Modelling the fluctuations of two small alpine glaciers (Glacier de Tsanfleuron and Glacier du Sex Rouge, western Swiss Alps) throughout the Holocene (11.5 ka – 2100 CE)

Due to ongoing glacier shrinkage caused by anthropogenic climate change, Col de Tsanfleuron (2803 m a.s.l.) in the western Swiss Alps, a former ice pass separating two neighbouring small mountain glaciers (Glacier de Tsanfleuron and Glacier du Sex Rouge), became ice-free in September 2022. The question arose as to whether or not the pass had – until 2022 – always been ice-covered throughout the Holocene. Both bedrock lithology of the now ice-free pass and anthropogenic disturbance likely impedes the application of surface exposure dating to answer this question. Therefore, the glaciers’ evolution was modelled from 11.5 ka until 2100 CE using the Instructed Glacier Model (IGM). Model calibration was carried out based on available glacier mass balance data. Due to the different quality and spatiotemporal resolution of available climate data, two different model runs were performed for the past (one from 11.5 ka to 2000 CE, and a second one from the Little Ice Age maximum (1850) to 2020). In addition, the future evolution of both glaciers was modelled until 2100 using an ensemble of 10 different GCM-RCM model chains for three different RCP scenarios. The three model runs were initialised with reconstructed or observational data for glacier extent, surface elevation and ice thickness. Validation of the modelling results was performed based on known evidence of Holocene glacier fluctuations in the Alps as well as using available 1850-present glacier area and volume data.

Even though the modelled fluctuations of Glacier de Tsanfleuron and Glacier du Sex Rouge show high temporal coherence with known advance and retreat phases for other alpine glaciers, modelled glacier extents and volumes are larger for the entire Holocene compared to in-situ measurements in 2019. According to preliminary modelling results, Col de Tsanfleuron was likely ice-covered throughout the Holocene until 2022. These results partly contrast with other studies, suggesting for instance that in the Alps various summits at higher altitude had been ice-free during the Holocene Thermal Maximum (~10.2 to ~4.2 ka). On the other hand, there is evidence that individual small alpine glaciers persisted during the entire Holocene. Our modelling results are subject to various uncertainties, e.g. related to the initial glacier area, surface elevation and volume, related to the climate data sets used, or related to the melt parameters applied and the modelling approach itself. For the period 1850-2020, our model is able to realistically trace the glaciers’ evolution at high spatiotemporal scale. Modelling results for the future predict the ultimate disappearance of both glaciers. Glacier du Sex Rouge will have completely vanished by around 2040, whereas, depending on the modelled climate scenario, the latest remnants of Glacier de Tsanfleuron will disappear between 2060 and 2080.