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In the Karakoram, dozens of glacier surges occurred in the past 2 decades, making the region a global hotspot. Detailed analyses of dense time series from optical and radar satellite images revealed a wide range of surge behaviour in this region: from slow advances longer than a decade at low flow velocities to short, pulse-like advances over 1 or 2 years with high velocities. In this study, we present an analysis of three currently surging glaciers in the central Karakoram: North and South Chongtar Glaciers and an unnamed glacier referred to as NN9. All three glaciers flow towards the same small region but differ strongly in surge behaviour. A full suite of satellites (e.g. Landsat, Sentinel-1 and 2, Planet, TerraSAR-X, ICESat-2) and digital elevation models (DEMs) from different sources (e.g. Shuttle Radar Topography Mission, SRTM; Satellite Pour l'Observation de la Terre, SPOT; High Mountain Asia DEM, HMA DEM) are used to (a) obtain comprehensive information about the evolution of the surges from 2000 to 2021 and (b) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring surges of relatively small glaciers in steep terrain. A strongly contrasting evolution of advance rates and flow velocities is found, though the elevation change pattern is more similar. For example, South Chongtar Glacier had short-lived advance rates above 10 km yr -1 , velocities up to 30 m d -1 , and surface elevations increasing by 170 m. In contrast, the neighbouring and 3-times-smaller North Chongtar Glacier had a slow and near-linear increase in advance rates (up to 500 m yr -1), flow velocities below 1 m d -1 and elevation increases up to 100 m. The even smaller glacier NN9 changed from a slow advance to a full surge within a year, reaching advance rates higher than 1 km yr -1. It seems that, despite a similar climatic setting, different surge mechanisms are at play, and a transition from one mechanism to another can occur during a single surge. The sensor inter-comparison revealed a high agreement across sensors for deriving flow velocities, but limitations are found on small and narrow glaciers in steep terrain, in particular for Sentinel-1. All investigated DEMs have the required accuracy to clearly show the volume changes during the surges, and elevations from ICESat-2 ATL03 data fit neatly to the other DEMs. We conclude that the available satellite data allow for a comprehensive observation of glacier surges from space when combining different sensors to determine the temporal evolution of length, elevation and velocity changes. [ABSTRACT FROM AUTHOR]

Whereas the termini of Hektoria and Green glaciers extend into a wide bay, the tongues of the other studied glaciers are narrower and laterally confined by mountain ridges. GLL - Location of the grounding line; CST - glacier front in 2011, 2013, 2016. c and d: Sections of Sentinel-1 image of 2017-04-01 showing the location of the velocity transects on Hektoria Glacier (c) and Crane Glacier (d). The Larsen outlet glaciers have cold snow/ice bodies in which a substantial portion of the meltwater released at the surface would freeze, in particular after cold periods[7]. [Extracted from the article]

We analysed volume change and mass balance of outlet glaciers on the northern Antarctic Peninsula over the periods 2011 to 2013 and 2013 to 2016, using highresolution topographic data from the bistatic interferometric radar satellite mission TanDEM-X. Complementary to the geodetic method that applies DEM differencing, we computed the net mass balance of the main outlet glaciers using the mass budget method, accounting for the difference between the surface mass balance (SMB) and the discharge of ice into an ocean or ice shelf. The SMB values are based on output of the regional climate model RACMO version 2.3p2. To study glacier flow and retrieve ice discharge we generated time series of ice velocity from data from different satellite radar sensors, with radar images of the satellites TerraSAR-X and TanDEM-X as the main source. The study area comprises tributaries to the Larsen A, Larsen Inlet and Prince Gustav Channel embayments (region A), the glaciers calving into the Larsen B embayment (region B) and the glaciers draining into the remnant part of the Larsen B ice shelf in Scar Inlet (region C). The glaciers of region A, where the buttressing ice shelf disintegrated in 1995, and of region B (ice shelf break-up in 2002) show continuing losses in ice mass, with significant reduction of losses after 2013. The mass balance numbers for the grounded glacier area of region A are -3.98±0.33 Gt a-1 from 2011 to 2013 and -2.38±0.18 Gt a-1 from 2013 to 2016. The corresponding numbers for region B are -5.75±0.45 and -2.32±0.25 Gt a-1. The mass balance in region C during the two periods was slightly negative, at -0.54±0.38 Gt a-1 and -0.58±0.25 Gt a-1. The main share in the overall mass losses of the region was contributed by two glaciers: Drygalski Glacier contributing 61% to the mass deficit of region A, and Hektoria and Green glaciers accounting for 67% to the mass deficit of region B. Hektoria and Green glaciers accelerated significantly in 2010-2011, triggering elevation losses up to 19.5ma-1 on the lower terminus during the period 2011 to 2013 and resulting in a mass balance of -3.88 Gt a-1. Slowdown of calving velocities and reduced calving fluxes in 2013 to 2016 coincided with years in which ice mélange and sea ice cover persisted in proglacial fjords and bays during summer. [ABSTRACT FROM AUTHOR]

The Greenland and Antarctic ice sheets are losing ice mass at an accelerating rate in response to atmospheric, oceanic and internal dynamic forcing. Detailed knowledge on ice flow dynamics and processes leading to the depletion of glacier mass is essential for future scenarios on contributions of polar ice masses to sea level rise. Satellite Earth observation data from Copernicus Sentinel-1, Sentinel-2 and TerraSAR-X/TanDEM-X play a key role hereby in providing spatially and temporally detailed climate data records on ice flow and surface elevation change. A recent and major advancement, emerging from the ESA CCI and Austrian ASAP/FFG programs, is the development and implementation of an automatic system for generation of ice velocity maps from repeat pass Sentinel-1 SAR data. In combination with a dedicated polar acquisition strategy, this marked a transition from traditional campaign-style observation to true operational monitoring of ice flow in polar regions.We generated time series of ice velocity from weekly repeat pass Sentinel-1 Interferometric Wide Swath and TerraSAR-X/TanDEM-X Stripmap data by applying customized iterative offset tracking tools. The ice velocity maps, in combination with high-resolution digital elevation models, derived from the bistatic interferometric radar mission TanDEM-X, and available ice thickness from radio echo sounding, form the basis for studying glacier dynamics, ice discharge and mass balance of Greenland and Antarctic outlet glaciers.Including the latest observational data, we present detailed ice sheet wide velocity maps of the Greenland and Antarctic mapping campaigns. The continuous 6 to 12-day repeat acquisitions of the ice sheet margins by Sentinel-1 are used to produce monthly-averaged ice velocity maps that, supplemented with ice thickness data, allow for calculating sub-annual discharge variations in order to distinguish short-term fluctuations from trends. Time series of ice velocity are combined with volumetric changes calculated from DEM differencing to analyse the evolution of outlet glaciers along the Weddell Coast of the Antarctic Peninsula. Our results reveal a complex and versatile pattern of mass balance and ice flow.This presentation highlights some of the capabilities of the satellite missions and emphasizes the importance of long-term comprehensive monitoring of polar ice masses, which is essential for understanding and predicting their response to climatic changes. [ABSTRACT FROM AUTHOR]

Satellite data provide a large range of information on glacier dynamics and changes. Results are often reported, provided and used without consideration of measurement accuracy (difference to a true value) and precision (variability of independent assessments). Whereas accuracy might be difficult to determine due to the limited availability of appropriate reference data and the complimentary nature of satellite measurements, precision can be obtained from a large range of measures with a variable effort for determination. This study provides a systematic overview on the factors influencing accuracy and precision of glacier area, elevation change (from altimetry and DEM differencing), and velocity products derived from satellite data, along with measures for calculating them. A tiered list of recommendations is provided (sorted for effort from Level 0 to 3) as a guide for analysts to apply what is possible given the datasets used and available to them. The more simple measures to describe product quality (Levels 0 and 1) can often easily be applied and should thus always be reported. Medium efforts (Level 2) require additional work but provide a more realistic assessment of product precision. Real accuracy assessment (Level 3) requires independent and coincidently acquired reference data with high accuracy. However, these are rarely available and their transformation into an unbiased source of information is challenging. This overview is based on the experiences and lessons learned in the ESA project Glaciers_cci rather than a review of the literature. [ABSTRACT FROM AUTHOR]