Quantifying Basal Melt of Swiss Glaciers

Glaciers retreating due to climate change have significant impacts both locally and globally. An essential part of understanding their evolution are mass balance measurements. Although the surface mass balance of glaciers is well known, non-surface components, more specifically basal and internal melt, are not well understood as they are inherently difficult to observe. Local maxima in basal melt on alpine glaciers are believed to result in the formation of large subglacial cavities, potentially leading to so called collapse features. The ice loss caused by these collapse features is likely to impact the retreat dynamics of glaciers. Using a parameterized model based on a complete consideration of factors of sub- and englacial energy exchange, basal melt for all 1400 Swiss glaciers was estimated. The model operates with data sets on surface mass balance and glacier geometry, as well as with simplified considerations of the relevant processes. Our model considers energy advection through ice-marginal streams and subglacial air flow, potential energy release from melt water, friction-induced heat release, geothermal heat flux and dissipation of heat uptake by surface melt water. Field observations were used to constrain some of the parameters. Additionally, high-resolution aerial imagery and digital elevation models (DEMs) were used to perform a geostatistical analysis to better understand glacio-hydrological relationships and processes. Besides modelling glacier-wide basal melt, we analyzed the spatial and temporal dynamics of individual collapse features on a selected group of glaciers in the Swiss Alps, using high resolution DEMs. The model results indicate that the advection of energy through ice-marginal streams and potential-energy release from melt water are the primary contributors to basal melt for Swiss glaciers. The relevance of the modelled components importantly varies between glaciers and depends on glacier size and topography among other factors. At the Swiss-wide scale, total basal melt is modelled to be in the range of a few millimeters to several tens of centimeters water equivalent per year (total mass balance of Swiss glaciers is on average -1 meter water equivalent per year). These results suggest that for some glaciers, basal melt is both a relevant fraction of total mass balance, as well as large enough to be measured using high-resolution in situ GNSS observations. The analysis of glacier collapse features yielded an average life span of 3.5 years and volumes of non-surface ice loss ranging from a few thousand to more than 175000 m3. These findings, along with the model results, emphasize the substantial role of basal melt in both local retreat dynamics and total glacier-wide mass balance.