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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 km2) 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 (R2 = 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: R2 = 0.84, Saint Sorlin R2 = 0.88). Our glacier phenology approach opens new perspectives for the remote sensing of alpine and polar glaciers. • The concept of glacier phenology is here introduced and applied to the European Alps. • Albedo-derived metrics were extracted from 20 years of MODIS satellite data. • 56% of glacier cells and 70% of glaciers show negative minimum albedo trends. • Summer glacier albedo significantly correlates with the mass balance of all glaciers. • Ablation season length increased (19% of glaciers) by 18 days/decade on average. [ABSTRACT FROM AUTHOR]

Several anthropogenic activities have undergone major changes following the spread of the COVID-19 pandemic, which in turn has had consequences on the environment. The effect on air pollution has been studied in detail in the literature, although some pollutants, such as ammonia (NH 3), have received comparatively less attention to date. Focusing on the case of Lombardy in Northern Italy, this study aimed to evaluate changes in NH 3 atmospheric concentration on a temporal scale (the years from 2013 to 2019 compared to 2020) and on a spatial scale (countryside, city, and mountain areas). For this purpose, ground-based (from public air quality control units scattered throughout the region) and satellite observations (from IASI sensors on board MetOp-A and MetOp-B) were collected and analyzed. For ground-based measurements, a marked spatial variability is observed between the different areas while, as regards the comparison between periods, statistically significant differences were observed only for the countryside areas (+31% in 2020 compared to previous years). The satellite data show similar patterns but do not present statistically significant differences neither between different areas, nor between the two periods. In general, there have been no reduction effects of atmospheric NH 3 as a consequence of COVID-19. This calls into question the role of the agricultural sector, which is known to be the largest responsible for NH 3 emissions. Even if the direct comparison between the two datasets shows little correlation, their contextual consideration allows making more robust considerations regarding air pollutants. [Display omitted] • We used ground and satellite data to estimate NH3 concentrations in Lombardy. • We analyzed the period before, during and after the COVID-19 lockdown. • We divided both datasets in three zones: city, countryside and mountain. • NH3 concentrations increased by 31% in countryside areas in 2020. • No difference was found between pre- and post-lockdown NH3 from satellite data. [ABSTRACT FROM AUTHOR]