Retrieval of firn thickness by means of polarisation phase differences in L-band SAR data

The knowledge of ice zones extent and their temporal variations is fundamental for the retrieval of surface mass balance of glaciers and ice sheets. In this context, a key parameter is the firn line (FL), the lower boundary of the firn zone, whose location is an indicator of time-integrated mass balance changes. Several approaches have been developed in the last decades to map the FL by means of Synthetic Aperture Radar (SAR) imagery, mainly exploiting backscatter intensities and their seasonal variations. In this paper, an alternative approach is proposed, based on co-polarization phase differences (CPDs). In particular, CPDs are interpreted as the result of propagation through anisotropic firn layers and are, therefore, proposed as an indicator of the presence of firn. A model is employed to demonstrate the link between CPDs and firn depth, indicating the potential of polarimetric SAR to improve firn characterization beyond spatial extent and FL detection. The proposed approach is demonstrated on L-band airborne data acquired by the F-SAR sensor of DLR in West Greenland during the ARCTIC15 campaign and validated with in-situ reference information available from other studies

Recent Changes to Langjökull Icecap, Iceland: An investigation integrating airborne LiDAR and satellite imagery

Langjökull, Iceland’s second largest icecap (~950 km$^{2}$), was the subject of an incomplete airborne LiDAR survey in August 2007. This study investigates and evaluates the application of photoclinometry, which employs visible light imagery (here, Landsat ETM+ band 4) to interpolate unmeasured sections of this fragmented data set. A complete digital elevation model (DEM) of Langjökull was produced, and photoclinometry was determined to be a satisfactory and robust technique for topographic interpolation (RMS error = 3.4 m over a 3 km section). Future applications of photoclinometry can ensure optimal results by focusing on the consistent ability of their imager to accurately represent low contrast surfaces; also, consideration of setting characteristic such as solar azimuth, solar elevation, and moderate surface slope will make photoclinometric interpolation more effective. Photoclinometry it is proven to be a current and valuable technique, it is confirmed as a secondary rather than primary tool, and other possible applications of photoclinometry are considered. Using the completed DEM of Langjökull for summer 2007 and a previously prepared corresponding 1997 data set, Langjökull was found to have a specific annual mass balance of -0.99$\pm$0.1 meters per year of water equivalence (m yr$^{-1}$ w.e.), a number which confirms published predictions that Langjökull will likely disappear in the next 200 years. Comparison of remotely-sensed mass balance values and traditional $\textit{in situ}$ measurements revealed a possible systematic disparity; it is hypothesized that field measurements may not be sufficiently constraining behavior of interior areas and that the signal from strongly receding outlet glaciers may be skewing the $\textit{in situ}$ mass balance value calculated for the entire icecap. An additional DEM of outlet Hagafellsjökull Vestari allowed for calculation of specific mass balances of -2.28 m yr$^{-1}$ w.e. for 1997-2001, -3.86 m yr$^{-1}$ w.e. for 2001-2007, and -3.23 m yr$^{-1}$ w.e. for 1997-2007. Similarly, visual inspection and tracing of Landsat images showed a recession of -3.42.5 km$^{2}$ yr$^{-1}$ from 1994 to 2007. The new 2007 DEM allowed for clear visualization of strong recession on several Langjökull outlets as well as interior mass loss and terminus advance witnessing to the 1998 surge event of outlet Hagafellsjökull Eystri. In addition, slight interior elevation increase and anti-correlated mass loss and terminal retreat potentially indicate a future surge of outlet Hagafellsjökull Vestari. In sum, the technological and glaciological information put forward in this study provides a method for innovative cryospheric research, presents a much needed benchmark and update on the state of Langjökull, and ultimately facilitates and encourages continued monitoring of highly important smaller glaciers and icecaps

Age stratigraphy and basal properties of the East Antarctic Ice Sheet from radio-echo sounding measurements : and inferences about ice dynamics

The ice sheet internal structure is an imprint of its history and dynamics, and, additionally, the ice and enclosed air provide valuable information about the past climate. An established continent-wide stratigraphy can constrain both paleo-climate reconstructions and ice-flow models. In this study, the internal stratigraphy and basal properties of the East Antarctic Ice Sheet (EAIS) are inferred from radio-echo sounding (RES) measurements and interpreted with respect to ice dynamics. The first part evaluates the compatibility of multiple RES data sets and investigates the physical cause for RES internal reflections, ensuring their isochronicity. These reflections are used to map the age-depth stratigraphy of the EAIS. The depth distribution of paleo surfaces provides first estimates about accumulation-rate distribution and maximum age of the ice. The third part analyzes the roughness of the EAIS' base. Relationships between basal roughness, ice-flow speed and basal temperatures are observed, which could serve as a method to infer the basal thermal condition or validate modeled temperate-bed locations

The Remotely and Directly Obtained Results of Glaciological Studies on King George Island: A Review

Climate warming has become indisputable, and it is now crucial to increase our understanding of both the mechanisms and consequences of climate change. The Antarctic region is subjected to substantial changes, the trends of which have been recognized for several decades. In the South Shetland Islands, the most visible effect of climate change is progressive deglaciation. The following review focuses on past glaciological studies conducted on King George Island (KGI). The results of collected cryosphere element observations are discussed herein in a comprehensive manner. Our analysis showed that there is a lack of temporal as well as spatial continuity for studies on the basic mass balance parameters on the entire KGI ice dome and only Bellingshausen Dome has a relatively long history of data collection. The methodologies of past work, which have improved over time, are also discussed. When studying the glacier front fluctuations, the authors most frequently use a 1956 aerial photography as reference ice coverage. This was the case for seven papers, while other sources are seldomly mentioned. In other papers as many as 41 other sources were used, and therefore comparison to photos taken up to 60 years later can give misleading trends, as small glaciers may have both advanced and retreated in that time. In the case of glacial velocities there is also an apparent lack of consistency, as different glaciers were indicated as the fastest on KGI. Only Lange, Anna, Crystal, Eldred, and eastern part of Usher glaciers were determined by more than one author as the fastest. Additionally, there are gaps in the KGI Ground Penetrating Radar (GPR) survey area, which includes three ice domes: the Warszawa Icefield, the Krakow Icefield, and eastern part of King George Island. Ideas for further work on the topic are also suggested, allowing for easier access to data and thus contributing to a better understanding of glacier development mechanisms

One Decade of Glacier Mass Changes on the Tibetan Plateau Derived from Multisensoral Remote Sensing Data

The Tibetan Plateau (TP) with an average altitude of 4,500 meters above sea level is characterized by many glaciers and ice caps. Glaciers are a natural indicator for climate variability in this high mountain environment where meteorological stations are rare or non-existent. In addition, the melt water released from the Tibetan glaciers is feeding the headwaters of the major Asian river systems and contributes to the rising levels of endorheic lakes on the plateau. As many people directly rely on the glacier melt water a continuous glacier monitoring program is necessary in this region. In situ measurements of glaciers are important, but are spatial limited due to large logistical efforts, physical constrains and high costs. Remote sensing techniques can overcome this gap and are suitable to complement in situ measurements on a larger scale. In the last decade several remote sensing studies dealt with areal changes of glaciers on the TP. However, glacier area changes only provide a delayed signal to a changing climate and the amount of melt water released from the glaciers cannot be quantified. Therefore it is important to measure the glacier mass balance. In order to estimate glacier mass balances and their spatial differences on the TP, several remote sensing techniques and sensors were synthesized in this thesis. In a first study data from the Ice Cloud and Elevation Satellite (ICESat) mission were employed. ICESat was in orbit between 2003 and 2009 and carried a laser altimeter which recorded highly accurate surface elevation measurements. As in mid-latitudes these measurements are rather sparse glaciers on the TP were grouped into eight climatological homogeneous sub-regions in order to perform a statistical sound analysis of glacier elevation changes. To assess surface elevation changes of a single mountain glacier from ICESat data, an adequate spatial sampling of ICESat measurements need to be present. This is the case for the Grosser Aletschgletscher, located in the Swiss Alps which served as a test site in this thesis. In another study data from the current TanDEM-X satellite mission and from the Shuttle Radar Topography Mission (SRTM) conducted in February 2000 were employed to calculate glacier elevation changes. In a co-authored study, these estimates could be compared with glacier elevation changes obtained from the current French Pléiades satellite mission. In order to calculate glacier mass balances, the derived elevation changes were combined with assumptions about glacier area and ice density in all studies. In this thesis contrasting patterns of glacier mass changes were found on the TP. With an ICESat derived estimate of -15.6±10.1 Gt/a between 2003 and 2009 the average glacier mass balance on the TP was clearly negative. However, some glaciers in the central and north-western part of the TP showed a neutral mass balance or a slightly positive anomaly which was also confirmed by data from the current TanDEM-X satellite mission. A possible explanation of this anomaly in mass balance could be a compensation of the temperature driven glacier melt due to an increase in precipitation

Elevation change and mass balance of Svalbard glaciers from geodetic data

This thesis uses ground-based, airborne and spaceborne elevation measurements to estimate elevation change and mass balance of glaciers and ice caps on the Svalbard archipelago in the Norwegian Arctic. Remote sensing data are validated against field measurements from annual campaigns at the Austfonna ice cap. A new and more accurate DEM of the ice cap is constucted by combining SAR interferometry with ICESat laser altimetry. The precision of the DEM is sufficient to correct ICESat near repeat-tracks for the cross-track topography such that multitemporal elevation profiles can be compared along each reference track. The calculated elevation changes agree well with more accurate elevation change data from airborne laser scanning and GNSS surface profiling. The average mass balance of Austfonna between 2002 and 2008 is estimated to -1.3 ± 0.5 Gt y-1, corresponding to an area-averaged water equivalent elevation change of -0.16 ± 0.06 m w.e. y-1. The entire net loss is due to a retreat of the tidewater fronts. Earlier time periods are difficult to assess due to limitations in the amount and quality of previous elevation data sets. Other Svalbard regions have been precisely mapped by aerial photogrammetry, so the ICESat profiles from 2003-2008 can be compared with existing topographic maps and DEMs from 1965-1990. The mass balance for this period is estimated to -9.7 ± 0.6 Gt y-1 (or -0.36 ± 0.02 m w.e. y-1), excluding Austfonna. Repeat-track ICESat data are also analysed for the entire Svalbard yielding an average 2003-2008 mass balance of -4.3 ± 1.4 Gt y-1 (or -0.12 ± 0.04 m w.e. y-1) when tidewater front retreat is not accounted for. The most accurate elevation change estimates are obtained using all available ICESat data in a joint regression where surface slope and elevation change are estimated for rectangular planes that are fitted to the data along each track. The good performance of the plane method implies that it can also be used in other Arctic regions where accurate DEMs typically are not available

Changes in surging outlet glaciers of the Langjökull Ice Cap, Iceland

This study investigated the surging Hagafellsjökull outlets of the Langjökull ice cap, Iceland. It utilises digital elevation models from 1986, 1997, 2004 and 2007 in order to assess topographic change. These changes are linked to the surging outlets in terms of alteration of the subglacial hydrological system. Flux of water through the subglacial system is considered using a degree day surface melt model. Possible mechanisms of surging are considered and linked to the apparent disparity in surging between the neighbouring outlets Hagafellsjökull Eystri and Hagafellsjökull Vestari. It is found that accumulation in the upper reaches of both outlets led to increased overburden pressure of ice. This resulted in a partial flow switch from the southern Hagafallsjokull outlets to more northern outlets. The loss of flow is considered to have led to instability in the subglacial drainage system resulting in a surge of Hagafellsjökull Eystri and a partial, but failed, surge of Hagafellsjökull Vestari in 1998. Modelled changes in neighbouring subglacial hydrological systems are linked to historic evidence that more outlets of Langjökull ice cap may be, or may have been, surge type. The possibility is suggested that Sudurjökull and Þrístapajökull may well have been subject to surging through alteration of their subglacial hydrological systems, most likely related to the Hagafellsjökull system. The future of Langjökull is considered and agreement is made that the ice cap is retreating with the potential to melt completely within the next 150 years. Future surges seem likely: primarily Hagafellsjökull Vestari is expected to surge within the next 5 years due to increasing imbalance and loss of subglacial meltwater flow. Hagafellsjökull Eystri, post 1998 surge, is also suggested to have returned to a period of quiescence and recent data shows moderate surface elevation increases characteristic of an outlet building up to a surge. Future surge behaviour may also be influenced by increased melting through climatic change and precipitation increases with the possibility of increased surge incidence suggested. The techniques employed are suggested to be useful and highly transable to other studies provided adequate data is available

Using Repeat Photography to Document the Effects of Climate Change on Glaciers in Iceland Change on Glaciers in Iceland

Climate change is a worldwide, multifaceted phenomenon that impacts our world today and will continue to impact our world in the future with even greater severity. Although climate change can sometimes be considered an abstract topic due to its being somewhat intangible, one direct way of observing the effects of climate change is by studying glaciers. This study combines a literature review with repeat photography in order to demonstrate the tangible effects of climate change on glaciers in Iceland and explore the secondary impacts on sea level elevation (SEL), water availability and distribution, hydropower, natural hazards, and tourism in Iceland. The literature review explores past research on both short-term and long-term glacial changes as well as future glacial change projections in Iceland. Results of the literature review showed a general consensus that Iceland’s glaciers have been steadily declining since the early 1990s, with one study even determining that over a 129-year study period, half of the observed mass change on Vatnajökull glacier in Iceland (-240 +/- 20 Gt) occurred during geological years 1994/1995 and 2018/2019 (Aðalgeirsdóttir, G. et al., 2020). For repeat photography, the outline of Vatnajökull glacier from photographs taken in 2013 and 2023 were compared, showing a general trend of glacial volume loss occurring throughout the 10-year period, aligning with the results of the literature review. By using the highly visual nature of repeat photography and combining it with the review of previous glacial research, this study allows for scientific research surrounding the effects of climate change on glaciers to be easily visible to the general public, thus rendering a previously considered intangible concept tangible. This study may, therefore, be seen as a starting point for bridging the gap between scientific discourse surrounding climate change and information for the general public

Multispectral classification and reflectance of glaciers: in situ data collection, satellite data algorithm development, and application in Iceland & Svalbard

Glaciers and ice caps (GIC) are central parts of the hydrological cycle, are key to understanding regional and global climate change, and are important contributors to global sea level rise, regional water resources and local biodiversity. Multispectral (visible and near-infrared) remote sensing has been used for studying GIC and their changing characteristics for several decades. Glacier surfaces can be classified into a range of facies, or zones, which can be used as proxies for annual mass balance and also play a significant role in understanding glacier energy balance. However, multispectral sensors were not designed explicitly for snow and ice observation, so it is not self-evident that they should be optimal for remote sensing of glaciers. There are no universal techniques for glacier surface classification which have been optimized with in situ reflectance spectra. Therefore, the roles that the various spectral, spatial, and radiometric properties of each sensor play in the success and output of resulting classifications remain largely unknown. Therefore, this study approaches the problem from an inverse perspective. Starting with in situ reflectance spectra from the full range of surfaces measured on two glaciers at the end of the melt season in order to capture the largest range of facies (Midtre Lovénbreen, Svalbard & Langjökull, Iceland), optimal wavelengths for glacier facies identification are investigated with principal component analysis. Two linear combinations are produced which capture the vast majority of variance in the data; the first highlights broadband albedo while the second emphasizes the difference in reflectance between blue and near-infrared wavelengths for glacier surface classification. The results confirm previous work which limited distinction to snow, slush, and ice facies. Based on these in situ data, a simple, and more importantly completely transferrable, classification scheme for glacier surfaces is presented for a range of satellite multispectral sensors. Again starting with in situ data, application of relative response functions, scaling factors, and calibration coefficients shows that almost all simulated multispectral sensors (at certain gain settings) are qualified to classify glacier accumulation and ablation areas but confuse classification of partly ash-covered glacier surfaces. In order to consider the spatial as well as the spectral properties of multispectral sensors, airborne data are spatially degraded to emulate satellite imagery; while medium-resolution sensors (~20-60 m) successfully reproduce high-resolution (2 m) observations, low-resolution sensors (i.e. 250 m+) are unable to do so. These results give confidence in results from current sensors such as ASTER and Landsat ETM+ as well as ESA’s upcoming Sentinel-2 and NASA’s recently launched LDCM. In addition, images from the Landsat data archive are used to classify glacier facies and calculate the albedo of glaciers on the Brøgger Peninsula, Svalbard. The time series is used to observe seasonal and interannual trends and investigate the role of melt-albedo feedback in thinning of Svalbard glaciers. The dissertation concludes with recommendations for glacier surface classification over a range of current and future multispectral sensors. Application of the classification schemes suggested should help to improve the understanding of recent and continuing change to GIC around the world.My doctoral studies were supported by a graduate studentship from Trinity College, Cambridge as well as by the National Science Foundation Graduate Research Fellowship Programme under Grant No. DGE-1038596. Further research support came from UK Natural Environment Research Council’s Field Spectroscopy Facility, ARCFAC (the European Centre for Arctic Environmental Research), Trinity College Cambridge, Sigma Xi, the Norwegian Marshall Fund, the Explorers Club, the National Geographic Society Young Explorers Program, the Scott Polar Research Institute, the Cambridge University Geography Department, the Cambridge University Department of Anglo-Saxon, Norse, and Celtic Studies, and the Cambridge University Worts Fund