Currently, about 12ĝ€¯%-13ĝ€¯% of High Mountain Asia's glacier area is debris-covered, which alters its surface mass balance. However, in regional-scale modelling approaches, debris-covered glaciers are typically treated as clean-ice glaciers, leading to a bias when modelling their future evolution. Here, we present a new approach for modelling debris area and thickness evolution, applicable from single glaciers to the global scale. We derive a parameterization and implement it as a module into the Global Glacier Evolution Model (GloGEMflow), a combined mass-balance ice-flow model. The module is initialized with both glacier-specific observations of the debris' spatial distribution and estimates of debris thickness. These data sets account for the fact that debris can either enhance or reduce surface melt depending on thickness. Our model approach also enables representing the spatiotemporal evolution of debris extent and thickness. We calibrate and evaluate the module on a selected subset of glaciers and apply GloGEMflow using different climate scenarios to project the future evolution of all glaciers in High Mountain Asia until 2100. Explicitly accounting for debris cover has only a minor effect on the projected mass loss, which is in line with previous projections. Despite this small effect, we argue that the improved process representation is of added value when aiming at capturing intra-glacier scales, i.e. spatial mass-balance distribution. Depending on the climate scenario, the mean debris-cover fraction is expected to increase, while mean debris thickness is projected to show only minor changes, although large local thickening is expected. To isolate the influence of explicitly accounting for supraglacial debris cover, we re-compute glacier evolution without the debris-cover module. We show that glacier geometry, area, volume, and flow velocity evolve differently, especially at the level of individual glaciers. This highlights the importance of accounting for debris cover and its spatiotemporal evolution when projecting future glacier changes.

Due to climate change, worldwide glaciers are rapidly declining. The trend will continue into the future, with consequences for sea level, water availability and tourism. Here, we assess the future evolution of all glaciers in Scandinavia and Iceland until 2100 using the coupled surface mass-balance ice-flow model GloGEMflow. The model is initialised with three distinct past climate data products (E-OBS, ERA-I, ERA-5), while future climate is prescribed by both global and regional climate models (GCMs and RCMs), in order to analyze their impact on glacier evolution. By 2100, we project Scandinavian glaciers to lose between 67 ± 18% and 90 ± 7% of their present-day (2018) volume under a low (RCP2.6) and a high (RCP8.5) emission scenario, respectively. Over the same period, losses for Icelandic glaciers are projected to be between 43 ± 11% (RCP2.6) and 85 ± 7% (RCP8.5). The projected evolution is only little impacted by both the choice of climate data products used in the past and the spatial resolution of the future climate projections, with differences in the ice volume remaining by 2100 of 7 and 5%, respectively. This small sensitivity is attributed to our model calibration strategy that relies on observed glacier-specific mass balances and thus compensates for differences between climate forcing products.

With the Paris Agreement, the urgency of limiting ongoing anthropogenic climate change has been recognised. More recent discussions have focused on the difference of limiting the increase in global average temperatures below 1.0, 1.5, or 2.0g C compared to preindustrial levels. Here, we assess the impacts that such different scenarios would have on both the future evolution of glaciers in the European Alps and the water resources they provide. Our results show that even half-degree differences in global temperature targets have important implications for the changes predicted until 2100, and that-for the most optimistic scenarios-glaciers might start to partially recover, owing to possibly decreasing temperatures after the end of the 21st century.