This paper provides an overview of a wide area wireless sensor network that was deployed on the c... more This paper provides an overview of a wide area wireless sensor network that was deployed on the calving front of the Helheim Glacier in Greenland during the summer of 2013. The purpose of the network was to measure the flow rate of the glacier using accurate satellite positioning data. The challenge in this extreme environment was to collect data in real time at the calving edge of the glacier. This was achieved using a solar powered 2.4 GHz Zigbee wireless sensor network operated in a novel hybrid cellular/mesh access architecture consisting of ice nodes communicating with base stations placed on the rock adjacent to the glacier. This highly challenging transmission environment creates substantial signal outage conditions which were successfully mitigated by a radio network diversity scheme. The network development and measurement campaign were highly successful yielding significant results on glacial dynamics associated with climate change.
Glaciers and their changes are increasingly monitored from a wide range of satellite sensors. Due... more Glaciers and their changes are increasingly monitored from a wide range of satellite sensors. Due to the often remote location of glaciers in inaccessible and high-mountain terrain, satellite observations complement ground-based measurements very well. Furthermore, satellite data provide observations of glacier characteristics that are difficult to monitor from ground. In the Glaciers_cci project three of these well-observable characteristics are investigated in detail: glacier area, elevation changes (ELC) and surface velocity (VEL). We use optical sensors to automatically derive area, digital elevation models from two points in time and repeat altimetry for ELC, and optical and microwave imaging sensors for VEL. For the latter, the two sensor types again provide complimentary information in regard to their spatio-temporal coverage. While some products can be generated largely automatically (ELC from altimetry and VEL), the others require interaction by the analyst. Largely based on round robin experiments for each of the products, we suggest in this contribution the most suitable algorithms for product creation and describe them here along with remarks on their practical implementation. In many cases the details on the latter are even more important for creation of high-quality products than the basic algorithm itself.
Ice shelves play a key role in the dynamics of marine ice sheets, by buttressing grounded ice and... more Ice shelves play a key role in the dynamics of marine ice sheets, by buttressing grounded ice and limiting rates of ice flux to the oceans. In response to recent climatic and oceanic change, ice shelves fringing the West Antarctic Ice Sheet (WAIS) have begun to fragment and retreat, with major implications for ice sheet stability. Here, we focus on the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned floating extension of Thwaites Glacier. We show that TEIS has undergone a process of fragmentation in the last five years, including brittle failure along a major shear zone, formation of tensile cracks on the main body of the shelf, and release of tabular bergs on both eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM) show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the shelf. We show that a significant zone of shear upstream of the main pinning point developed in response to the rapid acceleration of the shelf between 2002 and 2006, seeding damage on the shelf. Subsequently, basal melting and positive feedbacks between damage and strain rates weakened TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished through time as the pinning point has shrunk, accumulation of damage has ensured that the ice in the shear zone has remained the weakest link in the system. Experiments with the BISICLES ice sheet model indicate that additional damage to or unpinning of TEIS are unlikely to trigger significantly increased ice loss from WAIS, but the calving response to loss of TEIS remains highly uncertain. It is widely recognised that iceshelf fragmentation and collapse can be triggered by hydrofracturing and/or unpinning from ice shelf margins or grounding points. Our results indicate a third mechanism, backstresstriggered failure, that can occur when ice ffractures in response to stresses associated with pinning points. In most circumstances, pinning points are essential for ice shelf stability, but as ice shelves thin and weaken the concentration of backstress in damaged ice upstream of a pinning point may provide the seeds of their demise.
We investigate three annual mass-balance cycles on Helheim Glacier in south-east Greenland using ... more We investigate three annual mass-balance cycles on Helheim Glacier in south-east Greenland using TanDEM-X interferometric digital elevation models (DEMs), bedrock GPS measurements, and ice velocity from feature-tracking. The DEMs exhibit seasonal surface elevation cycles at elevations up to 800 m.a.s.l. with amplitudes of up to 19 m, from a maximum in July to a minimum in October or November, concentrated on the fast-flowing areas of the glacier indicating that the elevation changes have a mostly dynamic origin. By modelling the detrended bedrock loading/unloading signal we estimate a mean density for the loss of 671 ± 70 kg m −3 and calculate that total water equivalent volume loss from the active part of the glacier (surface flow speeds >1md a y −1 ) ranges from 0.5 km 3 in 2011 to 1.6 km 3 in 2013. A rough ice-flux divergence analysis shows that at lower elevations (<200 m) mass loss by dynamic thinning fully explains seasonal elevation changes. In addition, surface elevations decrease by a greater amount than field observations of surface ablation or surface-energy-balance modelling predict, emphasising the dynamic nature of the mass loss. We conclude, on the basis of ice-front position observations through the time series, that melt-induced acceleration is most likely the main driver of the seasonal dynamic thinning, as opposed to changes triggered by retreat.
Satellite data are increasingly used to provide observation-based estimates of the effects of aer... more Satellite data are increasingly used to provide observation-based estimates of the effects of aerosols on climate. The Aerosol-cci project, part of the European Space Agency's Climate Change Initiative (CCI), was designed to provide essential climate variables for aerosols from satellite data. Eight algorithms, developed for the retrieval of aerosol properties using data from AATSR (4), MERIS (3) and POLDER, were evaluated to determine their suitability for climate studies. The primary result from each of these algorithms is the aerosol optical depth (AOD) at several wavelengths, together with the Ångström exponent (AE) which describes the spectral variation of the AOD for a given wavelength pair. Other aerosol parameters which are possibly retrieved from satellite observations are not considered in this paper. The AOD and AE (AE only for Level 2) were evaluated against independent collocated observations from the ground-based AERONET sun photometer network and against "reference" satellite data provided by MODIS and MISR. Tools used for the evaluation were developed for daily products as produced by the retrieval with a spatial resolution of 10 × 10 km 2 (Level 2) and daily or monthly aggregates (Level 3). These tools include statistics for L2 and L3 products compared with AERONET, as well as scoring based on spatial and temporal correlations. In this paper we describe their use in a round robin (RR) evaluation of four months of data, one month for each season in 2008. The amount of data was restricted to only four months because of the large effort made to improve the algorithms, and to evaluate the improvement and current status, before larger data sets will be processed. Evaluation criteria are discussed. Results presented show the current status of the European aerosol algorithms in comparison to both AERONET and MODIS and MISR data. The comparison leads to a preliminary conclusion that the scores are similar, including those for the references, but the coverage of AATSR needs to be enhanced and further improvements are possible for most algorithms. None of the algorithms, including the references, outperforms all others everywhere. AATSR data can be used for the retrieval of AOD and AE over land and ocean. PARASOL and one of the MERIS algorithms have been evaluated over ocean only and both algorithms provide good results.
Many elements of the cryosphere respond to changes in climate, but mountain glaciers are particul... more Many elements of the cryosphere respond to changes in climate, but mountain glaciers are particularly good indicators of climate change, because they respond more quickly than most other ice bodies on Earth. Changes in glaciers are easily noticed by specialists and non-specialists alike, in ways that other climate indicators, such as ocean temperature or statistics of atmospheric circulation indices, are not. Remote sensing methods are capable of measuring many parameters of mountain glaciers and the changes they exhibit, leading to greater insight into processes affecting changes in glaciers and, hence, climate.
Over the last decades, improvements in remote sensing methods, especially new satellite systems, ... more Over the last decades, improvements in remote sensing methods, especially new satellite systems, have revealed accelerating mass loss, increased melt rates, thinning of the ice sheet margins, and increased discharge from many outlet glaciers due to rapid changes in ice dynamics. However, the lack of an historical context for these recent changes makes it difficult to determine whether they are a response to a warming trend or simply a reflection of the ice sheet's natural variability. We use historical stereo photography and modern lidar surveys to create a time series of digital elevation models (DEMs) and ice margin positions for glaciers in GRACE anomaly regions in both the north and south-east of Greenland. The earliest stereo photographs date from the 1940s making these timeseries up to 70 years long. All of this work has been carried out with the support of NASA's Airborne Thematic Mapper (ATM) missions to collect data to control the historical photography from which high resolution DEMs are generated.
ABSTRACT We present results of global aerosol retrieval from the ESA ATSR instrument series on ER... more ABSTRACT We present results of global aerosol retrieval from the ESA ATSR instrument series on ERS-2 and ENVISAT (1995-2010), and initial testing of a new algorithm developed for Sentinel-3, with planned operation 2013-2030. The ATSR instruments on ERS-2 and ENVISAT together provide one of the longest available, well-calibrated datasets of satellite radiance measurements. The dual-angle viewing capability gives two near-simultaneous images at differing slant paths though the atmosphere, allowing global retrieval of aerosol optical thickness without assumptions on surface spectral properties. We present the global ATSR time series and analysis of trends, and give comparison with AERONET and with MODIS and MISR global datasets. The algorithm has been developed for application to Sentinel-3 to make use of synergistic retrieval from two sensors, OLCI and SLSTR. The research explores the gain by using information from both instruments simultaneously to constrain atmospheric profile, characterise cloud, and provide improved atmospheric correction to surface reflectance. The algorithm has been implemented on the ESA BEAM system and tested on MERIS and AATSR data, and compared with existing algorithms. Recent developments within the ESA Aerosol Climate Change Initiative (CCI) towards a common aerosol retrieval framework are reported. References Bevan, S.L., North, P.R.J., Grey, W.M.F., Los, S.O. and Plummer, S.E. (2009). Impact of atmospheric aerosol from biomass burning on Amazon dry-season drought. Journal of Geophysical Research, 114, D09204, doi:10.1029/2008JD011112. Bevan, S.L., et al. (2011). A global dataset of atmospheric aerosol optical depth and surface reflectance from AATSR, Remote Sensing of Environment, in press. North, P.R.J. et al. (2010) Sentinel-3 L2 Products and Algorithm Definition: OLCI/SLSTR Level 2 and 3 Synergy Products, S3-L203S2-SU-ATBD.
ABSTRACT Remote sensing data show that over the last decade the greatest mass loss from the Green... more ABSTRACT Remote sensing data show that over the last decade the greatest mass loss from the Greenland Ice Sheet (GrIS) has been from the south-east (SE) region, primarily due to synchronous acceleration, thinning and retreat of marine-terminating outlet glaciers. These large dynamic changes have raised concerns about the future stability of the GrIS. We present an 80-year time-series of glacier margin positions for SE Greenland from the 1930s to 2010, together with snapshots of glacial thicknesses from trimlines and DEMs. Calving fronts were digitised from satellite imagery (Landsat, DISP, SAR), aerial photographs, as well as maps and oblique photographs from various expeditions. To extend the time-series beyond the observational record we additionally infer maximum Little Ice Age (LIA; 1150-1850) dimensions from vegetational trimlines. The two largest glaciers in the SE, Helheim and Kangerdlugssuaq, behave very differently over this timescale: Kangerdlugssuaq Glacier has gradually retreated ~20 km, whereas Helheim Glacier&#39;s August 2010 margin is within 2 km of the August 1933 position. Over the 80-year period Helheim was most retreated in 2005 and most advanced in 1985. From trimline evidence, the 1980s extent is comparable with the Little Ice Age maximum. Based on these data it is unlikely that Helheim Glacier has contributed to overall mass loss from the ice sheet since the LIA, whereas Kangerdlugssuaq has exhibited significant steady retreat. We compare the frontal positions of other glaciers in the SE including Gyldenlove, Bernstorff and Ikertivaq to determine whether the regionally widespread 2000s mass loss in SE Greenland is indicative of a longer-term contribution to sea-level rise from the region, or simply a rapid but short-lived fluctuation.
ABSTRACT We use sequences of TanDEM-X acquisitions over &#39;supersites&#39; Helheim and ... more ABSTRACT We use sequences of TanDEM-X acquisitions over &#39;supersites&#39; Helheim and Kangerdlugssuaq glaciers in south-east Greenland to generate interferometric digital elevation models (DEMs) and to feature-track surface displacement between image acquisitions. The high spatial resolution, day/night, and cloud-penetrating capabilities of the X-band SAR system enabled the production of more than 20 DEMs for each glacier with a spatial resolution of 8 m or better. The DEMs span the period June 2011 to March 2012, at 11-day intervals, with a few breaks. Time-lapse animations of Helheim DEMs reveal the development of troughs in surface elevation close to the front. The troughs propagate down flow and develop into the rifts from which calving takes place. On both glaciers, regions of high variance in elevation can be identified caused by the transit of crevasses. In addition, on Helheim, a 1 km wide band of high variance adjacent to the calving front may be interpreted as the response to tidal forcing of a partially floating tongue. In addition to the DEMs we will also present featured tracked high-quality surface velocity fields at a spatial resolution of 2 m coincident with the DEMs. On Helheim these velocity fields indicate a winter deceleration of less than 10% at a point 4 km behind the calving front.
The effects on climate of land-cover change, predominantly from the conversion of forests to crop... more The effects on climate of land-cover change, predominantly from the conversion of forests to crops or grassland, are reasonably well understood for low and high latitudes but are largely unknown for temperate latitudes. The main reason for this gap in our knowledge is that there are compensating effects on the energy and water balance that are related to changes in land-surface albedo, soil evaporation and plant transpiration. We analyse how vegetation height affected the response of vegetation during the 2003 European drought using precipitation data, temperature data, normalized difference vegetation index data and a new vegetation height data set obtained from the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat). At the height of the 2003 drought we find for tall vegetation a significantly smaller decrease in vegetation index and a smaller diurnal temperature (DTR) range, indicating less water stress and drought impacts on tall vegetation. Over Germany for example, 98 % of significant correlations showed a smaller anomaly in vegetation index anomaly with greater height, and 95 % of significant correlations showed a smaller DTR with greater vegetation height. Over France the equivalent percentages were 94 and 88 %, respectively. Vegetation height is likely associated with greater rooting depth, canopy heat capacity or both. Our results suggest that land-surface models can be improved by better estimates of vegetation height and associated with this a more realistic response to drought.
Determining whether increasing temperature or precipitation will dominate the cryospheric respons... more Determining whether increasing temperature or precipitation will dominate the cryospheric response to climate change is key to forecasting future sea-level rise. The volume of ice contained in the ice caps and glaciers of the Arctic archipelago of Svalbard is small compared with that of the Greenland or Antarctic ice sheets, but is likely to be affected much more rapidly in
This paper provides an overview of a wide area wireless sensor network that was deployed on the c... more This paper provides an overview of a wide area wireless sensor network that was deployed on the calving front of the Helheim Glacier in Greenland during the summer of 2013. The purpose of the network was to measure the flow rate of the glacier using accurate satellite positioning data. The challenge in this extreme environment was to collect data in real time at the calving edge of the glacier. This was achieved using a solar powered 2.4 GHz Zigbee wireless sensor network operated in a novel hybrid cellular/mesh access architecture consisting of ice nodes communicating with base stations placed on the rock adjacent to the glacier. This highly challenging transmission environment creates substantial signal outage conditions which were successfully mitigated by a radio network diversity scheme. The network development and measurement campaign were highly successful yielding significant results on glacial dynamics associated with climate change.
Glaciers and their changes are increasingly monitored from a wide range of satellite sensors. Due... more Glaciers and their changes are increasingly monitored from a wide range of satellite sensors. Due to the often remote location of glaciers in inaccessible and high-mountain terrain, satellite observations complement ground-based measurements very well. Furthermore, satellite data provide observations of glacier characteristics that are difficult to monitor from ground. In the Glaciers_cci project three of these well-observable characteristics are investigated in detail: glacier area, elevation changes (ELC) and surface velocity (VEL). We use optical sensors to automatically derive area, digital elevation models from two points in time and repeat altimetry for ELC, and optical and microwave imaging sensors for VEL. For the latter, the two sensor types again provide complimentary information in regard to their spatio-temporal coverage. While some products can be generated largely automatically (ELC from altimetry and VEL), the others require interaction by the analyst. Largely based on round robin experiments for each of the products, we suggest in this contribution the most suitable algorithms for product creation and describe them here along with remarks on their practical implementation. In many cases the details on the latter are even more important for creation of high-quality products than the basic algorithm itself.
Ice shelves play a key role in the dynamics of marine ice sheets, by buttressing grounded ice and... more Ice shelves play a key role in the dynamics of marine ice sheets, by buttressing grounded ice and limiting rates of ice flux to the oceans. In response to recent climatic and oceanic change, ice shelves fringing the West Antarctic Ice Sheet (WAIS) have begun to fragment and retreat, with major implications for ice sheet stability. Here, we focus on the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned floating extension of Thwaites Glacier. We show that TEIS has undergone a process of fragmentation in the last five years, including brittle failure along a major shear zone, formation of tensile cracks on the main body of the shelf, and release of tabular bergs on both eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM) show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the shelf. We show that a significant zone of shear upstream of the main pinning point developed in response to the rapid acceleration of the shelf between 2002 and 2006, seeding damage on the shelf. Subsequently, basal melting and positive feedbacks between damage and strain rates weakened TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished through time as the pinning point has shrunk, accumulation of damage has ensured that the ice in the shear zone has remained the weakest link in the system. Experiments with the BISICLES ice sheet model indicate that additional damage to or unpinning of TEIS are unlikely to trigger significantly increased ice loss from WAIS, but the calving response to loss of TEIS remains highly uncertain. It is widely recognised that iceshelf fragmentation and collapse can be triggered by hydrofracturing and/or unpinning from ice shelf margins or grounding points. Our results indicate a third mechanism, backstresstriggered failure, that can occur when ice ffractures in response to stresses associated with pinning points. In most circumstances, pinning points are essential for ice shelf stability, but as ice shelves thin and weaken the concentration of backstress in damaged ice upstream of a pinning point may provide the seeds of their demise.
We investigate three annual mass-balance cycles on Helheim Glacier in south-east Greenland using ... more We investigate three annual mass-balance cycles on Helheim Glacier in south-east Greenland using TanDEM-X interferometric digital elevation models (DEMs), bedrock GPS measurements, and ice velocity from feature-tracking. The DEMs exhibit seasonal surface elevation cycles at elevations up to 800 m.a.s.l. with amplitudes of up to 19 m, from a maximum in July to a minimum in October or November, concentrated on the fast-flowing areas of the glacier indicating that the elevation changes have a mostly dynamic origin. By modelling the detrended bedrock loading/unloading signal we estimate a mean density for the loss of 671 ± 70 kg m −3 and calculate that total water equivalent volume loss from the active part of the glacier (surface flow speeds >1md a y −1 ) ranges from 0.5 km 3 in 2011 to 1.6 km 3 in 2013. A rough ice-flux divergence analysis shows that at lower elevations (<200 m) mass loss by dynamic thinning fully explains seasonal elevation changes. In addition, surface elevations decrease by a greater amount than field observations of surface ablation or surface-energy-balance modelling predict, emphasising the dynamic nature of the mass loss. We conclude, on the basis of ice-front position observations through the time series, that melt-induced acceleration is most likely the main driver of the seasonal dynamic thinning, as opposed to changes triggered by retreat.
Satellite data are increasingly used to provide observation-based estimates of the effects of aer... more Satellite data are increasingly used to provide observation-based estimates of the effects of aerosols on climate. The Aerosol-cci project, part of the European Space Agency's Climate Change Initiative (CCI), was designed to provide essential climate variables for aerosols from satellite data. Eight algorithms, developed for the retrieval of aerosol properties using data from AATSR (4), MERIS (3) and POLDER, were evaluated to determine their suitability for climate studies. The primary result from each of these algorithms is the aerosol optical depth (AOD) at several wavelengths, together with the Ångström exponent (AE) which describes the spectral variation of the AOD for a given wavelength pair. Other aerosol parameters which are possibly retrieved from satellite observations are not considered in this paper. The AOD and AE (AE only for Level 2) were evaluated against independent collocated observations from the ground-based AERONET sun photometer network and against "reference" satellite data provided by MODIS and MISR. Tools used for the evaluation were developed for daily products as produced by the retrieval with a spatial resolution of 10 × 10 km 2 (Level 2) and daily or monthly aggregates (Level 3). These tools include statistics for L2 and L3 products compared with AERONET, as well as scoring based on spatial and temporal correlations. In this paper we describe their use in a round robin (RR) evaluation of four months of data, one month for each season in 2008. The amount of data was restricted to only four months because of the large effort made to improve the algorithms, and to evaluate the improvement and current status, before larger data sets will be processed. Evaluation criteria are discussed. Results presented show the current status of the European aerosol algorithms in comparison to both AERONET and MODIS and MISR data. The comparison leads to a preliminary conclusion that the scores are similar, including those for the references, but the coverage of AATSR needs to be enhanced and further improvements are possible for most algorithms. None of the algorithms, including the references, outperforms all others everywhere. AATSR data can be used for the retrieval of AOD and AE over land and ocean. PARASOL and one of the MERIS algorithms have been evaluated over ocean only and both algorithms provide good results.
Many elements of the cryosphere respond to changes in climate, but mountain glaciers are particul... more Many elements of the cryosphere respond to changes in climate, but mountain glaciers are particularly good indicators of climate change, because they respond more quickly than most other ice bodies on Earth. Changes in glaciers are easily noticed by specialists and non-specialists alike, in ways that other climate indicators, such as ocean temperature or statistics of atmospheric circulation indices, are not. Remote sensing methods are capable of measuring many parameters of mountain glaciers and the changes they exhibit, leading to greater insight into processes affecting changes in glaciers and, hence, climate.
Over the last decades, improvements in remote sensing methods, especially new satellite systems, ... more Over the last decades, improvements in remote sensing methods, especially new satellite systems, have revealed accelerating mass loss, increased melt rates, thinning of the ice sheet margins, and increased discharge from many outlet glaciers due to rapid changes in ice dynamics. However, the lack of an historical context for these recent changes makes it difficult to determine whether they are a response to a warming trend or simply a reflection of the ice sheet's natural variability. We use historical stereo photography and modern lidar surveys to create a time series of digital elevation models (DEMs) and ice margin positions for glaciers in GRACE anomaly regions in both the north and south-east of Greenland. The earliest stereo photographs date from the 1940s making these timeseries up to 70 years long. All of this work has been carried out with the support of NASA's Airborne Thematic Mapper (ATM) missions to collect data to control the historical photography from which high resolution DEMs are generated.
ABSTRACT We present results of global aerosol retrieval from the ESA ATSR instrument series on ER... more ABSTRACT We present results of global aerosol retrieval from the ESA ATSR instrument series on ERS-2 and ENVISAT (1995-2010), and initial testing of a new algorithm developed for Sentinel-3, with planned operation 2013-2030. The ATSR instruments on ERS-2 and ENVISAT together provide one of the longest available, well-calibrated datasets of satellite radiance measurements. The dual-angle viewing capability gives two near-simultaneous images at differing slant paths though the atmosphere, allowing global retrieval of aerosol optical thickness without assumptions on surface spectral properties. We present the global ATSR time series and analysis of trends, and give comparison with AERONET and with MODIS and MISR global datasets. The algorithm has been developed for application to Sentinel-3 to make use of synergistic retrieval from two sensors, OLCI and SLSTR. The research explores the gain by using information from both instruments simultaneously to constrain atmospheric profile, characterise cloud, and provide improved atmospheric correction to surface reflectance. The algorithm has been implemented on the ESA BEAM system and tested on MERIS and AATSR data, and compared with existing algorithms. Recent developments within the ESA Aerosol Climate Change Initiative (CCI) towards a common aerosol retrieval framework are reported. References Bevan, S.L., North, P.R.J., Grey, W.M.F., Los, S.O. and Plummer, S.E. (2009). Impact of atmospheric aerosol from biomass burning on Amazon dry-season drought. Journal of Geophysical Research, 114, D09204, doi:10.1029/2008JD011112. Bevan, S.L., et al. (2011). A global dataset of atmospheric aerosol optical depth and surface reflectance from AATSR, Remote Sensing of Environment, in press. North, P.R.J. et al. (2010) Sentinel-3 L2 Products and Algorithm Definition: OLCI/SLSTR Level 2 and 3 Synergy Products, S3-L203S2-SU-ATBD.
ABSTRACT Remote sensing data show that over the last decade the greatest mass loss from the Green... more ABSTRACT Remote sensing data show that over the last decade the greatest mass loss from the Greenland Ice Sheet (GrIS) has been from the south-east (SE) region, primarily due to synchronous acceleration, thinning and retreat of marine-terminating outlet glaciers. These large dynamic changes have raised concerns about the future stability of the GrIS. We present an 80-year time-series of glacier margin positions for SE Greenland from the 1930s to 2010, together with snapshots of glacial thicknesses from trimlines and DEMs. Calving fronts were digitised from satellite imagery (Landsat, DISP, SAR), aerial photographs, as well as maps and oblique photographs from various expeditions. To extend the time-series beyond the observational record we additionally infer maximum Little Ice Age (LIA; 1150-1850) dimensions from vegetational trimlines. The two largest glaciers in the SE, Helheim and Kangerdlugssuaq, behave very differently over this timescale: Kangerdlugssuaq Glacier has gradually retreated ~20 km, whereas Helheim Glacier&#39;s August 2010 margin is within 2 km of the August 1933 position. Over the 80-year period Helheim was most retreated in 2005 and most advanced in 1985. From trimline evidence, the 1980s extent is comparable with the Little Ice Age maximum. Based on these data it is unlikely that Helheim Glacier has contributed to overall mass loss from the ice sheet since the LIA, whereas Kangerdlugssuaq has exhibited significant steady retreat. We compare the frontal positions of other glaciers in the SE including Gyldenlove, Bernstorff and Ikertivaq to determine whether the regionally widespread 2000s mass loss in SE Greenland is indicative of a longer-term contribution to sea-level rise from the region, or simply a rapid but short-lived fluctuation.
ABSTRACT We use sequences of TanDEM-X acquisitions over &#39;supersites&#39; Helheim and ... more ABSTRACT We use sequences of TanDEM-X acquisitions over &#39;supersites&#39; Helheim and Kangerdlugssuaq glaciers in south-east Greenland to generate interferometric digital elevation models (DEMs) and to feature-track surface displacement between image acquisitions. The high spatial resolution, day/night, and cloud-penetrating capabilities of the X-band SAR system enabled the production of more than 20 DEMs for each glacier with a spatial resolution of 8 m or better. The DEMs span the period June 2011 to March 2012, at 11-day intervals, with a few breaks. Time-lapse animations of Helheim DEMs reveal the development of troughs in surface elevation close to the front. The troughs propagate down flow and develop into the rifts from which calving takes place. On both glaciers, regions of high variance in elevation can be identified caused by the transit of crevasses. In addition, on Helheim, a 1 km wide band of high variance adjacent to the calving front may be interpreted as the response to tidal forcing of a partially floating tongue. In addition to the DEMs we will also present featured tracked high-quality surface velocity fields at a spatial resolution of 2 m coincident with the DEMs. On Helheim these velocity fields indicate a winter deceleration of less than 10% at a point 4 km behind the calving front.
The effects on climate of land-cover change, predominantly from the conversion of forests to crop... more The effects on climate of land-cover change, predominantly from the conversion of forests to crops or grassland, are reasonably well understood for low and high latitudes but are largely unknown for temperate latitudes. The main reason for this gap in our knowledge is that there are compensating effects on the energy and water balance that are related to changes in land-surface albedo, soil evaporation and plant transpiration. We analyse how vegetation height affected the response of vegetation during the 2003 European drought using precipitation data, temperature data, normalized difference vegetation index data and a new vegetation height data set obtained from the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat). At the height of the 2003 drought we find for tall vegetation a significantly smaller decrease in vegetation index and a smaller diurnal temperature (DTR) range, indicating less water stress and drought impacts on tall vegetation. Over Germany for example, 98 % of significant correlations showed a smaller anomaly in vegetation index anomaly with greater height, and 95 % of significant correlations showed a smaller DTR with greater vegetation height. Over France the equivalent percentages were 94 and 88 %, respectively. Vegetation height is likely associated with greater rooting depth, canopy heat capacity or both. Our results suggest that land-surface models can be improved by better estimates of vegetation height and associated with this a more realistic response to drought.
Determining whether increasing temperature or precipitation will dominate the cryospheric respons... more Determining whether increasing temperature or precipitation will dominate the cryospheric response to climate change is key to forecasting future sea-level rise. The volume of ice contained in the ice caps and glaciers of the Arctic archipelago of Svalbard is small compared with that of the Greenland or Antarctic ice sheets, but is likely to be affected much more rapidly in
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Papers by Suzanne Bevan