The importance of Glaciers in High Mountain Asia as fresh water resource is indisputable. During the dry season, melt water runoff from glaciers contributes strongly to stream discharge and supports irrigation, hydro power and drinking water supply for several hundreds of million people. The future of this resource needs to be modeled with most precise constraints possible. Therefore, a continuous region-wide mountain glacier monitoring is in great demand, whereas remote sensing is in a leading role to meet it. In this study, we calculate glacier specific mass loss rates and the possible sea level rise contribution using the geodetic mass balance method based on spaceborne SAR observations over entire High Mountain Asia (Himalaya, Karakoram, Hindu Kush, Kunlun, Tien Shan, Pamir, Alai and the complete Tibetan Plateau).
The ongoing TanDEM-X mission, a bi-static SAR interferometer acquiring datasets flying in a helical orbit formation with controlled baseline, builds the foundation for this analysis. The mission goal to acquire a comprehensive coverage of the earths surface between 2011-2013 for a global digital elevation model (DEM) led to an advantageous data collection for the study. This archive of SAR imagery enables us to generate high resolution DEMs by an InSAR approach for all of the study area at 30 m ground resolution. We differentiate this elevation information derived from over 1600 TanDEM-X imagery pairs to the Shuttle Radar Topography Mission (SRTM) non-void-filled C-band DEM (V3) from February 2000. For this purpose we compiled a processing routine that generates highly accurate, vertically and horizontally adjusted DEMs and elevation change rate mosaic datasets of several raw image pairs. These products enable to tackle monitoring tasks on various scales: from individual glacier observations to regional trends analysis. But they also form a consistent baseline for future multi-temporal analysis with ongoing TanDEM-X observations. The preliminary results indicate an overall mass loss rate of 13.46 Gt a-¹ on a glacier area of 80,000km² with varying patterns across the study area. The largest diminution is observed on valley glaciers in the mountain ranges seaming the Tibetan Plateau to the south. For continuous accomplishment of this important monitoring task in changing climatic conditions, we highly recommend regular acquisitions of TanDEM-X imagery over glacierized regions in order to improve and consistently expand the temporal coverage for geodetic glacier mass balances for High Mountain Asia and other glaciated regions around the world.

Unlike most glaciers experience mass lost in recent decades, Karakoram and its surroundings seem rather stable. The phenomenon is claimed as ‘Karakoram anomaly’ and/or ‘Karakoram-Pamir anomaly’. Given its remote, the field observations for glacier mass balance and flow rates are hard and archive data are rare. Remote sensing technique plays an essential role in alpine glaciers observations. Previous studies yielded a positive glacier mass balance for the Karakoram and later extended to the Pamir Plateau between 2000 and ~2012, however the laser altimetry observation at almost the same period (2003 - 2009) found slight glacier mass loss but claimed that the anomaly centred at the West Kunlun.

To make a more accurate observation and to understand the ‘Karakoram anomaly’, we applied Differential SAR Interferometry (D-InSAR) technique to a set of X-band bistatic TerraSAR-SAR-X/TanDEM-X (TSX/TDX) images observed at ~2013 by respecting to SRTM DEM observed in 2000 to derive glacier mass balance. The topographic residual phase of D-InSAR is unwrapped and then transferred to height changes. By presuming density of 850 Kg/m3, the volume changes of glaciers are converted to glacier mass balance. We compare quasi-simultaneously observed C-band and X-band SRTM (both in February of 2000) to evaluate and to remove the penetration depth differences at different elevation bins. The possible seasonal variation in terms of glacier mass balance was evaluated at an adjacent site by using TSX/TDX images observed in different months in one year. The standard deviation of differential processing between SRTM and TSX/TDX is about 6.27m, which is more accurate than the previous study using SPOT DEM and SRTM. Besides, it noticed that TSX/TDX make more efficient observations at accumulating area than optical observations. The result found that both east and west part of the Karakoram present almost zero glacier mass balances, which were −0.020 ± 0.064 m w.e. yr−1 and −0.101 ± 0.058 m w.e. yr−1, respectively. Most negative glacier mass balance was contributed by southern slope of the Karakoram while the northern slope was rather stable. At the most northeastern part of the Karakoram, where are very close to the edge of Tarim basin, the glaciers presented thickening in also most every elevation levels. The glacier mass balance at the Karakoram presented a decreasing gradient from the edge of Tarim basin to the southwest of Karakoram. The mass balance for 2000 to ~2013 is almost identical comparing to 1974 to 2000, which implies that the stable environment at the Karakoram despite the global warming trend.

Flow rates can also suggest the status of glaciers. We obtained the long time supported Landsat 5, 7 SLC-on/SLC-off, 8 images observed from 1990 to 2015 and applied them to feature tracking technique to derive annual glacier flow rates variation for the Karakoram area. The Landsat 7 SLC-off images are fixed by using temporal adjacent obtained SLC-off images. Normalized cross-correlations algorithm is employed for feature tracking. Surging glaciers are common in the Karakoram, especially at its northern slope. After manually removing those surging glaciers identified by joint referring to glacier height change maps and temporal variation of glacier flow rates, we derived average annual glacier flow rates for the Karakoram. The time series can be separated into three stages, the first stage was from 1990 to 2000, the second was from 2000 to 2010 and the last stage was from 2010 to 2015. The first and third stage presented flow rate increases when the second stage was rather stable, which totally suggested flow rate increasing during 1990 and 2015. This implied more glacier mass has been transported to the lower area despite the stable glacier mass balance, which means more accumulation and also more melting. The re-analysis monthly GHCN_CAMS Gridded 2m Temperature data found an increasing start from 1995 to 2000 for about 1 degree and kept stable after then. Monthly precipitation Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) suggest rather stable annual and seasonal precipitation from 1980 to 2000, followed by an increasing trend. The precipitation increased from ~260mm/yr before 2000 to ~350mm/yr by 2010. Both glacier flow rate and meteorological data suggested that a warming and wetting trend after 2000 for the Karakoram, which possibly explains the ‘Karakoram anomaly’.

The Baltoro Glacier is one of the largest glaciers in the Karakoram mountain range. Long-term monitoring of glacier dynamics provides key information on glacier evolution in a changing climate, which is essential for regional water resource and natural hazard management. On large glaciers, detailed field based mass balance is not feasible. Ice dynamic variations quantify changes in mass transport and possibly the influence of environmental parameters on the evolution of the glacier. Although velocity variations of Baltoro Glacier during winter and summer are linked to seasonally enhanced basal sliding, little is known about differences in timing and magnitude of (intra-)seasonal velocity variations and their determining mechanisms. We present time series of annual, seasonal, and intra-seasonal glacier surface velocities by means of intensity offset tracking applied on multimission Synthetic Aperture Radar (SAR) data acquired by ERS-1/2, Envisat ASAR, ALOS PALSAR-1, TerraSAR-X, TanDEM-X, and Sentinel-1 with a repeat pass interval as short as 11 days, in order
analyze (intra-)seasonal and annual surface velocities of Baltoro Glacier from 1992 to 2017. Supraglacial lakes forming on the downstream glacier surface in summer were mapped from 1991 to 2017 based on the Normalized Difference Water Index (NDWI), calculated from multi-spectral Landsat and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) imagery. Additionally, precipitation data of the Tropical Rainfall Measurement Mission (TRMM) and temperature data of ERA-Interim were used to derive the Standardized Precipitation Index (SPI) and Standardized Temperature Index (STI) from 1998 to 2017. Linking surface velocities to the SPI confirmed a strong correlation between heavy precipitation events in winter and the magnitude and the timing of glacier acceleration in summer. Downstream extensions of summer acceleration that have been found since 2015 may be explained by additional water draining from an increased number of supraglacial lakes through crevasses that have been formed in consequence of higher initial velocities, evoked by strong winter precipitation. The warmer melt seasons observed in the years 2015 to 2017 additionally affects the formation of a supraglacial lake, so stronger summer acceleration events in recent years may be indirectly related to global warming.

One of the most valuable parameters to evaluate the impact of climatic changes in a glacier is the total mass balance and its components, which represents the sum of all processes that add or remove ice mass. Several methods exist for inferring the mass balance of a glacier, but from a remote sensing perspective, the geodetic and input/output (IOM) techniques represent reliable, complementary and well-documented procedures. This work combines the geodetic and IOM methods to obtain not only the total mass balance but also ice discharge parameters and the surface mass balance (SMB).

The geodetic mass balance is a method based on measurements of the surface elevation dynamics of a glacier determined during a time epoch. The elevation change rates are obtained subtracting two digital elevation models; by integrating this result over the glacier area, the volume change rate is obtained. The input/output method consists of calculating the surface mass balance imposed in a gate (or more) and compare it to the discharged flux coming out of that gate. The ice flux is calculated by means of the ice thickness and the correspondent velocity. Then, an estimation of the ice discharged at the front (calving flux) is computed also using the glacier retreat.

As region of interest for this study we were looking for a glacier region which has a significant number of calving glaciers. On the South Patagonia Icefield (SPI) in the southern part of the American Andes, only 2 of the 48 main outlet glaciers are terminating on land, 16 in fjords of the Pacific Ocean, and 30 in freshwater lakes. On SPI there are still many open questions regarding the relationship between mass balance and flow dynamics, in particular regarding the partly quite contrasting behavior of individual glaciers. The present study is complementing our work on the total mass balance determination of the entire icefield in between 2000-2012 and 2012-2016, using the geodetic technique. The estimated volume loss of SPI decreased by 20% between these epochs. Moreover the DEM differencing revealed the heterogeneity of the spatial pattern of the surface elevation change of the icefield as well as the changes in the temporal trend of individual basins.

We are deriving time series of parameters needed to calculate ice export due to calving from SAR data: calving flux, calving rate, calving front on major outlet glaciers of SPI. The data used for this work was carefully selected to match as close as possible the second mass balance epoch, using TerraSAR-X and Sentinel-1 acquisitions close to 2011-2016. The first epoch could be not completely covered due to the unavailability of SAR sensors providing the high resolution data needed for terminus velocity calculations. However we monitored the calving rates since 2008 which support the interpretation of the mass balance for some individual major glaciers since 2000. We will give examples of decreasing rate of surface lowering like of Upsala Glacier associated with slowdown of the terminus velocity as well as of accelerating downwasting like on Jorge Montt Glacier. The comparison of the total mass change and mass loss due to calving can be used for validation of the surface mass balance values resulted from modelling.

Outlet glaciers of the northern Antarctic Peninsula have been subject to flow acceleration and retreat in response to enhanced regional warming during the last decades, causing significant increase ice discharge to the ocean and major loss of grounded ice mass. The patterns of flow acceleration, glacier thinning and changes in the position of calving fronts have been variable in time and space. On the east coast the main trigger for increased ice discharge and mass losses was the disintegration of the ice shelves in Prince Gustav Channel and the Larsen A embayment in January 1995 and the break-up of the northern and central sections of the Larsen B ice shelf in March 2002. The glaciers on the west coast have been calving directly into the ocean since the start of observations, but also for these glaciers the fluctuations in mass balance are not directly linked to atmospheric forcing because ice dynamics and oceanic forcing play a main role.

In order to reveal the role of ice flow dynamics on ice export and to estimate the current contributions to sea level rise, we analysed volume change and mass balance of the outlet glaciers on the northern Antarctic Peninsula between 63.5° S and 67° S over two epochs, 2011 to 2013 and 2013 to 2016, and generated time series of ice flow velocities. We derived high resolution surface elevation models from data of the bistatic interferometric radar mission TanDEM-X and applied DEM differencing to map surface elevation change (SEC) and mass balance. The comparison of the TanDEM-X SEC data with airborne lidar data, acquired by NASA IceBridge over the epoch 2011 to 2015 along central flowlines of several glaciers, shows excellent agreement, confirming the high quality of both data sets. We generated time series of glacier velocities over the study region from SAR data of the TerraSAR-X, TanDEM-X and Sentinel-1 missions and derived calving fluxes for the main outlet glaciers. High temporal repeat sequence data on velocity are available since 2014, at first based on Sentinel-1A with 12-day repeat observation period, reduced to 6 days after the launch of Sentinel-1B in March 2016.

The data show a complex and versatile pattern of surface elevation change and ice flow over the study area. The glaciers on the east coast north of Sjögren-Boydell inlet and those on the west coast show only small velocity fluctuations during the study period and no significant deviations from balanced mass budget. This is in concordance with the regional trend of a modest atmospheric cooling during the last decade. The main glaciers draining into the embayments of the former Larsen A and Larsen B ice shelves, on the other hand, show significant continuing losses in ice mass. The spatial pattern of drawdown and mass losses shows large differences between individual glaciers, traceable back to differences in dynamic response shortly after disintegration of the buttressing ice shelf, as well as to the different disposition regarding the alignment to the changed boundary conditions afterwards. Still, these glaciers show a common temporal trend when comparing the data of the two study epochs. Calving velocities slowed down and calving fluxes decreased during the period 2013 to 2016 compared to the years before, aided by a dense and persistent cover of ice melange and sea ice in the pro-glacial fjords year-round.

In its 8 years in space, ESA’s CryoSat-2 SIRAL radar altimeter has facilitated improved mapping of the rate of thickness change of polar glaciers and sea ice. Over glaciers, ice caps, and the margins of the Greenland and Antarctic ice sheets, the instrument is operated in SARIn mode – whereby the Doppler shift of the return signal is used to divide the footprint into along-track slices, and the phase shift between the two receiving antennas used to calculate the off-nadir angle of the point of closest approach (POCA). Where the across-track slope of the sub-satellite topography lies within an appropriate range, additional ‘swath’ points can also be retrieved. The increased data density this affords, and the ability to accurately geolocate the elevation estimates, represent a considerable step forward compared to previous generations of radar altimeters aboard satellites such as ENVISAT and ERS-1/2. However, Ku-band signals are known to penetrate into cold snowpacks, resulting in a ‘radar elevation’ that is below the physical surface of a glacier. It is important to quantify this bias and assess the impact it has on further processing.

We compare elevation estimates from swath processing of roll angle corrected CryoSat-2 SIRAL SARIn-mode L1b BaselineC radar altimetry with snowscooter-mounted GNSS and airborne laser altimetry transects across the Austfonna ice cap, Svalbard, collected as part of CryoVEx calibration and validation campaigns. POCA and swath elevation estimates are compared to GNSS and Laser elevations with small spatiotemporal offsets (< 500m, < 1 month), using an external DEM to account for intervening slopes. We find that POCA points are a good approximation for the physical surface (negligible bias), whereas swath points have a penetration bias of around one metre. The GNSS and Laser transects were collected at the end of the accumulation season and shortly before the onset of melt, and hence approximate the elevation of the ice cap at the onset of summer melt. Comparing swath points from before and after the onset of melt suggests the penetration bias is significantly reduced when meltwater in the upper snowpack masks out subsurface backscatter. Given the differing bias between POCA and swath points, we investigate different processing strategies, and conclude that widely used methods appear robust, although similar comparisons from other areas would be beneficial. Finally, we present mass balance estimates for the Svalbard archipelago, demonstrating a considerable increase in mass loss since the ICESat-1 period. There is a general pattern of slight elevation gain in the interior of icefields and ice caps, and thinning at the margins, punctuated by a number of surges. The greatest mass losses originate from the south and west, as well as the surge of Storisstraumen (Basin-3), Austfonna.