Glaciers are a major contributor to current sea level change and are projected to remain so until the end of the 21st century. Global monitoring of glaciers remains a challenging task since global estimates rely on a variety of observations and models to achieve the required spatial and temporal coverage, and significant differences remain between current estimates. Glaciers are losing ice in response to atmospheric and oceanic warming via increase in surface run-off and ice discharge. The relative contribution of increased run-off and discharge is still poorly known, an information that is key for process understanding and to constrain projections of glacier loss into the future.
We generate for the first time a high spatial and temporal record of ice loss across glaciers globally from CryoSat-2 swath interferometric radar altimetry, demonstrating that radar altimetry can now be used alongside GRACE and DEM differencing for global glacier mass balance assessments. We show that between 2010 and 2020, glaciers lost a total of 277 ± 10 Gt yr−1 of ice, equivalent to a loss of 2.1% of their total volume during the 10-year study period. All years observed experienced ice loss, however there is considerable variation in the rates of loss from year to year. Between 2010 and 2020, glaciers have contributed 0.76 ± 0.03 mm yr−1 to SLR, equivalent to the loss of both ice sheets combined over the same period, and equivalent to about 25% of global sea-level budget.
Using a simple parameterization, we demonstrate that during this period, surface mass balance dominated the mass budget. We find that locally, in regions where the ocean is known to undergo rapid changes, dynamic imbalance plays a significant role. Our findings imply that it is key for models projecting future glacier response to climatic changes to represent the dynamic response of glacier to atmospheric and oceanic forcing.

Retreating and thinning glaciers are icons of climate change and impact the local hazard situation, regional runoff as well as global sea level. For past reports of the Intergovernmental Panel on Climate Change (IPCC), regional glacier change assessments were challenged by the small number and heterogeneous spatio-temporal distribution of in situ measurement series and uncertain representativeness for the respective mountain range as well as by spatial limitations of current satellite altimetry (only point data) and gravimetry (coarse resolution). Towards IPCC SROCC and AR6, there have been considerable improvements with respect to available geodetic datasets. Geodetic volume change assessments for entire mountain ranges have become possible thanks to recently available and comparably accurate digital elevation models (e.g., from ASTER or TanDEM-X). At the same time, new spaceborne altimetry (CryoSat-2, IceSat-2) and gravimetry (GRACE-FO) missions are in orbit and about to release data products to the science community. This opens new opportunities for regional evaluations of results from different methods as well as for truly global assessments of glacier mass changes and related contributions to sea-level rise. At the same time, the glacier research and monitoring community is facing new challenges related to data size, formats, and availability as well as new questions with regard to best practices for data processing chains and for related uncertainty assessments.

In this presentation, we introduce the working group on Regional Assessments of Glacier Mass Change (RAGMAC) of the International Association of Cryospheric Sciences. RAGMAC was established to tackle these challenges in a community effort. We will present our approach to develop a common framework for regional-scale glacier mass-change estimates towards a new consensus estimate of regional and global mass changes from glaciological, geodetic, altimetric, and gravimetric methods.

In the framework of the ESA CCI programme we developed an automatic system for ice sheet velocity and discharge monitoring. Applying customized iterative offset tracking tools we generate surface velocity maps from repeat pass Sentinel-1 Interferometric Wide Swath (IWS) data. These data, based on the Terrain Observation by Progressive Scans (TOPS) technology, with a spatial resolution of 4 m by 22 m in slant range and azimuth, respectively, and a swath width of 250 km, are well suited for comprehensive velocity retrievals over large ice bodies. Since 2019 additional Sentinel tracks were added to the regular acquisition scheme, covering the slow-moving interior of the Greenland Ice Sheet by crossing ascending and descending acquisitions. This offers the opportunity for regular application of the InSAR technique for improving ice velocity products particular in slow moving sections of ice sheets. A major challenge for TOPS interferometry is the correction of phase jumps at burst boundaries affecting the displacement in along-track direction and phase unwrapping of long data tracks of several 100 km lengths. We developed and implemented an InSAR processing line for generation of ice velocity maps from crossing orbits of Sentinel-1 IW TOPS data, which are not affected by ionospheric strikes which are evident especially in slow moving areas in corresponding offset tracking ice velocity products. On the tongues of major outlet glaciers, the high velocity causes decorrelation of the interferomeric phase signal. Therefore, the ice velocity product from InSAR is combined with offset tracking data to fill these data gaps and generate optimized ice sheet wide velocity maps. Combined with ice thickness, derived from airborne radio echo sounding, the velocity maps form the basis for studying glacier dynamics, calculating the ice discharge, and estimating mass balance.
We will present advanced combined InSAR and offset tracking ice velocity products for the Greenland ice sheet and for selected key ice streams in Antarctica covered by S1 crossing orbits and will report on the performance of the product using in-situ GPS data as benchmark. Additionally, we show time series of velocity variations of major outlet glaciers of the ice sheets and other polar ice bodies and their evolution over time. Based on extended time series including velocity products from other missions (ERS, ALOS, TerraSAR-X), we show how velocity and ice discharge varies spatially and temporally over time scales ranging from days to years. The continuous repeat observation capability of Sentinel-1 with 6-day time intervals offers also excellent capabilities for mapping of grounding lines and monitoring their migration by means of SAR interferometry.
During winter 2020/21 the 7th ice sheet wide Greenland mapping campaign is planned for the first time with complete coverage of crossing orbits as needed for InSAR ice velocity retrievals. Furthermore, the monitoring of the Antarctic margins is going on. Sentinel-1 continues delivering essential information for comprehensive monitoring of polar ice masses, a prerequisite for understanding and predicting the response of the ice sheets and glaciers to climatic change.

Accurate measurement of seasonal snow mass, or Snow Water Equivalent (SWE), from space over the boreal region is still a challenging task. Repeat-pass Interferometric Synthetic Aperture Radar (InSAR) is a promising technique for retrieval of SWE changes over large areas. The repeat-pass InSAR retrieval technique is based on the relation between the interferometric phase and changes in the SWE (Leinss et al, 2015). However, this technique needs to overcome some limitations. Retrieval is constrained by the instrument wavelength, as SWE increases which would infer a phase shift greater than a fringe, will yield to ambiguities in the calculation. Moreover, high frequency bands are more susceptible to suffer from temporal decorrelation (Rott et al, 2003), which is a major cause of degradation in repeat-pass InSAR SWE retrieval. For these reasons, L-band emerges as a solid candidate for the task, since it accounts for a relatively long wavelength and good temporal properties.

SodSAR (Sodankylä SAR) is a tower-based 1-18 GHz fully polarimetric SAR system located in Sodankylä, Northern Finland (Jorge Ruiz et al. 2020). Since October 2019 several acquisitions have been made daily for later reconstruction of the SWE accumulation profiles for the winter season and analysis of the different temporal decorrelation sources. The results have been validated using in-situ ancillary data. Results indicate that SWE profiles can be reconstructing summing up the retrieved SWE changes from high coherence acquisitions. The study of temporal decorrelation shows that melting down events drastically lowers the coherence and that both wind and precipitation also cause decorrelation, since they change the snowpack properties.

Up to date, several L-band satellite SAR missions have been operating, such as SAOCOM or ALOS/ALOS2. Additionally, in the upcoming years, more L-band satellites missions will be launched, such as NISAR, ALOS-4 or Tandem-L. The future deployment of these SAR instruments will open new possibilities for repeat-pass InSAR SWE retrieval. However, satellites face several challenges such as atmospheric phase delay and temporal baselines of several days. ALOS2 imagery from 2019 to 2021 over Sodankylä area have been used to generate interferograms. In this analysis, SWE maps derived from SnowModel (Liston et al, 2006), both with and without assimilation of in-situ snow measurements, have been used. These data is used to validate the retrieval technique and analyze the interferometric products (both in terms of coherence and phase) behaviour for different land covers, in relation with the accumulated SWE.


S. Leinss, A. Wiesmann, J. Lemmetyinen and I. Hajnsek, "Snow Water Equivalent of Dry Snow Measured by Differential Interferometry," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 8, no. 8, pp. 3773-3790, Aug. 2015.


J. Jorge Ruiz, R. Vehmas, J. Lemmetyinen, J. Uusitalo, J. Lahtinen, K. Lehtinen, A. Kontu, K. Rautiainen, R. Tarvainen, J. Pulliainen, J.Praks, "SodSAR: A Tower-Based 1–10 GHz SAR System for Snow, Soil and Vegetation Studies". Sensors. 2020; 20(22):6702.


H. Rott, T. Nagler, R. Scheiber, "Snow mass retrieval by means of SAR interferometry", (2003).


G. E. Liston, K. Elder, "A Distributed Snow-Evolution Modeling System (SnowModel)", Journal of Hydrometeorology, 7(6), 1259-1276, (2006).

Glaciers are sensitive and reactive to climate change. There is evidence of rapid glacier recession around the world and therefore the interest in the monitoring of glaciers has grown. The loss of glacier mass has significant implications on global sea level, water resources and hydropower potential in various regions which has significantly increased the importance of knowledge of glacier volume and spatial distribution for quantifying its contribution to sea level rise and projections of future glacial runoff. Remote sensing is a rapidly expanding and evolving technique of monitoring and assessing the changes in glacier dynamics. Various methods have been developed and refined for this purpose.
This study focusses on the changes in Caucasus mountains. They stretch between the Black Sea and the Caspian Sea with an elevation reaching up to 5600 m asl. The glaciers cover an area of about 1200 sq. km. as per the Randolph Glacier Inventory. The west has a semiarid climate, and the east is characterised with desert like conditions. The northern slopes are colder than the southern slopes and there is a sharp contrast between summer and winter temperatures due to a continental climate. Precipitation is higher in the western parts and the southern slopes. The southwestern slopes also receive heavy snowfall.
There have been published assessments suggesting decrease in area, retreat and acceleration in retreat of the glaciers in the region at the end of 20th century but large-scale and long-term assessments of changes in mass balance are not reported. The area is well known for various disastrous events related to glacier dynamics such as rock-ice avalanches, debris flows, landslides and outburst floods which makes the regular monitoring of the area a necessity. The glaciers are an important source of run off for agriculture and hydropower generation in the area. Therefore, knowledge regarding the present-day state of the glaciers is required to manage the resources for the near future.
The current study uses Digital Elevation Models (DEM) (Shuttle Radar Topographic Mission (SRTM), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)) at different time intervals and the DEM differencing technique to calculate the change in elevation of the glaciers over time. Additionally, satellite altimetry data of Ice, Cloud and land Elevation Satellite-2 (ICESat-2) has been utilised to obtain the most recent elevation. The change in area of glaciers has been obtained using optical images of Sentinel-2 and Landsat series. The change in volume and mass of the Caucasus mountains could have been thus calculated.
The preliminary results indicate a loss in elevation, area and an overall decrease in ice mass. There is a further requirement to understand the association of these changes with the increasing global temperatures and altering precipitation patterns to estimate the impact of the climate change on the glaciers accurately. The study proves the utility of DEMs along with the altimetry measurements in region specific research.

Svalbard is an Arctic archipelago characterized by a high latitude and high relief glacial and periglacial landscape. In the lowlands, the uppermost part of the ground above the permafrost, called the active layer, thaws in summer and refreezes in winter. This can induce cm-scale subsidence and heave due to the phase change of the water/ice present in the ground. On valley sides, various mass-wasting processes induce downslope creeping processes. Ground displacements in Svalbard are important to take into account for the management of infrastructure stability and for the assessment of geohazards to ensure the safety of the population. In addition, the displacement rates vary spatially and temporally depending on various environmental factors. They indirectly document the dynamics of the ground thermal regime, which influences a large set of hydrological and biological processes occurring in the upper part of the ground.

Although ground dynamics in Svalbard has practical implications and is important to document in a context of climate change, measurements of displacements in Svalbard are currently mainly based on in-situ instrumentation and provide sparse and unevenly distributed observations. The European Commission Copernicus Sentinel-1 SAR satellites has since 2015 provided capability for large-scale monitoring of surface movement using Synthetic Aperture Radar Interferometry (InSAR). In mainland Norway, the openly available “InSAR Norway” ground motion mapping service (https://insar.ngu.no) provides InSAR displacement time series over the whole country and is operationally used to identify and monitor unstable areas.

In Svalbard, we have demonstrated that InSAR is valuable to:
• Identify fast moving areas around Longyeardalen that can potentially affect infrastructure stability or safety of the population (Rouyet et al., 2017);
• Map the timing of the active layer freeze and thaw transition, as a correspondence between seasonal subsidence/heave patterns and ground temperature has been shown (Rouyet et al., 2019; 2021a);
• Document the kinematics of creeping landforms (e.g. rock glaciers) and monitor their changes, as acceleration due to permafrost warming has been evidenced (Eriksen et al., 2018; Rouyet et al., 2021b; ESA CCI Permafrost, 2021).

InSAR in Svalbard has both a practical geohazard relevance and a scientific relevance to develop climate change indicators related to the Essential Climate Variable (ECV) Permafrost, as supported by the ESA Climate Change Initiative (CCI) Permafrost (https://climate.esa.int/en/projects/permafrost/). In addition, InSAR products may complement existing data coordinated by the Svalbard Integrated Arctic Earth Observing System (SIOS) and fulfill specific needs from the diverse scientific community. However, technical challenges must be considered to develop operational upscaling strategies of Sentinel-1 InSAR to the whole Svalbard archipelago. Specific methods and algorithms must be tailored to solve polar challenges (long winter season with snow cover, extensive glacial surfaces, very dynamic surficial conditions, seasonal cyclic displacement patterns, etc.). In this presentation, we will discuss the potential and challenges to develop an InSAR ground motion service in Svalbard.

References:
- Eriksen, H.Ø., Rouyet, L., Lauknes, T.R., Berthling, I., Isaksen, K., Hindberg, H., Larsen, Y. and Corner, G.D. (2018). Recent acceleration of a rock glacier complex, Adjet, Norway, documented by 62 years of remote sensing observations. Geophysical Research Letters, 45(16), pp.8314-8323. https://doi.org/10.1029/2018GL077605.
- ESA CCI Permafrost (2021). Rock glacier kinematics as new associated parameter of ECV Permafrost. Deliverables 4-5: Product Validation and Intercomparison Report (PVIR); Climate Research Data Package (CRDP); Product User Guide (PUG); Climate Assessment Report (CAR). https://climate.esa.int/en/projects/permafrost/key-documents/#rock-glacier-kinematics-as-new-associated-parameter-of-ecv-permafrost.
- Rouyet L., Eckerstorfer, M., Lauknes, T.R., Riise, T. (2017). Deformasjonskartlegging rundt Longyearbyen ved bruk av satellittbasert radarinterferometri. Norut report 13/2017. https://www.miljovernfondet.no/wp-content/uploads/2020/02/17-59-terrengstabilitet-lyr.pdf.
- Rouyet L., Lauknes T.R., Christiansen H.H., Strand S.M., Larsen Y. (2019) Seasonal dynamics of a permafrost landscape, Adventdalen, Svalbard, investigated by InSAR. Remote Sensing of Environment 231:111236. https://doi.org/10.1016/j.rse.2019.111236.
- Rouyet, L., Liu, L., Strand, S.M., Christiansen, H.H., Lauknes, T.R., Larsen, Y. (2021a). Seasonal InSAR Displacements Documenting the Active Layer Freeze and Thaw Progression in Central-Western Spitsbergen, Svalbard. Remote Sensing, 13(15), p.2977, https://doi.org/10.3390/rs13152977.
- Rouyet, L., Lilleøren, K.S., Böhme, M., Vick, L.M., Delaloye, R., Etzelmüller, B., Lauknes, T.R., Larsen, Y., Blikra, L.H. (2021b). Regional morpho-kinematic inventory of slope movements in Northern Norway. Frontiers in Earth Science: Cryospheric Sciences, 9:6810881. https://www.frontiersin.org/articles/10.3389/feart.2021.681088/full.