Abstract
Mountain areas have raised a lot of attention in the past years, as they are considered sentinel of climate changes. Cryosphere is one of the most affected domains. Up to now, several studies have investigated snow cover changes at continental scale and there are several indications of snow cover decline over the Northern Hemisphere (Chen et al., 2015; Bormann et al., 2018), despite no study has analyzed snow behavior specifically in mountain areas at global level. In this context, this study investigates the changes in the main snow cover parameters (snow cover area, snow cover duration, snow onset, snow melt and snow line altitude) over global mountain areas from 2000 to 2018 (Notarnicola, 2020). In order to proper monitor the evolution of snow changes at global mountain areas and interlinkages with meteorological drivers (snowfall, and temperature), automatic procedures were developed based on MODIS imagery in global mountain areas over the period 2000-2018 by exploiting Google Earth Engine where the whole time series of MODIS is available at a global scale. MODIS snow cover products have the highest resolution available, 500 m, and with daily global acquisitions. Only recently with almost 20 years of data, these imageries have become useful to start assessing tends in snow cover and snow phenology. The results of the trend analysis carried with Man-Kendall statistics indicate that around 78% of the global mountain areas are undergoing a snow decline. More in details, snow cover duration has decreased up to 43 days, and a snow cover area up to 13%. Few areas show positive changes with snow cover duration increase up to 32 days, and snow cover area increase up to 11%. This latter behavior can be found during wintertime and in areas mainly located in Northern Hemisphere. Significant snow cover duration changes can be linked in 58% of the areas to both delayed snow onset, and advanced melt. Furthermore, snowmelt advance is going faster than snow onset delay in the analyzed time period. While snow cover, and the snow phenology parameters are quite variable at mid-elevations, from 4000 m upward only negative changes are detected. In the correlation with meteorological parameters, air temperature is found as the main driver for snow onset and melt, while a combined effect of air temperature and precipitation dominates the winter season.