debris cover evolution pattern differ significantly for Terek and Kuban glacierized basins. Lower elevated Kuban basin glaciers undergo a rapid retreat and lose the debris covered glacier tongues while the Terek basin glaciers experience supraglacial debris expansion with a six times larger effect of debris cover on glacier volume evolution. From 2000 to 2016 the mass loss in the Terek ice basin reached 47834 Mt with an influence of the debris cover module and 50435 Mt  under debris-free conditions. Therefore, we can expect that by the end of the current century the mass loss of the Terek glaciers will be significantly overestimated in case debris cover influence will be ignored in model calculations. On the contrary, in the Kuban basin, calculated mass loss in 2000-2016 with and without debris cover were 1249 Mt  and 1258 Mt  respectfully. Committed loss experiments (constant mean climate for 1990-2015) show that the glaciers of the Terek basin lose ~35% of ice if debris cover is not taken into account and ~29% if debris cover module is turned on (~2  ice volume difference). For the Kuban basin glaciers, the difference of ice volume is only ~0.1  in debris-free vs. debris-covered modes.

The reported study was funded by the RFBR and RS grant 21-55-10003, the work of T. Postnikova was supported by the RFBR grant 20-35-90042.