41 research outputs found
Quantification and Change Assessment Benjamin Aubrey Robson 2016 Dissertation date: 31st October 2016 of Debris-Covered Glaciers using Remote Sensing
No full textThis thesis investigates how remote sensing data can be used to assess the changing state of debris-covered ice. The principal study areas are the Manaslu Region in Nepal (papers I and III) and the Hohe Tauern National Park, Austria (paper II). Clean glacier ice is straightforward to semi-automatically classify using multi-spectral satellite imagery owing to the strong spectral signature of clean ice in the visible and near-infrared sections of the electromagnetic spectrum. Since the ablation zones of clean ice glaciers are at the pressure melting point, a change in terminus position or glacier area can be directly linked to a change in climate. Debris-covered ice is however more complicated to map and to interpret temporal change. Supraglacial debris is spectrally indistinguishable from the surrounding paraglacial terrain, and requires auxiliary data such as a Digital Elevation Model (DEM), thermal band data, or flow data. Object-Based Image Analysis (OBIA) provides a framework for combining multiple datasets in one analysis, while additionally allowing shape, contextual, hierarchical and textural criteria to be used to classify imagery. Paper I combines optical (Landsat-8), topographic (void-filled SRTM) and SAR coherence (ALOS PALSAR) data within an OBIA workflow to semi-automatically classify both clean ice and debris-covered ice in the challenging area surrounding Mount Manaslu in Nepal. When compared with manually delineated outlines, the classification achieved an accuracy of 91% (93% for clean ice and 83% for debriscovered ice). The classification was affected by seasonal snow and shadows while the debris-covered ice mapping was influenced by the datasets being temporally inconsistent, and the mountainous topography causing inconsistencies in the SAR coherence data. The method compares well with other automated techniques for classifying debris-covered ice, but has two additional advantages: firstly, that SAR coherence data can distinguish active ice from stagnant ice based on whether motion or significant downwasting has occured, and secondly, that the method is applicable over a large study area using just space-borne data. Paper II explores the potential of using high-resolution (10 m) topographic data and an edge detection algorithm to morphologically map the extent of debris-covered ice. The method was applied in the Hohe Tauern National Park, Austria, using a 10 m DEM derived from airborne Light Detection and Radar (LiDAR) acquisitions. Additionally, the end-of-summer transient snowline (TSL) was also mapped, which approximates the annual Equilibrium Line Altitude (ELA). Our classification was applied on three Landsat satellite images from 1985, 2003 and 2013 and compared the results to the Austrian Glacier Inventories from 1969 and 1998 to derive decadal-scale glacial changes. A mean rate of glacier area reduction of 1.4 km2a-1 was calculated between 1969 and 2013 with a total reduction in area of 33%. The TSL rose by 92 m between 1985 and 2013 to an altitude of 3005 m. By comparing our results with manually delineated outlines an accuracy of 97.5% was determined. When a confusion matrix was calculated it could be seen that the results contained few false positives but some false negatives which were attributed to seasonal snow, shadows and misclassified debris. Our results correspond broadly with those found in other areas of the European Alps although a heterogeneity in glacier change is observable. We recommend that future glacier mapping investigations should utilise a combination of both SAR coherence data and high-resolution topographic data in order to delineate the extent of both active and stagnant glacier ice. Paper III investigates decadal scale changes in glacier area, velocity and volume in the previously undocumented Manaslu Region, Nepal. Between 2001 and 2013 the glacier area reduced by 8.2% (-0.68% a-1). Simultaneously, the glaciers lowered by -0.21 ± 0.08 m a-1 and had a slightly negative specific mass balance of -0.05 ± 016 m w.e a-1 although mass balances ranged -2.49 ± 2.24 to +0.27 ± 0.30 m w.e a-1 throughout the region. The geodetic mass balance for select glaciers covered by a Corona DEM between 1970 and 2013 was -0.24 ± 0.12 m w.e a-1 which became more negative (-0.51 ± 0.12 m w.e. a-1) between 2005 and 2013. Rates of surface lowering over debriscovered ice increasing by 168% between 1970 – 2000 (0.40 ± 0.18 m a-1) and 2005 – 2013 (1.07 ±0.48 m a-1). The rate of glacier melt varies due to presumed increases in debris thickness at the upper and lower boundaries of the ablation zone, while an area of enhanced glacier downwasting corresponds to the presence of supraglacial lakes and exposed ice. The glacier velocity varies across the region. Many glaciers have stagnant sections towards the glacier termini, and a trend of ongoing stagnation is observable. No relationship exists between trends in glacier area and glacier volume or velocity, although a weak relationship exists between trends in the changes of volume and velocity. The rates of glacier area and velocity change appear to be similar, although the number of glaciers that had records of area, velocity, and volume was few. Our results are comparable to studies looking at mean surface lowerings and geodetic mass balances in other areas of the Himalayas, and point towards heterogeneous yet pronounced mass losses across the Himalaya region
Long‐term impact of the proglacial lake Jökulsárlón on the flow velocity and stability of Breiðamerkurjökull glacier, Iceland
No full textProglacial lakes are becoming ubiquitous at the termini of many glaciers worldwide due to continued climate warming and glacier retreat, and such lakes have important consequences for the dynamics and future stability of these glaciers. In light of this, we quantified decadal changes in glacier velocity since 1991 using satellite remote sensing for Breiðamerkurjökull, a large lake-terminating glacier in Iceland. We investigated its frontal retreat, lake area change and ice surface elevation change, combined with bed topography data, to understand its recent rapid retreat and future stability. We observed highly spatially variable velocity change from 1991 to 2015, with a substantial increase in peak velocity observed at the terminus of the lake-terminating eastern arm from ~1.00±0.36m day1 in 1991 to 3.50±0.25m day1 in 2015, with mean velocities remaining elevated from 2008 onwards. This is in stark comparison to the predominately land-terminating arms, which saw no discernible change in their velocity over the same period. We also observed a substantial increase in the area of the main proglacial lake (Jökulsárlón) since 1982 of ~20 km2, equating to an annual growth rate of 0.55km2 year1. Over the same period, the eastern arm retreated by ~3.50km, which is significantly greater than the other arms. Such discrepancies between the different arms are due to the growth and, importantly, depth increase of Jökulsárlón, as the eastern arm has retreated into its ~300m-deep reverse-sloping subglacial trough. We suggest that this growth in lake area, forced initially by rising air temperatures, combined with the increase in lake depth, triggered an increase in flow acceleration, leading to further rapid retreat and the initiation of a positive feedback mechanism. These findings may have important implications for how increased melt and calving forced by climate change will affect the future stability of large soft-bedded, reverse-sloped, subaqueous-terminating glaciers elsewhere
Decadal Scale Changes in Glacier Area in the Hohe Tauern National Park (Austria) Determined by Object-Based Image Analysis
No full textIn this paper, we semi-automatically classify clean and debris-covered ice for 145 glaciers within Hohe Tauern National Park in the Austrian Alps for the years 1985, 2003, and 2013. We also map the end-summer transient snowline (TSL), which approximates the annual Equilibrium Line Altitude (ELA). By comparing our results with the Austrian Glacier Inventories from 1969 and 1998, we calculate a mean reduction in glacier area of 33% between 1969 and 2013. The total ice area reduced at a mean rate of 1.4 km2 per year. This TSL rose by 92 m between 1985 and 2013 to an altitude of 3005 m. Despite some limitations, such as some seasonal snow being present at higher elevations, as well as uncertainties related to the range of years that the LiDAR DEM was collected, our results show that the glaciers within Hohe Tauern National Park conform to the heavy shrinkage experienced in other areas of the European Alps. Moreover, we believe that Object-Based Image Analysis (OBIA) is a promising methodology for future glacier mapping
The seasonal evolution of subglacial drainage pathways beneath a soft-bedded glacier
No full textSubglacial hydrology is a key element in glacier response to climate change, but investigations of this environment are logistically difficult. Most models are based on summer data from glaciers resting on rigid bedrocks. However a significant number of glaciers rest on soft (unconsolidated sedimentary) beds. Here we present a unique multi-year instrumented record of the development of seasonal subglacial behavior associated with an Icelandic temperate glacier resting on a deformable sediment layer. We observe a distinct annual pattern in the subglacial hydrology based on self-organizing anastomosing braided channels. Water is stored within the subglacial system itself (till, braided system and ‘ponds’), allowing the rapid access of water to enable glacier speed-up events to occur throughout the year, particularly in winter
Glacier and rock glacier changes since the 1950s in the La Laguna catchment, Chile
No full textBenjamin Aubrey Robson was supported by a University of Bergen mobility grant for this work. This work was also supported by ANID and Concurso de Fortalecimiento al Desarrollo Científico de Centros Regionales (grant no. 2020-R20F0008-CEAZA), and Álvaro Ayala was supported by ANID and FONDECYT (grant no. 3190732).Glaciers and rock glaciers play an important role in the hydrology of the semi-arid northern Chile. Several studies show that glaciers have rapidly lost mass in response to climate change during the last decades. The response of rock glaciers to climate change in this region is, however, less known. In this study we use a combination of historical aerial photography, stereo satellite imagery, airborne lidar, and the Shuttle Radar Topography Mission (SRTM) DEM to report glacier changes for the Tapado Glacier-rock glacier complex from the 1950s to 2020 and to report mass balances for the glacier component of the complex, Tapado Glacier. Furthermore, we examine high-resolution elevation changes and surface velocities between 2012 and 2020 for 35 rock glaciers in the La Laguna catchment. Our results show how Tapado Glacier has shrunk by -25.2 +/- 4.6 % between 1956 and 2020, while the mass balance of Tapado Glacier has become steadily more negative, from being approximately in balance between 1956 and 1978 (-0.04 +/- 0.08 m w.e. a(-1)) to showing increased losses between 2015 and 2020 (-0.32 +/- 0.08 m w.e. a(-1)). Climatological (re-)analyses reveal a general increase in air temperature, decrease in humidity, and variable precipitation since the 1980s in the region. In particular, the severe droughts starting in 2010 resulted in a negative mass balance of -0.54 +/- 0.10 m w.e. a(-1) between 2012 and 2015. The rock glaciers within the La Laguna catchment show heterogenous changes, with some sections of landforms exhibiting pronounced elevation changes and surface velocities exceeding that of Tapado Glacier. This could be indicative of high ice contents within the landforms and also highlights the importance of considering how landforms can transition from more glacial landforms to more periglacial features under permafrost conditions. As such, we believe high-resolution (sub-metre) elevation changes and surface velocities are a useful first step for identifying ice-rich landforms.Peer reviewe
Spatial variability in patterns of glacier change across the Manaslu range, central Himalaya
No full textThis study assesses changes in glacier area, velocity, and geodetic mass balance for the glaciers in the Manaslu region of Nepal, a previously undocumented region of the Himalayas. We studied changes between 1970 (for select glaciers), 2000, 2005, and 2013 using freely available Landsat satellite imagery, the SRTM Digital Elevation Model (DEM) and a DEM based on Worldview imagery. Our results show a complex pattern of mass changes across the region, with glaciers lowering on average by 0.25 ±0.08ma−1 between 2000 and 2013 which equates to a geodetic mass balance of −0.21 ± 0.16m w.e.a−1. Over approximately the same time period (1999 to 2013) the glaciers underwent a 16.0% decrease in mean surface velocity over their debris-covered tongues as well as a reduction in glacier area of 8.2%. The rates of glacier change appear to vary between the different time periods, with glacier losses increasing in most cases. The glaciers on Manaslu itself underwent a change in surface elevation of −0.46 ± 0.03m a−1 between 1970 and 2000 and −0.99 ±0.08ma−1 between 2000 and 2013. Rates of glacier area change for the same glaciers increased from−0.36 km2 a−1 between 1970 and 2001 to −2.28 km2 a−1 between 2005 and 2013. Glacier change varies across the region and seems to relate to a combination of glacier hypsometry, glacier elevation range and the presence and distribution of supraglacial debris. Lower-elevation, debris-free glaciers with bottom-heavy hypsometries are losing most mass. As the glaciers in the Manaslu region continue to stagnate, an accumulation and thickening of the debris-cover is likely, thereby insulating the glacier and further complicating future glacier responses to climate.</p
A Remote Sensing Investigation into the evolution of Folgefonna Glacier over the last 150 years
No full textThe evolution of Folgefonna, three large maritime ice masses in Hardanger, Western Norway has been assessed over the last 150 years using a variety of remote sensing datasets (optical and microwave satellite images, aerial photography, digital elevation models (DEMs) and old maps). Changes in glacier area, volume and elevation of the transient snowline (TSL), a commonly used glacier mass balance proxy are determined. All three parameters show a similar trend, although the scarcity of glacier volume data points means that changes cannot be resolved in as much detail as the measurements of glacier area or the TSL elevation. Since the Little Ice Age (LIA) maxima at the end of the nineteenth century Folgefonna has been retreating and losing mass with noticeable glacier advances in the 1960s/70s and the 1990s. Since the turn of the millennium Folgefonna has retreated rapidly interrupted only by a short lived advance between 2005 and 2008. In 2011 Nordfonna, Midtfonna and Sørfonna had respective areas of 24.8 km², 9.1 km² and 156.7 km², reductions of 47%, 68% and 20% compared with their LIA maxima sizes in 1860. The TSL mirrors this trend albeit with less magnitude compared with the other observations, it is therefore assumed that in actual fact it the firn line being measured and not the TSL. Absolute ice volume calculations are only possible for Nordfonna where the subglacial topography is known; Nordfonna measured 1.84 km³ in 2010, a reduction of 43% of its 1937 volume. If planar bedrock surfaces beneath 95% of the ice surfaces are assumed then rudimentary percentage losses can be calculated. Over the same time span Midtfonna lost 1441 million kg (50%) of mass, while Sørfonna lost 8268 million kg (18%) between 1987 and 2010, the portion of Sørfonna visible on the 1937 topographic map lost 5658 million kg (21%) between then and 2010. The changes observed remotely in Folgefonna relate well to the in-situ data as well as the climatic data, it is evident that winter precipitation has traditionally been the principle driver of Folgefonna, however recent increases in summer temperature have been responsible for the acceleration in glacier shrinkage. Folgefonna is found to have advanced and retreated roughly in synchronisation with ice masses in Scandinavia, Europe and further afield suggesting that a global force is partly responsible for driving the glacier
Surface melt driven summer diurnal and winter multi-day stick-slip motion and till sedimentology
No full textFluctuations in glacier motion are very common and are thought to be controlled by subglacial hydrology and till deformation. There are few instrumented studies that have monitored seasonal changes. We use the innovative Glacsweb subglacial in situ wireless probes, combined with dGPS and custom geophone data from an Icelandic soft-bedded temperate glacier, to show that there are two distinct seasonal styles of speed-up events. Relatively small diurnal events occur during the melt season, whilst during winter there are larger multi-day events related to positive degree days. These events are accompanied by a distinct pattern of till deformation and basal icequakes. We argue these reflect stick-slip motion which occurs when the glacier hydrological system is unable to accommodate the melt water flux generated by surface melt episodes. We show a rare fully instrumented coupled glacier/till record of contrasting summer and winter stick-slip motion and discuss its implication for till sedimentology
Monitoring glacial lake outburst flood susceptibility using Sentinel-1 SAR data, Google Earth Engine, and persistent scatterer interferometry
No full textFunding to support this research from the University of St Andrews and the School of Geography and Sustainable Development is gratefully acknowledged.Continuous monitoring of glacial lakes, their parent glaciers and their surroundings is crucial because possible outbursts of these lakes pose a serious hazard to downstream areas. Ongoing climate change increases the risk of this hazard globally due to recession of glaciers leading to formation and expansion of glacial lakes, and permafrost degradation which impacts the stability of glaciers, slopes and moraines. Here, we demonstrate the capability of our approach for monitoring lake outburst susceptibility using time-series of Sentinel-1 Synthetic Aperture Radar (S-1 SAR) data. We selected Lunana in the Bhutanese Himalayas as an example region as it is highly susceptible to glacial lake outburst floods and suitable baseline data were available. We used Google Earth Engine (GEE) to calculate average radar backscatter intensity (ARBI) of glaciers, lakes, basins, and moraines. To determine the periodicity of the highest and the lowest radar backscatter intensity, we denoised the ARBI data using a Fast Fourier Transform and autocorrelated using a Pearson correlation function. Additionally, we determined glacier melt area, basin melt area, lake area, open water area, and lake ice area using radar backscatter intensity data. The Persistent Scatterer Interferometry (PSI) technique was used to investigate the stability of moraines and slopes around glacial lakes. The PSI results were qualitatively validated by comparison with high-resolution digital elevation model differencing results. Our approach showed that glaciers and basins in the region underwent seasonal and periodic changes in their radar backscatter intensity related to changes in ice and snow melt. Lakes also showed seasonal changes in their radar backscatter intensity related to the variation of lake ice and open water area, but the radar backscatter intensity change was not periodic. We could also infer lake area change using a time-series radar backscatter intensity data such as the rapid expansion of Bechung Tsho. The PSI analysis showed that all the terminal moraines were stable except Drukchung Tsho. Its terminal moraine showed subsidence at the rate of –5.18 mm/yr. Sidewalls of lakes were also stable with the exception of Lugge Tsho at site 4. Due to the free availability of S-1 SAR data, the efficiency of processing a large amount of imagery within GEE, and the PSI technique, we were able to understand the outburst susceptibility of glacial lakes in the region at great detail. The regular acquisition of S-1 SAR data enables continuous monitoring of glacial lakes. A similar approach and concept can be transferred to any geographic region on earth that shares similar challenges in glacial lake monitoring.Peer reviewe
