Ice Surface Elevation Research Articles

Assessing the status of glaciers in part of the Chandra basin, Himachal Himalaya: A multiparametric approach

This study investigates the change in multiple glacier parameters of three valley-type glaciers (Sakchum (SC), Chhota Shigri (CS), and Bara Shigri (BS)) located in Chandra basin, Himachal Himalaya, sharing the same climatic regime, and assesses the control of nonclimatic factors on wholesome glacier response. Multitemporal satellite remote sensing data from Landsat-TM/ETM/OLI (1993–2014), and Terra-ASTER (2002–2014) along with an SRTM digital elevation model were used for extraction of the glacier parameters. Results show that while SC and BS retreated (SC: 10.65±2.52m/y; BS: 15.51±2.52m/y) and lost area (SC: 0.49±0.0032km2, BS: 1.18±0.0032km2), the CS remained relatively stable (retreat rate: 4.06±2.52m/y, area loss: 0.19±0.0032km2) during 1993–2014. However, results of surface ice velocities (SIV) change (SC: 24.41%, CS: 21.60%, and BS: 28.49%) and surface elevation change (SC: −1.22m/y, CS: −0.91m/y and BS: −1.21m/y) suggest a comparable slowing down and surface lowering from 2002 to 2014. Debris cover also varied substantially (SC: 30.25%, CS: 11.96%, BS: 19.61%) on these glaciers. Results reveal that higher retreat/deglaciation of glaciers was associated with higher altitudinal range, slow SIV in lower ablation zones (LAZ), and glacier hypsometry. Debris cover on glaciers was found to be controlled by slope, higher deglaciation rates, higher SIV in the upper ablation zone (UAZ) coupled with lower SIV in LAZ and surface lowering. Glacier SIV was primarily governed by slope gradient, differential surface lowering, and size of accumulation zone (ACZ). The SIV results confirm the presence of stagnant zones in the lower ablations of SC (<1km upstream) and BS (<6km upstream) and a major SIV dropdown in the UAZ (SC: 34.48%, BS: 37.35%), indicating the expansion of stagnant zones farther up. Glacier surface lowering seemed to be influenced by ice-flux, changing spatial distribution of debris cover, presence of supraglacial lakes, and ice cliffs bordering them. Melting around supraglacial lakes and backwasting of ice cliffs may be the prime reasons behind intense mass loss observed in the stagnant zone of the Bara Shigri glacier, where surface lowering was ~32% higher than glacier-wide thinning. Differential surface lowering in glaciers reduced the surface gradient and hence the SIV. Also, rapid surface gradient changes and SIV dropdown in the Sakchum glacier indicate possible development of supraglacial lakes on it in the future. Interestingly, despite close proximity, SC, CS, and BS demonstrate three distinct ablation patterns, reemphasizing the role of nonclimatic factors. Overall, the study states that mutual variation of glacier parameters together with nonclimatic factors exercise a great control on glacier response.

Detailed dynamic, geometric and supraglacial moraine data for Glaciar Pio XI, the only surge-type glacier of the Southern Patagonia Icefield

ABSTRACTIn contrast to the general trend for glaciers of the Southern Patagonia Icefield, Glaciar Pio XI has experienced a large cumulative frontal advance since 1945. In an effort to better understand this advancing behaviour, this paper presents a synoptic analysis of frontal fluctuations (1998–2014), ice velocities (1986–2014), ice-surface elevations (1975–2007) and supraglacial moraines (1945–2014) derived from geospatial datasets. These analyses reveal changes in the ice flow of Glaciar Pio XI's freshwater calving northern terminus and tidewater calving southern terminus over recent decades. Between 1986 and 2000, ice flow speed generally accelerated reaching peaks of &gt;15 m d−1 at the frontal edge of the southern terminus. Following this period, flow speed decreased, reducing to &lt;1 m d−1 for the central part of the southern terminus in 2014, despite advancing to a neoglacial maximum. From 2000 to 2014 the reduction in speed was accompanied by a shift in maximum velocity away from the southern terminus, towards the central glacier trunk. As a result, the northern terminus, which accelerated during this period, represented the new primary flow path in 2014. Notably, the moraine maps presented highlight surges occurring around 1981 and again between 1997 and 2000, marked by arcuate moraine features on the southern terminus.

A century of geometry and velocity evolution at Eqip Sermia, West Greenland

ABSTRACTRapid dynamic changes of ocean-terminating outlet glaciers of the Greenland ice sheet are related to atmospheric and oceanic warming but the detailed link to external forcing is not well understood. Observations show high variability in dynamic changes and are mainly limited to the past three decades with dense satellite observations. Here we compile a long-term record of dynamic changes of Eqip Sermia Glacier, West Greenland. Starting in 1912, we combine historical measurements of terminus positions, ice-surface elevation and flow velocity together with more recent in-situ and remote-sensing observations. In the 20th century, the glacier underwent small variations in terminus position and flow speed. Between 2000 and 2003, the terminus retreated substantially, but stabilized thereafter. In 2009, the northern terminus lobe started to retreat very rapidly; the southern lobe collapsed in 2013. The present terminus position, which has retreated by 4 km since 1920, is unprecedented in the historical record. Flow velocities were relatively stable until 2010. The recent acceleration reached threefold velocities in 2014 and rapidly affected the whole terminus region up to 15 km inland. Comparison with forcings from the atmosphere and the ocean over the past century shows that no dominant cause can be identified, and that local effects of bed geometry modulate the glacier response.

Direct Reconstruction of Three-dimensional Glacier Bedrock and Surface Elevation from Free Surface Velocity

This study presents a new algorithm to reconstruct both the ice-surface elevation and the altitude of the bedrock of a glacier from the knowledge of the ice-surface velocity which could potentially be obtained from satellite data. It requires the prior knowledge of the surface mass balance and basal conditions. The algorithm is realized in two steps: the first one involves the solution of a partial differential equation obtained from a rearrangement of the shallow-ice approximation and the second one involves the mere downslope integration of a non-linear function of the ice velocity and ice thickness. It is therefore an efficient algorithm which is in principle easy to implement. The algorithm is tested on synthetic data and is shown to be very successful with an ideal dataset and robust even when significant noise is added to the input data. Importantly, the inversion algorithm does not appear to amplify the input error in the data

Direct Reconstruction of Three-dimensional Glacier Bedrock and Surface Elevation from Free Surface Velocity

This study presents a new algorithm to reconstruct both the ice-surface elevation and the altitude of the bedrock of a glacier from the knowledge of the ice-surface velocity which could potentially be obtained from satellite data. It requires the prior knowledge of the surface mass balance and basal conditions. The algorithm is realized in two steps: the first one involves the solution of a partial differential equation obtained from a rearrangement of the shallow-ice approximation and the second one involves the mere downslope integration of a non-linear function of the ice velocity and ice thickness. It is therefore an efficient algorithm which is in principle easy to implement. The algorithm is tested on synthetic data and is shown to be very successful with an ideal dataset and robust even when significant noise is added to the input data. Importantly, the inversion algorithm does not appear to amplify the input error in the data

Fractals in fluctuations of Antarctic ice sheet surface elevation

Progress in reconstructing the mechanism of the influence of drifting snow on changes in the ice surface elevation, when it moves with velocity of 2–5 m/year, was achieved owing to space-based laser altimetry and the discovery of fractal patterns of the elevation fluctuations. They consist in the growth of variance of elevation changes by an increased lag. The variance grows in the power law dependence in the scaling range of 70 km. The wavelet analysis revealed a wide spectrum of spatial frequencies and demonstrated that they occur in the range of 1.4–22.5 km–1. The discovered properties of changes in the surface elevation subordinate to the Gaussian distribution with zero mean indicate dynamic equilibrium of the present-day Antarctic ice sheet.

Outlet glacier response to the 2012 collapse of the Matusevich Ice Shelf, Severnaya Zemlya, Russian Arctic

AbstractThe Matusevich Ice Shelf (MIS), located within the Severnaya Zemlya Archipelago in the Russian Arctic, rapidly broke apart between 10 August and 7 September 2012. We examine the response of the outlet glaciers that fed the MIS from local ice caps to the removal of the ice shelf. We use spaceborne laser altimetry and multiple optically derived digital elevation models to track ice surface elevation change rates (dh/dt) between 1984 and 2014. Glacier speeds are measured by pixel‐tracking from optical and RADAR imagery between 2010 and 2014 and interferometric synthetic aperture radar in 1995 to compare precollapse and postcollapse velocities. We find that the three main outlet glaciers that fed the MIS are thinning an order of magnitude more rapidly than most of the rest of Severnaya Zemyla, based upon ICESat data from 2003 to 2009. Recent, 2012 to 2014 thinning rates are three to four times faster than the 30 year average thinning rate, calculated between 1984 and 2014. The springtime speeds of the largest outlet glacier (Issledovateley) have increased more than 200% at the terminus between April 2010 and April 2014. To date, changes in surface elevation (dh/dt) and velocity at the outlet glaciers near MIS are smaller than glacier responses to ice shelf collapse in Antarctica. It is possible that the MIS was already very weak prior to the 2012 collapse and unable to support back stress. Further observations are required to assess whether the thinning and nonmelt season glacier speeds are continuing to accelerate.

Hybrid glacier Inventory, Gravimetry and Altimetry (HIGA) mass balance product for Greenland and the Canadian Arctic

We present a novel inversion algorithm that generates a mass balance field that is simultaneously consistent with independent observations of glacier inventory derived from optical imagery, cryosphere-attributed mass trends derived from satellite gravimetry, and ice surface elevation trends derived from airborne and satellite altimetry. We use this algorithm to assess mass balance across Greenland and the Canadian Arctic over the Sep-2003 to Oct-2009 period at 26km resolution. We evaluate local algorithm-inferred mass balance against forty in situ point observations. This evaluation yields an RMSE of 0.15mWE/a, and highlights a paucity of in situ observations from regions of high dynamic mass loss and peripheral glaciers. We assess mass losses of 212±67Gt/a to the Greenland ice sheet proper, 38±11Gt/a to peripheral glaciers in Greenland, and 42±11Gt/a to glaciers in the Canadian Arctic. These magnitudes of mass loss are dependent on the gravimetry-derived spherical harmonic mass trend we invert. We spatially partition the transient glacier continuity equation by differencing algorithm-inferred mass balance from modeled surface mass balance, in order to solve the horizontal divergence of ice flux as a residual. This residual ice dynamic field infers flux divergence (or submergent flow) in the ice sheet accumulation area and at tidewater margins, and flux convergence (or emergent flow) in land-terminating ablation areas, which is consistent with continuum mechanics theory.

Surface mass balance and its climate significance from the coast to Dome A, East Antarctica

Based on stake measurements conducted along the Chinese Antarctic traverse since Jan. 1999, we investigated the characteristics of surface mass balance (SMB) and related climate consequences from Zhongshan Station to Dome A, East Antarctica. Spatial analysis suggests that post-depositional processes have a great impact on surface morphology; thus, the representativeness of a single measurement should be discussed in conjunction with local climate features. The comparison among snow accumulation, ice sheet thickness, surface elevation, and ice velocity indicates that the bedrock topography has an indirect connection with the SMB patterns through controlling the surface topography and local climate. The observation reveals that the Lambert Glacier Basin has been experiencing increasing mass input (4.5%), whereas the inland area has experienced a 6% loss, since 2005. An overall estimation of the SMB along the route is 71.3±44.3 kg m−2 a−1, but the annual and regional variation is considerable. Tendency analysis shows that there are four sections with different SMB patterns as a result of three moisture sources and surface climatic discrepancy in the Antarctic inland. This study is the first to identify four SMB patterns from the coast to the Dome area and should provide a valuable contribution to modeling and remote sensing on a continental scale.

Fine-Resolution Radar Altimeter Measurements on Land and Sea Ice

Satellite radar altimeter (RA) measurements are important for continued monitoring of rapidly changing polar regions. In 2010, the European Space Agency launched CryoSat-2 carrying SIRAL, a Ku-band RA with objectives of determining the thickness and extent of sea ice and the topography of the ice sheets. One difficulty with Ku-band radar surveys over snow and ice is unknown penetration of RA signal into snow cover. Improving our understanding of the interactions of RA signals with snow and ice is needed to produce accurate elevation products. To this end, we developed a low-power, ultrawideband (12-18 GHz) RA for airborne surveys to provide fine resolution measurements capable of detecting both scattering from the surface and layers within sea ice and ice sheets. These measurements provide a means of identifying the dominant scattering location of lower resolution RA measurements comparable to satellite-based instruments. We generated two products: a full-bandwidth waveform (FBW) to identify scattering targets at fine resolution and a reduced-bandwidth waveform (RBW) to represent conventional RA measurements. Retrackers are used to generate height estimates over various surface conditions for comparisons. Over ice sheets, the leading-edge tracker provided consistent ice-surface elevation measurements between the FBW and RBW results; however, there were significant differences between the results from the centroid tracker. Over sea ice, the location of the dominant return between the results from snow-covered sea ice is highly variable. This paper provides an overview of RA surveys in polar regions, a description of the CReSIS system, and a discussion of the results.

Five decades of changes in the glaciers on the Friendship Peak in the Altai Mountains, China: Changes in area and ice surface elevation

Mountain glaciers are indicators of climate change and of current water resources. They are important ecological systems and can be used to support sustainable development of industry and agriculture. However, due to climate warming, most glaciers are in a state of rapid retreat. Using topographic maps in 1959, ASTER remote sensing data in 2008 and ASTER digital elevation models (DEMs), area, ice surface elevation, and volume changes of glaciers on the Friendship Peak in the Chinese Altai Mountains were analyzed. Results showed that the collective area of all 201 glaciers investigated was reduced by 30.4% from 1959 to 2008. Fifty-five glaciers disappeared entirely. The average rates of reduction in area of glaciers with sizes <0.5, 0.5–1, 1–4, 4–10, and >10km2 were 25.9%, 30.8%, 30.9%, 35.9%, and 27.4%, respectively. From 1959 to 2008, the elevation of the glacier surface decreased by 20m at an average rate of 0.4ma−1. For the Kanas Glacier, the changes in ice surface elevation ranged from −101 to +38m. Results showed that glaciers at lower altitudes and smaller sizes experienced more extensive changes in elevation. The intensive glacier ablation over the Friendship Peak in the Altai Mountains was found to be caused by increases in the regional temperature, which occurred at an average rate of 0.52°C per decade.

Change in glacier area and thickness in the Tomur Peak, western Chinese Tien Shan over the past four decades

This paper looks at glacier area change in the Tomur Peak for the 1964/71–2000 and 2000–2011 periods. The results show that the total area of the selected 78 glaciers has decreased from 555.45 km2 in 1964/71 to 530.39 km2 in 2000, and further reduced to 521.89 km2 in 2011. Overall, the selected glaciers have lost 4.50% of their surface area between 1964/71 and 2000, and have lost 1.60% more between 2000 and 2011. The area reduction rate of the 78 glaciers increased from 0.12%/a during 1964/71–2000 to 0.13%/a during 2000–2011. In addition, this paper describes a method for estimating the ice surface elevation change using the SRTM (2000) and elevation data generated from topographic maps (1964/71) to quantify the ice thickness change for the 1964/71–2000 period. The surface elevation for about 73.50% of grid area decreased, mostly in the glacier ablation zone. The overall average thickness loss is 22.35 m which yields an annual loss of 0.60 m/a.

Regional glacier mass loss estimated by ICESat-GLAS data and SRTM digital elevation model in the West Kunlun Mountains, Tibetan Plateau, 2003–2009

The Ice, Cloud and land Elevation Satellite (ICESat) altimetry and the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) data are used to estimate glacier mass loss changes in the West Kunlun Mountains, Tibetan Plateau (TP), 2003 to 2009. We integrated Landsat thematic mapper (TM)/enhanced TM images and GIS spatial analysis to map glacier surface elevation changes during 2003 to 2009. The ICESat-GLAS data can be used as baselines for surface elevation changes in altimetry as well as for TM imagery changes for depicting glacier area. Least-squares regression of an ICESat-derived thickness series shows the regional glacier mass decreased at an average rate of 1.41±0.23 km3/year water equivalent from 2003 to 2009, during the period of 2003 to 2009, the trend in thickness change became weaker. The ICESat-GLAS and SRTM DEM elevation differences between February 2003 and October 2009 show that the glacier surface elevations were decreasing below 5800 m but increased by 1.1±0.7 m above 6000 m a.s.l. region over that period. Thickness changes in the lower reaches of the glaciers indicate that the glacier ice ablation was mainly due to the summer temperature increases of 0.23°C and the summer precipitation decreases of 47.8 mm as measured at four stations, as well as the effects of sand and dust sources from the Tarim Basin. Meanwhile, in the upper parts of the glaciers (above 5800 m a.s.l.), ice surface elevation was increasing even though the Hetian station summer temperature at the 500 hPa level showed an obvious decrease of 0.78°C compared to 2003. In the upper part of the glacier-covered region, firn compaction and surface density should be taken into account. The glacier surges and bedrock movement may also lead to an underestimate of the volume loss due to the reduced mass conversion.

Five decades of glacier changes in the Hulugou Basin of central Qilian Mountains, Northwest China

The Heihe River Basin is the second largest inland river basin in the arid regions of Northwest China. Glaciers provide a large proportion of water resources for human production and living. Studies of glacier changes and their impact on water resources in the arid lands are of vital importance. A joint expedition was carried out in 2010 for investigating glaciers in the Hulugou Basin, which is located in the upper reaches of Heihe River. Therefore, glacier changes in the Hulugou Basin of central Qilian Mountains during the past 50 years were analyzed in this study by comparing topographic maps, satellite images, digital elevation models and field observation data from different periods. Results showed that the total area of the 6 glaciers in the Hulugou Basin decreased by 0.590±0.005 km2 during the period 1956–2011, corresponding to a loss of 40.7% over the total area in 1956. The average area reduction rate of the 6 glaciers is 0.011 km2/a. During the past five decades, the glacier shrinkage was accelerated. The changes in glacier ice surface elevation ranged from −15 to 3 m with an average thinning of 10±8 m or an annual decrease of 0.23±0.18 m (0.20±0.15 m/a water equivalent) for the period 1956–2000. The area of Shiyi Glacier in the Hulugou Basin decreased from 0.64 km2 in 1956 to 0.53 km2 in 2011 with a reduction rate of 17.2%. The Shiyi Glacier had been divided into two separated glaciers because of severe melting. Comparative analysis showed that glacier shrinkage in the Hulugou Basin is more serious than that in the other regions of Qilian Mountains.

Recent variations in the terminus position, ice velocity and surface elevation of Langhovde Glacier, East Antarctica

AbstractTo improve the understanding of the mechanism driving recent changes in outlet glaciers in East Antarctica, we measured changes in the terminus position, ice flow velocity and surface elevation of the Langhovde Glacier located on the Sôya Coast. From satellite images from 2000–12 and field measurements taken in 2012 the glacier terminus position and flow velocity showed little change between 2003 and 2007. After this quiescent period, the glacier progressively advanced by 380 m and the flow velocity increased near the calving front by 10 m a-1 from 2007–10. No significant change was observed in surface elevation during the study period. The changes in the terminus position and flow velocity imply a reduction in the calving rate from 93 m a-1 (2003–07) to 16 m a-1 (2007–10). This suggests that calving was inhibited by stable sea ice conditions in the ocean. Theses results indicate that the Langhovde Glacier was in a relatively stable condition during the study period, and its terminus position was controlled by the rate of calving under the influence of sea ice conditions.

The Greenland Ice Mapping Project (GIMP) land classification and surface elevation data sets

Abstract. As part of the Greenland Ice Mapping Project (GIMP) we have produced three geospatial data sets for the entire ice sheet and periphery. These are (1) a complete, 15 m resolution image mosaic, (2) ice-covered and ice-free terrain classification masks, also posted to 15 m resolution, and (3) a complete, altimeter-registered digital elevation model posted at 30 m. The image mosaic was created from a combination of Landsat-7 and RADARSAT-1 imagery acquired between 1999 and 2002. Each pixel in the image is stamped with the acquisition date and geo-registration error to facilitate change detection. This mosaic was then used to manually produce complete ice-covered and ice-free land classification masks. Finally, we used satellite altimetry and stereo-photogrammetric digital elevation models (DEMs) to enhance an existing DEM for Greenland, substantially improving resolution and accuracy over the ice margin and periphery.

Ocean variability contributing to basal melt rate near the ice front of Ross Ice Shelf, Antarctica

AbstractBasal melting of ice shelves is an important, but poorly understood, cause of Antarctic ice sheet mass loss and freshwater production. We use data from two moorings deployed through Ross Ice Shelf, ∼6 and ∼16 km south of the ice front east of Ross Island, and numerical models to show how the basal melting rate near the ice front depends on sub‐ice‐shelf ocean variability. The moorings measured water velocity, conductivity, and temperature for ∼2 months starting in late November 2010. About half of the current velocity variance was due to tides, predominantly diurnal components, with the remainder due to subtidal oscillations with periods of a few days. Subtidal variability was dominated by barotropic currents that were large until mid‐December and significantly reduced afterward. Subtidal currents were correlated between moorings but uncorrelated with local winds, suggesting the presence of waves or eddies that may be associated with the abrupt change in water column thickness and strong hydrographic gradients at the ice front. Estimated melt rate was ∼1.2 ± 0.5 m a−1 at each site during the deployment period, consistent with measured trends in ice surface elevation from GPS time series. The models predicted similar annual‐averaged melt rates with a strong annual cycle related to seasonal provision of warm water to the ice base. These results show that accurately modeling the high spatial and temporal ocean variability close to the ice‐shelf front is critical to predicting time‐dependent and mean values of meltwater production and ice‐shelf thinning.

Surface reconstruction and derivation of erosion rates over several glaciations (1 Ma) in an alpine setting (Sinks Canyon, Wyoming, USA)

At middle to high latitudes, many alpine valleys have been shaped by glaciers associated with periods of Pleistocene glaciation. Present glaciated valleys are characterised by broadened valley floors and U-shaped cross sections, continuously formed by glacial activity from initially V-shaped, fluvial cross sections. Sinks Canyon (Wind River Range, USA) is a glaciated valley characterised by a typical U-shaped cross section, containing till from several glacial advances over a range of at least 1Ma. The morphostratigraphic records indicate a fourfold difference in ice surface elevation between the youngest and oldest glacial periods, which is not easily explained by the present-day valley topography. To assess possible evolution scenarios of Sinks Canyon, we modelled the palaeovalley topography using a geographic information system (GIS) filtering technique in combination with temporal reference points from relative and numerically dated glacial deposits. Ice thicknesses were calculated using the shallow ice approximation.In our model, the valley became shallower and the topography smoother with increasing years back in time. The results suggest that valley topography with ages between 640 and 1000ka can clearly be distinguished from the present-day topography. Surfaces with ages of 130–200ka (attributable to MIS 6; Bull Lake glaciation) still could be discerned from present-day topography, but with relatively high uncertainties. The method did not work for topography less than ~100ka or older than ~1Ma. Erosion depths were calculated using the differences between present-day elevation and the modelled surfaces. Calculated erosion rates were within the range of reference values for glacial erosion (0.001 to 1mm a−1). Glacial erosion appears to have removed 0.52 to 0.72mm a−1 of rock within a time frame of 1Ma, assuming 200ka of aggregated glacial flow. If the glacial occupation was longer or the impact of fluvial erosion was not negligible (as assumed), then the calculated rate is lower. If the model assumes a less extended time of glacial flow (<100ka), the rate exceeds 1mm a−1. The method used for surface reconstruction and erosion rates calculation is highly dependent on (i) the reliability of the temporal anchor points and (ii) the assumption of physical glacier conditions (basal shear stress).

Evidence for accelerating glacier ice loss in the Takht’e Solaiman Mountains of Iran from 1955 to 2010

This study reports on the clean ice area and surface elevation changes of the Khersan and Merjikesh glaciers in the north of Iran between 1955 and 2010 based on several high to medium spatial resolution remote sensing data. The object-oriented classification technique has been applied to nine remote sensing images to estimate the debris-free areas. The satellite-based analysis revealed that the clean ice areas of Khersan and Merjikesh glaciers shrank since 2010 with an overall area decrease of about 45% and 60% respectively. It means that the dramatic proportions of 1955 glaciers surface area are covered with debris during the last five decades. Although the general trend is a clean ice area decrease, some advancement is observed over the period of 1997–2004. During 1987–1991 the maximum decrease in the clean ice area was observed. However, the clean ice area had steadily increased between 1997 and 2010. To quantify the elevation changes besides the debris-free change analysis, several Digital Elevation Models (DEMs) were extracted from aerial photo (1955), topographic map (1997), ASTER image (2002) and Worldview-2 image (2010) and after it a 3-D Coregistration and a linear relationship adjustments techniques were used to remove the systematic shifts and elevation dependent biases. Unlike the sinusoidal variation of our case studies which was inferred from planimetric analysis, the elevation change results revealed that the glacier surface lowering has occurred during 1955–2010 continuously without any thickening with the mean annual thinning of about 0.4 ± 0.04 m per year and 0.3 ± 0.026 m per year for Khersan and Merjikesh glaciers, respectively. The maximum thinning rate has been observed during 1997–2002 (about 1.1 ± 0.09 per year and 0.96 ± 0.01 mper year, respectively), which was compatible partially with debris-free change analysis. The present result demonstrates that although in debris-covered glaciers clean ice area change analysis can illustrate the direction of changes (retreat or advance), due to the high uncertainty in glacier area delineation in such glaciers, it cannot reveal the actual glacier changes. Thus, both planimetric and volumetric change analyses are very critical to obtain accurate glacier variation results.

Boundary conditions of an active West Antarctic subglacial lake: implications for storage of water beneath the ice sheet

Abstract. Repeat-pass ICESat altimetry has revealed 124 discrete surface height changes across the Antarctic Ice Sheet, interpreted to be caused by subglacial lake discharges (surface lowering) and inputs (surface uplift). Few of these active lakes have been confirmed by radio-echo sounding (RES) despite several attempts (notable exceptions are Lake Whillans and three in the Adventure Subglacial Trench). Here we present targeted RES and radar altimeter data from an "active lake" location within the upstream Institute Ice Stream, into which at least 0.12 km3 of water was previously calculated to have flowed between October 2003 and February 2008. We use a series of transects to establish an accurate depiction of the influences of bed topography and ice surface elevation on water storage potential. The location of surface height change is downstream of a subglacial hill on the flank of a distinct topographic hollow, where RES reveals no obvious evidence for deep (&gt; 10 m) water. The regional hydropotential reveals a sink coincident with the surface change, however. Governed by the location of the hydrological sink, basal water will likely "drape" over topography in a manner dissimilar to subglacial lakes where flat strong specular RES reflections are measured. The inability of RES to detect the active lake means that more of the Antarctic ice sheet bed may contain stored water than is currently appreciated. Variation in ice surface elevation data sets leads to significant alteration in calculations of the local flow of basal water indicating the value of, and need for, high-resolution altimetry data in both space and time to establish and characterise subglacial hydrological processes.