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  • 1
    Publication Date: 2010-01-15
    Description: We present velocity observations of a glacier outlet in Vatnajökull, Iceland, deduced from interferometric SAR (InSAR) data obtained during the ERS1/2 tandem mission in 1995–2000. More than a 50% decrease in glacier velocity was observed subsequent to a large jökulhlaup from the subglacial lake Grímsvötn in 1996. The glacier had not reached its former flow rate in 2000. The jökulhlaup damaged the lake's ice-dam causing persistent drainage from the lake. InSAR based studies of water accumulation within Grímsvötn suggest that a leakage of 〉3 m3 s−1 prevailed throughout our study period. We suggest that the lake leakage kept open a tunnel at low water pressure underneath the whole length of the glacier. The tunnel flow drained water from its surroundings, hence lowering the water pressure of a distributed drainage system, underneath the upper and centre parts of the glacier, which prior to the jökulhlaup sustained significant basal sliding. This is in accordance with theoretical prediction that tunnel flow in a steady state may cause slow-down in glacier velocity by reducing the subglacial water pressure. The width of the affected areas was ~5 km on the upper part of the glacier and ~8 km on the centre part of the glacier. This indicates that the water pressure reduction propagates laterally from the tunnel over a distance of a few km.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 2
    Publication Date: 2015-09-09
    Description: In this paper we describe how recent high resolution Digital Elevation Models (DEMs) can be used as constraints for extracting glacier surface DEMs from old aerial photographs and to evaluate the uncertainty of the mass balance record derived from the DEMs. We present a case study for Drangajökull ice cap, NW-Iceland. This ice cap covered an area of 144 km2 when it was surveyed with airborne LiDAR in 2011. Aerial photographs spanning all or most of the ice cap are available from survey flights in 1946, 1960, 1975, 1985, 1994 and 2005. All ground control points used to constrain the orientation of the aerial photographs were obtained from the high resolution LiDAR DEM (2 m × 2 m cell size and vertical accuracy 〈 0.5 m). The LiDAR DEM was also used to estimate errors of the extracted photogrammetric DEMs in ice and snow free areas, at nunataks and outside the glacier margin. The derived errors of each DEM were used to constrain a spherical variogram model, which along with the derived errors in ice and snow free areas were used as inputs into 1000 Sequential Gaussian Simulations (SGSim). The simulations were used to estimate the possible bias in the entire glaciated part of the DEM. The derived bias correction, varying in magnitude between DEMs from 0.03 to 1.66 m (1946 DEM) was then applied. The simulation results were also used to calculate the 95 % confidence level of this bias, resulting in values between ±0.21 m (in 2005) and ±1.58 m (in 1946). Error estimation methods based on more simple proxies would typically yield 2–4 times larger error estimates. The aerial photographs used were acquired between late June and early October. An additional bias correction was therefore estimated using a degree day model to obtain the volume change between the start of two hydrological years (1 October). This correction corresponds to an average elevation change of ~ −3 m in the worst case for 1960, or about ~ 2/3 of volume change between the 1960 and the 1975 DEMs. The total uncertainty of the derived mass balance record is mostly due to uncertainty of the SGSim bias correction, the uncertainty of the seasonal bias correction and the uncertainty of the interpolated glacier surface where data is lacking. The record shows a glacier-wide mass balance rate of Ḃ = −0.250 ± 0.040 m w.e. a−1 for the entire study period (1946–2011). We observe significant decadal variability including positive periods, peaking in 1985–1994 with Ḃ = 0.26 ± 0.11 m w.e. a−1. There is a striking difference if Ḃ is calculated separately for the western and eastern halves of Drangajökull, with a reduction of eastern part on average ~ 3 times faster than the western part. Our study emphasises the need of applying rigorous geostatistical methods for obtaining uncertainty estimates of geodetic mass balance, the importance of seasonal corrections of DEMs from glaciers with high mass turnover and the risk of extrapolating mass balance record from one glacier to another even over short distances.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 3
    Publication Date: 2014-09-05
    Description: Area and volume changes and the average geodetic mass balance of the non-surging outlet glaciers of southeast Vatnajökull ice cap, Iceland, during different time periods between ~1890 and 2010, are derived from a multi-temporal glacier inventory. A series of digital elevation models (DEMs) (∼1890, 1904, 1936, 1945, 1989, 2002, 2010) have been compiled from glacial geomorphological features, historical photographs, maps, aerial images, DGPS measurements and a LiDAR survey. Given the mapped bedrock topography we estimate relative volume changes since the end of the Little Ice Age (LIA) ~1890. The variable dynamic response of the outlets, assumed to have experienced similar climate forcing, is related to their different hypsometry, bedrock topography, and the presence of proglacial lakes. In the post-LIA period the glacierized area decreased by 164 km2 and the glaciers had lost 10–30% of their ~1890 area by 2010. The glacier surface lowered by 150–270 m near the terminus and the outlet glaciers collectively lost 60 ± 8 km3 of ice, which is equivalent to 0.154 ± 0.02 mm of sea level rise. The relative volume loss of individual glaciers was in the range of 15–50%, corresponding to a geodetic mass balance between −0.70 and −0.32 m w.e. a−1. The rate of mass loss was most negative in the period 2002–2010, on average −1.34 ± 0.12 m w.e. a−1, which lists among the most negative mass balance values recorded worldwide in the early 21st century. From the data set of volume and area of the outlets, spanning the 120 years post-LIA period, we evaluate the parameters of a volume-area power law scaling relationship.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 4
    Publication Date: 2011-04-06
    Description: The Little Ice Age maximum extent of glaciers in Iceland was reached about 1890 AD and most glaciers in the country have retreated during the 20th century. A model for the surface mass balance and the flow of glaciers is used to reconstruct the 20th century retreat history of Hoffellsjökull, a south-flowing outlet glacier of Vatnajökull, which is located close to the southeast coast of Iceland. The bedrock topography was surveyed with radio-echo soundings in 2001. A wealth of data are available to force and constrain the model, e.g. surface elevation maps from ~1890, 1936, 1946, 1986, 2001, 2008 and 2010, mass balance observations conducted in 1936–1938 and after 2001, energy balance measurements after 2001, and glacier surface velocity derived by DGPS and correlation of SPOT5 images. The 21% volume loss of this glacier in the period 1895–2010 is realistically simulated with the model. After calibration of the model with past observations, it is used to simulate the future response of the glacier during the 21st century. The mass balance model was forced with an ensemble of temperature and precipitation scenarios from a study of the effect of climate change on energy production in the Nordic countries (the CES project). If the average climate of 2000–2009 is maintained into the future, the volume of the glacier is projected to be reduced by 30% with respect to the present at the end of this century, and the glacier will almost disappear if the climate warms as suggested by most of the climate change scenarios. Runoff from the glacier is predicted to increase for the next 30–40 years and decrease after that as a consequence of the diminishing ice-covered area.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 5
    Publication Date: 2009-08-06
    Description: We present velocity observations of a glacier outlet in Vatnajökull, Iceland, deduced from interferometric SAR (InSAR) data obtained during the ERS1/2 tandem mission in 1995–2000. More than 50% decrease in glacier motion was observed subsequent to a large jökulhlaup from the subglacial lake Grímsvötn in 1996. The glacier had not reached its former flow rate in 2000. The jökulhlaup damaged the lake's ice-dam causing persistent drainage from the lake. InSAR based studies of water accumulation within Grímsvötn suggest that a leakage of 〉3 m3 s−1 prevailed throughout our study period. We suggest that the lake leakage kept open a tunnel at low water pressure underneath the whole length of the glacier. The tunnel flow drained water from its surroundings, hence lowering the water pressure of a distributed drainage system, underneath the upper and centre part of the glacier, which prior to the jökulhlaup sustained significant basal sliding. This is in accordance with theoretical prediction that tunnel flow in a steady state may cause slow-down in glacier motion by reducing the subglacial water pressure. The width of the affected areas was ~5 km on the upper part of the glacier and ~8 km on the centre part of the glacier. This indicates that the water pressure reduction propagates laterally from the tunnel over a distance of a few km.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 6
    Publication Date: 2014-09-11
    Description: In response to climate change, most glaciers are losing mass and hence contribute to sea-level rise. Repeated and accurate mapping of their surface topography is required to estimate their mass balance and to extrapolate/calibrate sparse field glaciological measurements. In this study we evaluate the potential of Pléiades sub-meter stereo imagery to derive digital elevation models (DEMs) of glaciers and their elevation changes. Our five validation sites are located in Iceland, the European Alps, the Central Andes, Nepal and Antarctica. For all sites, nearly simultaneous field measurements were collected to evaluate the Pléiades DEMs. For Iceland, the Pléiades DEM is also compared to a Lidar DEM. The vertical biases of the Pléiades DEMs are less than 1 m if ground control points (GCPs) are used, but reach up to 6 m without GCPs. Even without GCPs, vertical biases can be reduced to a few decimetres by horizontal and vertical co-registration of the DEMs to reference altimetric data on ice-free terrain. Around these biases, the vertical precision of the Pléiades DEMs is ±1 m and even ±0.5 m on the flat glacier tongues (1-sigma confidence level). We also demonstrate the high potential of Pléiades DEMs for measuring seasonal, annual and multi-annual elevation changes with an accuracy of 1 m or better. The negative glacier-wide mass balances of the Argentière Glacier and Mer de Glace (−1.21 ± 0.16 and −1.19 ± 0.16 m.w.e. yr−1, respectively) are revealed by differencing SPOT5 and Pléiades DEMs acquired in August 2003 and 2012 demonstrating the continuing rapid glacial wastage in the Mont-Blanc area.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 7
    Publication Date: 2016-01-19
    Description: In this paper we describe how recent high-resolution digital elevation models (DEMs) can be used to extract glacier surface DEMs from old aerial photographs and to evaluate the uncertainty of the mass balance record derived from the DEMs. We present a case study for Drangajökull ice cap, NW Iceland. This ice cap covered an area of 144 km2 when it was surveyed with airborne lidar in 2011. Aerial photographs spanning all or most of the ice cap are available from survey flights in 1946, 1960, 1975, 1985, 1994 and 2005. All ground control points used to constrain the orientation of the aerial photographs were obtained from the high-resolution lidar DEM. The lidar DEM was also used to estimate errors of the extracted photogrammetric DEMs in ice- and snow-free areas, at nunataks and outside the glacier margin. The derived errors of each DEM were used to constrain a spherical semivariogram model, which along with the derived errors in ice- and snow-free areas were used as inputs into 1000 sequential Gaussian simulations (SGSims). The simulations were used to estimate the possible bias in the entire glaciated part of the DEM and the 95 % confidence level of this bias. This results in bias correction varying in magnitude between 0.03 m (in 1975) and 1.66 m (in 1946) and uncertainty values between ±0.21 m (in 2005) and ±1.58 m (in 1946). Error estimation methods based on more simple proxies would typically yield 2–4 times larger error estimates. The aerial photographs used were acquired between late June and early October. An additional seasonal bias correction was therefore estimated using a degree-day model to obtain the volume change between the start of 2 glaciological years (1 October). This correction was largest for the 1960 DEM, corresponding to an average elevation change of −3.5 m or approx. three-quarters of the volume change between the 1960 and the 1975 DEMs. The total uncertainty of the derived mass balance record is dominated by uncertainty in the volume changes caused by uncertainties of the SGSim bias correction, the seasonal bias correction and the interpolation of glacier surface where data are lacking. The record shows a glacier-wide mass balance rate of Ḃ  = −0.26 ± 0.04 m w.e. a−1 for the entire study period (1946–2011). We observe significant decadal variability including periods of mass gain, peaking in 1985–1994 with Ḃ  = 0.27 ± 0.11 m w.e. a−1. There is a striking difference when Ḃ  is calculated separately for the western and eastern halves of Drangajökull, with a reduction of eastern part on average  ∼  3 times faster than the western part. Our study emphasizes the need for applying rigorous geostatistical methods for obtaining uncertainty estimates of geodetic mass balance, the importance of seasonal corrections of DEMs from glaciers with high mass turnover and the risk of extrapolating mass balance record from one glacier to another even over short distances.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 8
    Publication Date: 2011-11-02
    Description: The Little Ice Age maximum extent of glaciers in Iceland was reached about 1890 AD and most glaciers in the country have retreated during the 20th century. A model for the surface mass balance and the flow of glaciers is used to reconstruct the 20th century retreat history of Hoffellsjökull, a south-flowing outlet glacier of the ice cap Vatnajökull, which is located close to the southeastern coast of Iceland. The bedrock topography was surveyed with radio-echo soundings in 2001. A wealth of data are available to force and constrain the model, e.g. surface elevation maps from ~1890, 1936, 1946, 1989, 2001, 2008 and 2010, mass balance observations conducted in 1936–1938 and after 2001, energy balance measurements after 2001, and glacier surface velocity derived by kinematic and differential GPS surveys and correlation of SPOT5 images. The approximately 20% volume loss of this glacier in the period 1895–2010 is realistically simulated with the model. After calibration of the model with past observations, it is used to simulate the future response of the glacier during the 21st century. The mass balance model was forced with an ensemble of temperature and precipitation scenarios derived from 10 global and 3 regional climate model simulations using the A1B emission scenario. If the average climate of 2000–2009 is maintained into the future, the volume of the glacier is projected to be reduced by 30% with respect to the present at the end of this century. If the climate warms, as suggested by most of the climate change scenarios, the model projects this glacier to almost disappear by the end of the 21st century. Runoff from the glacier is predicted to increase for the next 30–40 yr and decrease after that as a consequence of the diminishing ice-covered area.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 9
    Publication Date: 2015-03-19
    Description: Area and volume changes and the average geodetic mass balance of the non-surging outlet glaciers of the southeast Vatnajökull ice cap, Iceland, during different time periods between ~ 1890 and 2010, are derived from a multi-temporal glacier inventory. A series of digital elevation models (DEMs) (~ 1890, 1904, 1936, 1945, 1989, 2002, 2010) are compiled from glacial geomorphological features, historical photographs, maps, aerial images, DGPS measurements and a lidar survey. Given the mapped basal topography, we estimate volume changes since the end of the Little Ice Age (LIA) ~ 1890. The variable volume loss of the outlets to similar climate forcing is related to their different hypsometry, basal topography, and the presence of proglacial lakes. In the post-LIA period, the glacierized area decreased by 164 km2 (or from 1014 to 851 km2) and the glaciers had lost 10–30 % of their ~ 1890 area by 2010 (anywhere from 3 to 36 km2). The glacier surface lowered by 150–270 m near the terminus and the outlet glaciers collectively lost 60 ± 8 km3 of ice, which is equivalent to 0.15 ± 0.02 mm of sea-level rise. The volume loss of individual glaciers was in the range of 15–50%, corresponding to a geodetic mass balance between −0.70 and −0.32 m w.e. a−1. The annual rate of mass change during the post-LIA period was most negative in 2002–2010, on average −1.34 ± 0.12 m w.e. a−1, which is among the most negative mass balance values recorded worldwide in the early 21st century.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 10
    Publication Date: 2014-12-10
    Description: In response to climate change, most glaciers are losing mass and hence contribute to sea-level rise. Repeated and accurate mapping of their surface topography is required to estimate their mass balance and to extrapolate/calibrate sparse field glaciological measurements. In this study we evaluate the potential of sub-meter stereo imagery from the recently launched Pléiades satellites to derive digital elevation models (DEMs) of glaciers and their elevation changes. Our five evaluation sites, where nearly simultaneous field measurements were collected, are located in Iceland, the European Alps, the central Andes, Nepal and Antarctica. For Iceland, the Pléiades DEM is also compared to a lidar DEM. The vertical biases of the Pléiades DEMs are less than 1 m if ground control points (GCPs) are used, but reach up to 7 m without GCPs. Even without GCPs, vertical biases can be reduced to a few decimetres by horizontal and vertical co-registration of the DEMs to reference altimetric data on ice-free terrain. Around these biases, the vertical precision of the Pléiades DEMs is ±1 m and even ±0.5 m on the flat glacier tongues (1σ confidence level). Similar precision levels are obtained in the accumulation areas of glaciers and in Antarctica. We also demonstrate the high potential of Pléiades DEMs for measuring seasonal, annual and multi-annual elevation changes with an accuracy of 1 m or better if cloud-free images are available. The negative region-wide mass balances of glaciers in the Mont-Blanc area (−1.04 ± 0.23 m a−1 water equivalent, w.e.) are revealed by differencing Satellite pour l'Observation de la Terre 5 (SPOT 5) and Pléiades DEMs acquired in August 2003 and 2012, confirming the accelerated glacial wastage in the European Alps.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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