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Observations of glacier mass changes are key to understanding the response of glaciers to climate change and related impacts, such as regional runoff, ecosystem changes, and global sea level rise. Spaceborne optical and radar sensors make it possible to quantify glacier elevation changes, and thus multi-annual mass changes, on a regional and global scale. However, estimates from a growing number of studies show a wide range of results with differences often beyond uncertainty bounds. Here, we present the outcome of a community-based inter-comparison experiment using spaceborne optical stereo (ASTER) and synthetic aperture radar interferometry (TanDEM-X) data to estimate elevation changes for defined glaciers and target periods that pose different assessment challenges. Using provided or self-processed digital elevation models (DEMs) for five test sites, 12 research groups provided a total of 97 spaceborne elevation-change datasets using various processing approaches. Validation with airborne data showed that using an ensemble estimate is promising to reduce random errors from different instruments and processing methods but still requires a more comprehensive investigation and correction of systematic errors. We found that scene selection, DEM processing, and co-registration have the biggest impact on the results. Other processing steps, such as treating spatial data voids, differences in survey periods, or radar penetration, can still be important for individual cases. Future research should focus on testing different implementations of individual processing steps (e.g. co-registration) and addressing issues related to temporal corrections, radar penetration, glacier area changes, and density conversion. Finally, there is a clear need for our community to develop best practices, use open, reproducible software, and assess overall uncertainty to enhance inter-comparison and empower physical process insights across glacier elevation-change studies. [ABSTRACT FROM AUTHOR]

Avalanches are important contributors to the mass balance of glaciers located in mountain ranges with steep topographies. Avalanches result in localized over-accumulation that is seldom accounted for in glacier models due to the difficulty of quantifying this contribution, let alone the occurrence of avalanches in these remote regions. Here, we developed an approach to semi-automatically map avalanche deposits over long time periods and at scales of multiple glaciers, utilizing imagery from Sentinel-1 synthetic aperture radar (SAR). This approach performs particularly well for scenes acquired in winter and in the morning but can also be used to identify avalanche events throughout the year. We applied this method to map 16 302 avalanche deposits over a period of 5 years at a 6 to 12 d interval over the Mt Blanc massif (European Alps), the Everest (central Himalaya) region, and the Hispar (Karakoram) region. These three survey areas are all characterized by steep mountain slopes but also present contrasting climatic characteristics. Our results enable the identification of avalanche hotspots on these glaciers and allow us to quantify the avalanche activity and its spatio-temporal variability across the three regions. The avalanche deposits are preferentially located at lower elevations relative to the hypsometry of the glacierized catchments and are also constrained to a smaller elevation range at the Asian sites, where they have a limited influence on their extensive debris-covered tongues. Avalanche events coincide with solid precipitation events, which explains the high avalanche activity in winter in the Mt Blanc massif and during the monsoon in the Everest region. However, there is also a time lag of 1–2 months, visible especially in the Everest region, between the precipitation and avalanche events, indicative of some snow retention on the mountain headwalls. This study therefore provides critical insights into these mass redistribution processes and tools to account for their influence on glacier mass balance. [ABSTRACT FROM AUTHOR]

Observations of glacier mass changes are key to understanding the response of glaciers to climate change and related impacts, such as regional runoff, ecosystem changes, and global sea-level rise. Spaceborne optical and radar sensors make it possible to quantify glacier elevation changes, and thus multi-annual mass changes, on a regional and global scale. However, estimates from a growing number of studies show a wide range of results with differences often beyond uncertainty bounds. Here, we present the outcome of a community-based inter-comparison experiment using spaceborne optical stereo (ASTER) and synthetic aperture radar interferometry (TanDEM-X) data to estimate elevation changes for defined glaciers and target periods that pose different assessment challenges. Using provided or self-processed digital elevation models (DEMs) for five test sites, 12 research groups provided a total of 97 spaceborne elevation-change datasets using various processing strategies. Validation with airborne data showed that using an ensemble estimate is promising to reduce random errors from different instruments and processing methods, but still requires a more comprehensive investigation and correction of systematic errors. We found that scene selection, DEM processing, and co-registration have the biggest impact on the results. Other processing steps, such as treating spatial data voids, differences in survey periods, or radar penetration, can still be important for individual cases. Future research should focus on testing different implementations of individual processing steps (e.g. co-registration) and addressing issues related to temporal corrections, radar penetration, glacier area changes, and density conversion. Finally, there is a clear need for our community to develop best practices, use open, reproducible software, and assess overall uncertainty in order to enhance inter-comparison and empower physical process insights across glacier elevation-change studies. [ABSTRACT FROM AUTHOR]

Avalanches are important contributors to the mass balance of glaciers located in mountain ranges with steep topographies. They result in localised over-accumulation that is seldom accounted for in glacier models, due to the difficulty to quantify this contribution, let alone the occurrence of avalanches in these remote regions. Here, we developed an approach to semi-automatically map avalanche deposits over long time periods and at scales of multiple glaciers, utilising imagery from Sentinel-1 Synthetic Aperture Radar (SAR). This approach performs particularly well for scenes acquired in winter and in the morning, but can also be used to identify avalanche events throughout the year. We applied this method to map 16,302 avalanche deposits over a period of five years at a 6 to 12 days interval over the Mt Blanc massif (European Alps), the Everest (Central Himalaya) and Hispar (Karakoram) regions. These three survey areas are all characterised by steep mountain slopes, but also present contrasting climatic characteristics. Our results enable the identification of avalanche hotspots at the surface of these glaciers and allow us to quantify the avalanche activity and its spatio-temporal variability across the three regions. The avalanche deposits are preferentially located at lower elevations relative to the hypsometry of the glacierized catchments, and are also constrained to a smaller elevation range at the Asian sites, where they have a limited influence on their extensive debris-covered tongues. Avalanche events coincide with solid precipitation events, which explains the high avalanche activity in winter in the Mt Blanc massif and during the monsoon in the Everest region. However, there is also a time lag of 1–2 months, visible especially in the Everest region, between the precipitation and avalanche events, indicative of some snow retention on the mountain headwalls. Ultimately, this study provides critical insights into these mass redistribution processes as well as tools to account for their influence on glacier mass balance. [ABSTRACT FROM AUTHOR]

Ice flow along land-terminating margins of the Greenland Ice Sheet (GIS) varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet. Such inputs lubricate the ice-bed interface, transiently speeding up the flow of ice (1,2). Greater melting results in faster ice motion during summer, but slower motion over the subsequent winter, owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice-sheet bed that have a high basal water pressure (2,3). However, the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained. Here we show that annual ice motion across an 8,000-[km.sup.2] land-terminating region of the west GIS margin, extending to 1,100 m above sea level, was 12% slower in 2007-14 compared with 198594, despite a 50% increase in surface meltwater production. Our findings suggest that, over these three decades, hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion (not a speed-up, as previously postulated (1)). Increases in meltwater production from projected climate warming may therefore further reduce the motion of land-terminating margins of the GIS. Our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought., The GIS is losing mass at an accelerating rate (4,5), as a result both of increased surface melting (6) and of enhanced ice discharge from accelerating marine-terminating glaciers (5). Enhanced [...]

The South Col Glacier is an iconic small body of ice and snow (approx. 0.2 km²), located on the southern ridge of Mt. Everest. A recent study proposed that South Col Glacier is rapidly losing mass. This seems in contradiction with our comparison of two digital elevation models derived from aerial photographs taken in 1984 and a stereo Pléiades satellite acquisition from 2017, from which we measure a mean elevation change of 0.01 ± 0.07 m a-1. To reconcile these results we investigate wind erosion and surface energy and mass balance, and find that melt is unlikely a dominant process, contrary to previous findings. [ABSTRACT FROM AUTHOR]

Surges are an important source of glacier hazards and complete surge-type glacier inventories are required for assessing glacier-related hazards. Glacier surge events in High Mountain Asia (HMA) are widely reported. However, the completeness of present inventories of HMA surge-type glaciers is constrained by the insufficient spatial and temporal coverage of glacier change observations, or by the limitations of the identification methods. In this paper, we established a new inventory of HMA surge-type glaciers based on the glacier surface elevation changes over four decades. Four kinds of elevation sources (KH-9 DEM, NASADEM, COP30 DEM, HMA8m DEM) were utilized to estimate the glacier surface elevation changes during two periods (1970s-2000 and 2000-2010s). In total 1015 surge-type glaciers were identified in HMA. Compared to the latest surge-type glacier inventory in HMA, our inventory incorporated 477 new surge-type glaciers. The number and area of surge-type glaciers accounted for ~2.49 % (excluding glaciers less than 0.3 km2) and ~23.32 % of the total glacier number and glacier area in HMA, respectively. Considering that glacier outlines are usually composed of multiple tributaries within a glacier complex, the proportion of surge-related area may be overestimated, and the number of surge-type glaciers could be even larger. Surge-type glaciers were found in 21 of the 22 subregions of HMA (except for the Dzhungarsky Alatau), however, the density of surge-type glaciers is highly uneven. Surge-type glaciers are common in the northwest subregions (e.g., Pamir and Karakoram), but scarce in the peripheral subregions (e.g., Eastern Tien Shan, Eastern Himalaya, and Hengduan Shan). The inventory indicates that surge activity is more likely to occur for larger and longer glaciers. Besides, we found a potential relationship between the frequency of surge activities andregional glacier mass balance. The subregions with slightly negative or positive mass balance hold large clusters of surge-type glaciers, while those with severe glacier mass loss hold very few surge-type glaciers. In some subregions where glacier mass loss accelerated, the frequency of surge activities that occurred before 2000 was much higher than that after 2000. [ABSTRACT FROM AUTHOR]