Pan-Alpine glacier phenology reveals lowering albedo and increase in ablation season length

Glaciers in many parts of the world are reported to be undergoing a darkening process (decrease in surface albedo). This has multiple causes, including the presence of impurities (such as mineral dust, carbonaceous particles, algae and cryoconite), and debris. Also glacier melt can induce an albedo decrease by initiating a melt-albedo feedback cycle. The extent of this albedo decrease and its spatial variability in different mountain ranges however are poorly known. In this study, we assess albedo changes on a selection of Alpine glaciers (542 glaciers > 0.1 km(2)) by deriving eight albedo metrics from MODIS MOD10A1 satellite observations over a time period of 20 years (2000-2019). Albedo metrics include the start and end of the ablation season and its length, calculated through the selection of a threshold (albedo = 0.4) to separate melting ice from snow. We further calculated the number of ice melt days, integral of the length of the ablation season, minimum annual albedo, mean summer albedo and mean annual albedo. We identified significant trends in all metrics for a large percentage of glacier cells. A negative trend in minimum albedo (-0.06 decade(-1) on average) was identified for 56% of glacier cells, and 70% of glaciers; a positive trend in the length of season (+18 days decade(-1) on average) was identified for 11% of glacier cells and 19% of glaciers. Changes in minimum albedo occur mostly around the glacier median elevation, while for other metrics (e.g. start and length of season) changes are more frequent at the glacier termini (i.e. below 25th elevation percentile). We further investigated the relationship between the metrics and climate (downscaled ERA5 reanalysis data) and topographic (ALOS AW3D30 DEM) variability; we found that average summer temperature and elevation are the most significant proxies for all metrics (maximum correlation = 0.40 between elevation and minimum albedo) compared to spring temperature and other topographic variables. Finally, we looked at the ability of the albedo metrics to represent annual and summertime glacier mass balance of 31 selected Alpine glaciers in the World Glacier Monitoring Service database, finding significant correlations between minimum and summertime albedo and annual mass balance (R-2 = 0.44 for minimum albedo and 0.40 for summertime albedo using all observations). Considering individual glaciers, all showed significant correlations with minimum albedo, and some of them are strongly correlated (e.g. Argentiere: R-2 = 0.84, Saint Sorlin R-2 = 0.88). Our glacier phenology approach opens new perspectives for the remote sensing of alpine and polar glaciers.