Satellite remote sensing of red algal blooms in the snow of the European Alps

In the European Alps, snow sometimes takes a blood-like color in late spring due to the presence of snow algal blooms. These blooms decrease snow albedo, accelerating snowmelt and potentially feeding back on snow and glacier decline caused by climate change. In the Alps, so far, only sparse information exists regarding the frequency and location of these blooms. We developed a methodology to identify red snow algal blooms in the European Alps on Sentinel-2 image that enabled to separate red blooms from similarly colored snow due to Saharan dust depositions that occurs frequently in the Alps. The methodology was evaluated using 4600 webcam images. We applied the methodology to 5 years of Sentinel-2 images to generate an atlas of snow algal blooms in the Alps.

The atlas was combined to detailed simulations of the snow and meteorological conditions to identify the drivers of the blooms in the Alps as well as to quantify the maximum contributions of red algal blooms to snow melt. Based on this analysis and on projections on the future snow and meteorological conditions under different emission scenarios, we finally conclude that the occurrences of red snow algal blooms in the European Alps by the end of the century will either stay stable or slightly decrease.