Article

Satellite-derived volume loss rates and glacier speeds for the Juneau Icefield, Alaska

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Abstract

We provide a high-resolution map of elevation change rates dh/dt at the Juneau Icefield (JIF), southeastern Alaska, in order to quantify its contribution to sea-level rise between 2000 and 2009/2013. We also produce the first high-resolution map of ice speeds at the JIF, which we use to constrain flux and look for acceleration. We calculate dh/dt using stacked digital elevation models (DEMs) from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument and the Shuttle Radar Topography Mission (SRTM), taking into account SRTM C-band penetration via comparison with SRTM X-band elevations. Overall, the JIF is losing mass less rapidly (0.13±0.12 m w.e.a–1) than other Alaskan icefields (0.79 m w.e.a–1). We determine glacier speeds using pixel-tracking on optical image pairs acquired from 2001 to 2010 by ASTER, from radar image pairs acquired between 2007 and 2011 and from radar interferometry in 1995. We detect seasonal speed variations but no interannual acceleration, ruling out dynamics as the cause of the observed thinning. Thinning must therefore be due to the documented warming in the region. Flux measurements confirm this for Mendenhall Glacier, showing that calving constitutes only 2.5–5% of mass loss there.

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... The Stikine Icefield is the furthest south of the major northern hemisphere icefields (Figure 1). We focus on this area because it has not received recent detailed study unlike the nearby regions (Figure 1) of Glacier Bay (e.g., Johnson et al., 2013) and the Juneau Icefield (e.g., Melkonian et al., 2014) and our study builds upon the coverage of Larsen et al. (2015) that surveyed about one quarter of the icefield. ...
... The Land Processes Distributed Active Archive Center (LP DAAC) uses this stereo-imagery to produce an on-demand DEM for each ASTER acquisition requested (e.g., Fujisada et al., 2005). Full details of the methodology for the feature tracking and dh dt processing are given in Willis et al. (2012a), Melkonian et al. (2013), and Melkonian et al. (2014), as well as in the Supplementary Material. ...
... The total frontal ablation flux from these four glaciers is high enough at both the beginning and end of the 2000 to 2013/2014 dh dt period to accommodate the estimated additional −0.7 Gt yr −1 mass loss from marine-terminating glaciers above what it would be if they had the same dh dt by elevation as land-terminating glaciers (−0.8 Gt yr −1 as calculated in the last paragraph). The disproportionately high mass loss from marineterminating glaciers is in stark contrast with mass loss from the adjacent Juneau Icefield to the north (e.g., Melkonian et al., 2014) where the flux from marine-terminating glaciers is essentially zero. This is in part because the only "marine-terminating" glacier on the Juneau Icefield, Taku, has developed a terminal moraine through excavation of proglacial sediments that prevents it from calving (e.g., Criscitiello et al., 2010). ...
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We calculate thinning rates (dhdt) at the 5800 km2 Stikine Icefield of southeast Alaska from stacked digital elevation models (DEMs) acquired between 2000 and 2013/2014, and glacier velocities between 1985 and 2014 from feature tracking on optical image pairs. We find a mass change rate of −3.3 ± 1.1 Gt yr−1 between 2000 and 2014, equivalent to an area-averaged elevation change rate of −0.57 ± 0.18 m w.e. yr−1. In 2014, land-terminating glaciers are 50% of the Stikine Icefield's glaciated area and contribute −0.9 ± 0.4 Gt yr−1 of mass change (27% of the total), while marine-terminating glaciers are only 30% of the total glaciated area, but contribute −1.5 ± 0.3 Gt yr−1 (or 45% of total mass change, with the remaining mass loss from lacustrine-terminating glaciers). We estimate the frontal ablation flux between 2000 and 2014 at the four largest marine-terminating glaciers on the Stikine Icefield (covering 90–95% of the marine-terminating glaciated area) using our glacier velocities and maps of fjord bathymetry to estimate terminus cross sections and glacier thicknesses. The combined 2014 frontal ablation flux of these four glaciers is 1.18 ± 0.14 Gt yr−1, which may account for the difference in average mass loss between marine- and land-terminating glaciers on the Stikine Icefield. The Stikine and adjacent Juneau Icefields have very different mass loss contributions from marine-terminating glaciers (45% vs. effectively 0%), but both have area-averaged elevation change rates that are less negative than Alaska-wide estimates, which is surprising for these southernmost icefields in Alaska.
... We assigned an initial 5 m uncertainty to the SRTM DEM following Carabajal and Harding (2005) and Rodriguez et al. (2006). Because the SRTM DEM is a C-band radar product, it is subject to snow and ice penetration that must be corrected before it is included in the DEM time series (e.g., Willis et al., 2012b;Melkonian et al., 2014;Berthier et al., 2016). Previous studies in the Juneau Icefield and Southern Patagonian Icefield found that the C-band SRTM DEM has a maximum penetration depth of 2-3 m in these regions compared to its X-band counterpart which, due to its smaller wavelength, is assumed to have a relatively small radar penetration depth (Willis et al., 2012b;Melkonian et al., 2014). ...
... Because the SRTM DEM is a C-band radar product, it is subject to snow and ice penetration that must be corrected before it is included in the DEM time series (e.g., Willis et al., 2012b;Melkonian et al., 2014;Berthier et al., 2016). Previous studies in the Juneau Icefield and Southern Patagonian Icefield found that the C-band SRTM DEM has a maximum penetration depth of 2-3 m in these regions compared to its X-band counterpart which, due to its smaller wavelength, is assumed to have a relatively small radar penetration depth (Willis et al., 2012b;Melkonian et al., 2014). Icy Bay, the Juneau Icefield, and the Southern Patagonian Icefield are similar in that they are regions of temperate glaciers with high mass-turnover rates. ...
... Raw ALOS SAR imagery were prepared for pixel-tracking using the Repeat Observation Interferometry PACkage (ROI_PAC: Rosen et al., 2004). Once the images were orthorectified and prepared, we performed pixel-tracking through normalized cross-correlation using the "ampcor" function of the ROI_PAC software as implemented by Melkonian et al. (2014). Uncertainties for each velocity pair were calculated as the median velocity of the off-ice portions, which should be zero (e.g., Willis et al., 2012a), and velocity maps with uncertainties >0.5 m day −1 were excluded from the time series. ...
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Since 1990, Yahtse Glacier in southern Alaska has advanced at an average rate of ~100 m year−1 despite a negative mass balance, widespread thinning in its accumulation area, and a low accumulation-area ratio. To better understand the interannual and seasonal changes at Yahtse and the processes driving these changes, we construct velocity and ice surface elevation time series spanning the years 1985–2016 and 2000–2014, respectively, using satellite optical and synthetic aperture radar (SAR) observations. We find contrasting seasonal dynamics above and below a steep (up to 35% slope) icefall located approximately 6 km from the terminus. Above the icefall, velocities peak in May and reach their minima in October synchronous with the development of a small embayment at the calving terminus. The up-glacier minimum speeds, embayment, and plume of turbid water that emerges from the embayment are consistent with an efficient, channelized subglacial drainage system that lowers basal water pressures and leads to focused submarine melt in the calving embayment. However, velocities near the terminus are fastest in the winter, following terminus retreat, possibly off of a terminal moraine that results in decreased backstress. Between 1996 and 2016 the terminus decelerated by ~40% at an average rate of ~0.4 m day−1 year−1, transitioned from tensile to compressive longitudinal strain rates, and dynamically thickened at rates of 1-6 m year−1, which we hypothesize is in response to the development and advance of a terminal moraine. The described interannual changes decay significantly upstream of the icefall, indicating that the icefall may inhibit the upstream transmission of stress perturbations. We suggest that diminished stress transmission across the icefall could allow moraine-enabled terminus advance despite mass loss in Yahtse's upper basin. Our work highlights the importance of glacier geometry in controlling tidewater glacier re-advance, particularly in a climate favoring increasing equilibrium line altitudes.
... Once the images in a pair are orthorectified, we use normalized crosscorrelation to perform feature tracking using the "ampcor" tool from the Repeat Orbit Interferometry PACkage ( ROI_PAC Rosen, Hensley, Peltzer, and Simons, 2004). Full details of the methodology are given in Willis, Melkonian, Pritchard, and Ramage (2012), Melkonian et al. (2013) and Melkonian, Willis, and Pritchard (2014). We use velocities from 57 image pairs in this study (date intervals shown in Fig. S1). ...
... We S2) and have comprehensive spatial coverage but are not as accurate as WorldView DEMs (e.g., (Fujisada et al., 2005;Berthier et al., 2010;Willis, Melkonian, Pritchard, and Ramage, 2012;Willis, Melkonian, Pritchard, and Rivera, 2012;Melkonian et al., 2013Melkonian et al., , 2014, particularly at higher elevations within the accumulation zone where snow cover predominates and is often featureless at the resolution of ASTER imagery (15 m per pixel). ASTER DEMs are, however, reliable over the exposed ice and bedrock at lower elevations, where we see the highest thinning rates. ...
... ASTER DEMs are, however, reliable over the exposed ice and bedrock at lower elevations, where we see the highest thinning rates. Willis, Melkonian, Pritchard, and Ramage (2012), Willis, Melkonian, Pritchard, and Rivera (2012), Melkonian et al. (2013Melkonian et al. ( , 2014 are examples of similar processing applied to ASTER DEMs over other glaciated regions. Details of processing the ASTER/WorldView dh dt are provided in the supplemental material. ...
Article
Ice loss from the glaciers of Novaya Zemlya (NVZ) is the dominant contributor to sea level for the Russian Arctic. Here we present maps of glacier elevation change rates (dhdt) and velocities at Novaya Zemlya that reveal evidence of the impact of both calving flux and surface melt on ice mass loss.dhdt are calculated by applying a weighted linear regression to stacked digital elevation models (DEMs) from a 1952 map and DEMs created from stereo-pairs of WorldView images acquired between 2012 and 2014. The ~60-year average mass change rate is -0.23±0.04 mw.e.yr-1. The recent, short-term (2012-2013/2014) average mass change rate is -0.40±0.09 m w.e. yr-1. We apply the same weighted linear regression to stacked WorldView DEMs and ICESat elevations from 2003 to 2009, as well as stacked WorldView DEMs and ASTER DEMs from 2000 to 2013. The dhdt from both datasets confirm that thinning has been more rapid than the 60-year average since 2000.Average mass loss has accelerated at the lower elevations of land-terminating glaciers, suggesting that recent near-surface warming and increased melt have led to greater thinning. Marine-terminating glaciers along the Barents Sea coast have substantially higher thinning rates near their termini in recent years than adjacent land-terminating glaciers, suggesting that calving flux is an important contributor to overall ice mass loss in the region.Flow velocities are generated by feature tracking on optical and radar image pairs. Marine-terminating glaciers along the Barents Sea coast have the highest frontal velocities of any glaciers on NVZ. Our surface velocity measurements constrain the calving flux at the two fastest-retreating glaciers, Inostrantseva (INO) and Vil'kitskogo (VIS), which are both on the Barents Sea coast. We find that the calving flux is high enough to drive frontal thinning and retreat at those two glaciers. Summer terminus speeds at Inostrantseva Glacier have accelerated from a 2006 maximum of 3 m day-1 to a 2012 maximum of 9 to 10 m day-1 and the front has retreated by more than 3km between 2006/06/27 and 2013/08/17.
... If correct, the estimates of Melkonian et al. ( , 2016 would imply a considerable slowdown of the mass loss of the Juneau and, to a smaller extent, Stikine icefields during the first decade of the 21st century. However, no clear trend in climate such as cooling or increased precipitation was found during this period to explain such a slowdown Ziemen et al., 2016). ...
... Field observations of the equilibrium line altitudes and surface mass balances on Lemon Creek and Taku glaciers (JIF) also do not support a slowdown (WGMS, 2017). The estimates of Melkonian et al. ( , 2016 used as a starting elevation measurement the C-band SRTM DEM acquired in February 2000, the core of winter in Alaska. The C-band radar signal is known to penetrate into the cold winter snow and firn such that SRTM maps a surface below the real glacier surface which can bias the elevation change measurements (e.g., Berthier et al., 2006;Rignot et al., 2001). ...
... The C-band radar signal is known to penetrate into the cold winter snow and firn such that SRTM maps a surface below the real glacier surface which can bias the elevation change measurements (e.g., Berthier et al., 2006;Rignot et al., 2001). Melkonian et al. ( , 2016 accounted for this penetration by subtracting the simultaneous C-band and X-band SRTM DEMs, assuming no penetration of the X-band DEM (Gardelle et al., 2012), the best available correction at the time of their study. However, this strategy may not be appropriate given that the X-band penetration depth has recently been recognized to reach several meters in cold and dry snow/firn (e.g., Dehecq et al., 2016;Round et al., 2017). ...
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The large Juneau and Stikine icefields (Alaska) lost mass rapidly in the second part of the 20th century. Laser altimetry, gravimetry and field measurements suggest continuing mass loss in the early 21st century. However, two recent studies based on time series of Shuttle Radar Topographic Mission (SRTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation models (DEMs) indicate a slowdown in mass loss after 2000. Here, the ASTER-based geodetic mass balances are recalculated carefully avoiding the use of the SRTM DEM because of the unknown penetration depth of the C-band radar signal. We find strongly negative mass balances from 2000 to 2016 (−0.68 ± 0.15 m w.e. a⁻¹ for the Juneau Icefield and −0.83 ± 0.12 m w.e. a⁻¹ for the Stikine Icefield), in agreement with laser altimetry, confirming that mass losses are continuing at unabated rates for both icefields. The SRTM DEM should be avoided or used very cautiously to estimate glacier volume change, especially in the North Hemisphere and over timescales of less than ∼ 20 years.
... Several different methods have been applied in the literature, and we briefly summarize them here. They include bilinear interpolation of elevation or elevation differences (e.g., Kääb, 2008), filling with an average value from a surrounding neighborhood (e.g., Melkonian et al., 2013Melkonian et al., , 2014, multiplying the average elevation change by the total glacier-covered area (e.g., Surazakov and Aizen, 2006;Paul and Haeberli, 2008;Fischer et al., 2015), and estimating elevation change as a function of elevation, integrating this curve with the glacier hypsometry (e.g., Arendt et al., 2002Arendt et al., , 2006Kohler et al., 2007;Berthier et al., 2010;Kronenberg et al., 2016). In addition, we can classify these methods into "global" (here meaning encompassing the whole of the dataset as opposed to worldwide) and "local" types, whereby global methods use data from an entire region or group of glaciers, while local methods fill voids using only information from an individual glacier or from data closely surrounding the voids. ...
... The SRTM was acquired in February 2000 aboard the Space Shuttle Endeavour, flying both C-band and X-band instruments (Van Zyl, 2001). This nearly global DEM is temporally consistent and therefore ideal and commonly used for geodetic mass balance estimation (e.g., Surazakov and Aizen, 2006;Larsen et al., 2007;Melkonian et al., 2013Melkonian et al., , 2014, though typically over longer time periods (> 10-year separation between DEMs). We have selected this dataset, and not the US National Elevation Dataset (NED) as other studies have used in the region (e.g., Arendt et al., 2002Arendt et al., , 2006Larsen et al., 2007;Berthier et al., 2010), as the NED DEM was produced by digitizing 1948 USGS contour maps (Larsen et al., 2007) that contained large biases at higher elevations on glaciers (e.g., Arendt et al., 2002). ...
... based on on-glacier pixels within a 1 km radius of the void pixel. Examples of this approach can be found in Melkonian et al. (2013Melkonian et al. ( , 2014. ...
Article
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Glacier mass balance has been estimated on individual glacier and regional scales using repeat digital elevation models (DEMs). DEMs often have gaps in coverage (“voids”), the properties of which depend on the nature of the sensor used and the surface being measured. The way that these voids are accounted for has a direct impact on the estimate of geodetic glacier mass balance, though a systematic comparison of different proposed methods has been heretofore lacking. In this study, we determine the impact and sensitivity of void interpolation methods on estimates of volume change. Using two spatially complete, high-resolution DEMs over southeast Alaska, USA, we artificially generate voids in one of the DEMs using correlation values derived from photogrammetric processing of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) scenes. We then compare 11 different void interpolation methods on a glacier-by-glacier and regional basis. We find that a few methods introduce biases of up to 20 % in the regional results, while other methods give results very close (<1 % difference) to the true, non-voided volume change estimates. By comparing results from a few of the best-performing methods, an estimate of the uncertainty introduced by interpolating voids can be obtained. Finally, by increasing the number of voids, we show that with these best-performing methods, reliable estimates of glacier-wide volume change can be obtained, even with sparse DEM coverage.
... In Alaska, Burgess et al. (2013) presented the first comprehensive flow map for glaciers in Central Alaska (including the Kenai Peninsula), but speeds were based on winter images only. Melkonian et al. (2014) derived surface velocities by SAR offset tracking, for the Juneau Icefield, while Armstrong et al. (2017) employed optical offset tracking to derive velocity fields for South Central Alaska glaciers, and Altena et al. (2019) extracted glacier velocity over southern Alaska and Yukon from Landsat data. Other studies have determined ice velocity fields of larger ice caps for the purpose of computing frontal ablation rates rather than a detailed analysis of ice motion (e.g., Antarctic periphery (Osmanoğlu et al. (2013); Osmanoglu et al. (2014)) and Canada's Queen Elizabeth Islands (Van Wychen et al., 2014)). ...
... Monthly variations over the 5-year period are largely synchronous among the three glacier types. In addition, annual mean speeds remained relatively stable with an interannual variability of ±10% of the period mean, consistent with findings on the Juneau Icefield, Alaska, between 1995 and 2011 (Melkonian et al., 2014), and Taku Glacier, Alaska, during 1950(M. S. Pelto et al., 2008. ...
Article
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To characterize the spatiotemporal variations of glacier surface speed on the Kenai Peninsula, Alaska (∼3,900 km²), we derived 92 surface speed fields between October 2014 and December 2019 using intensity offset tracking on Sentinel‐1 data. On average, speeds are 50% greater in spring (March‐May) than the annual mean (69 m a⁻¹) while winter speeds are close to the annual mean. While marine‐terminating glaciers have their maximum speed near the terminus, both land‐ and lake‐terminating glaciers flow fastest around the median glacier elevation. On average, the lake‐terminating and tidewater glaciers flow 1.7 and 2.3 times faster than the land‐terminating glaciers, respectively. Monthly variations over the 5‐year period are strikingly synchronous regardless of terminus type suggesting that regional‐scale meteorological drivers govern the temporal variability. Mean annual speeds fluctuate roughly ±10% of the period mean without an apparent trend. At lake‐terminating Bear Glacier, a short‐term tripling in ice speed in fall 2019 over the area below an ice‐dammed lake coincides with an observed glacier lake outburst flood (GLOF). An earlier GLOF caused a persistent breach of the beach barrier between the proglacial lake and ocean which likely led to overall speed‐up of the lower glacier part throughout 2019. A significant speedup was also observed at the lower part of the lake‐terminating Ellsworth Glacier and attributed to rapid glacier retreat and lake expansion, probably further amplified by the terminus area becoming buoyant and a large tabular iceberg breaking off. Our results highlight the impact of GLOFs and proglacial characteristics in spatial and temporal glacier speed variations.
... For each void pixel, we calculate the average elevation difference based on on-glacier pixels within a 1 km radius of the void pixel. Examples of this approach can be found in Melkonian et al. (2013Melkonian et al. ( , 2014. ...
... general, the pattern of elevation change is negative, especially at lower elevations, as noted in other studies (e.g., Larsen et al., 2007;Johnson et al., 2013;Melkonian et al., 2013Melkonian et al., , 2014Berthier et al., 2018). Some exceptions include Margerie, Johns ...
Article
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Glacier mass balance is a direct expression of climate change, with implications for sea level, ocean chemistry, oceanic and terrestrial ecosystems, and water resources. Traditionally, glacier mass balance has been estimated using in-situ measurements of changes in surface height and density at select locations on the glacier surface, or by comparing changes in surface height using repeat, full-coverage digital elevation models (DEMs), also called the geodetic method. DEMs often have gaps in coverage (voids) based on the nature of the sensor used and the surface being measured. The way that these voids are accounted for has a direct impact on the estimate of geodetic glacier mass balance, though a systematic comparison of different proposed methods has been heretofore lacking. In this study, we determine the impact and sensitivity of void-filling methods on estimates of volume change. Using two spatially complete, high-resolution DEMs over Southeast Alaska, USA, we compare 11 different void-filling methods on a glacier-by-glacier and regional basis. We find that a few methods introduce biases of up to 20% in the regional results, while other methods give results very close (1% difference) to the true, non-voided volume change estimates. Finally, we independently show using ASTER DEMs that some of best-performing methods are more robust than others, depending on the properties of the original DEMs, and therefore recommend that studies compare a few of these methods to estimate the uncertainty introduced by filling DEM voids.
... Recently, this limitation has partly been overcome by processing less precise but numerous DEMs derived from optical stereo-imagery acquired by the Advanced Spaceborne Thermal Emission and Reflection (ASTER) sensor onboard the TERRA satellite. This multi-temporal ASTER DEM strategy has been pioneered over the Everest area (Nuimura et al., 2012) and the Northern Patagonian Icefield (Willis et al., 2012a), and later applied to the Southern Patagonian and Cordillera Darwin icefields (Willis et al., 2012b;Melkonian et al., 2013), to the Juneau Icefield in Alaska (Melkonian et al., 2014) and to two glaciers in New-Zealand (Wang and Kääb, 2015). Among these studies, only Nuimura et al. (2012) had some reference Global Navigation Satellite System (GNSS) data to evaluate the accuracy of their elevation change measurements on glaciers. ...
... The mean penetration depth (about 9 ± 3 m) is in remarkable agreement with a value of 8 m estimated independently for all Swiss glaciers (Fischer et al., 2015) and with the 5 to 10 m underestimation of the SRTM elevations already reported over Mont-Blanc glaciers (Berthier et al., 2006). An implication of these large penetration depths is that, contrary to what was done in some earlier studies (e.g., Melkonian et al., 2014), the SRTM DEM should not be used in the linear trend analysis to extract dh/dt from ASTER DEMs. This recommendation applies mostly to the northern hemisphere where SRTM was acquired in the core of winter (February 2000). ...
Article
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Since 2000, a vast archive of stereo-images has been built by the Advanced Spaceborne Thermal Emission and Reflection (ASTER) satellite. Several studies already extracted glacier mass balances from multi-temporal ASTER digital elevation models (DEMs) but they lacked accurate independent data for validation. Here, we apply a linear regression to a time series of 3D-coregistered ASTER DEMs to estimate the rate of surface elevation changes (dh/dtASTER) and geodetic mass balances of Mont-Blanc glaciers (155 km²) between 2000 and 2014. Validation using field and spaceborne geodetic measurements reveals large errors at the individual pixel level (> 1 m a-1) and an accuracy of 0.2-0.3 m a-1 for dh/dtASTER averaged over areas larger than 1 km². For all Mont-Blanc glaciers, the ASTER region-wide mass balance (-1.05±0.37 m water equivalent (w.e.) a-1) agrees remarkably with the one measured using Spot5 and Pléiades DEMs (-1.06±0.23 m w.e. a-1) over their common 2003-2012 period. This multi-temporal ASTER DEM strategy leads to smaller errors than the simple differencing of two ASTER DEMs. By extrapolating dh/dtASTER to mid-February 2000, we infer a mean penetration depth of about 9±3 m for the C-band Shuttle Radar Topographic Mission (SRTM) radar signal, with a strong altitudinal dependency (range 0-12 m). This methodology thus reveals the regional pattern of glacier surface elevation changes and improves our knowledge of the penetration of the radar signal into snow and ice.
... Whereas optical images portray the surface of glaciers and snow, however, radar signals penetrate ice and dry snow to varying depths dependent on snow and ice properties (i.e., moisture content and purity), as well as the properties of the signal itself (e.g., Rignot et al., 2001;Shugar et al., 2010). With simultaneously-acquired data of different frequency (i.e., SRTM Cband and X-band data), it is possible to estimate and correct for penetration effects locally, though these approaches are limited in extent and not universally applicable Melkonian et al., 2014). Accuracy of radar interferometric DEMs is also dependent on precise knowledge of satellite orbital parameters, which tends to be lacking in earlier interferometric missions. ...
... As noted by Paul (2008), these effects are most likely related to resampling of elevation data, introduced because of the curvature of high-elevation terrain, and not because of elevation per se. Further studies have extended these findings (e.g., Gardelle et al., 2012) to correct elevation biases using the maximum terrain curvature, and implemented in other studies using the SRTM data (e.g., Willis et al., 2012;Gardelle et al., 2013;Melkonian et al., 2013Melkonian et al., , 2014. ...
Article
Satellite data provide a large range of information on glacier dynamics and changes. Results are often reported, provided and used without consideration of measurement accuracy (difference to a true value) and precision (variability of independent assessments). Whereas accuracy might be difficult to determine due to the limited availability of appropriate reference data and the complimentary nature of satellite measurements, precision can be obtained from a large range of measures with a variable effort for determination. This study provides a systematic overview on the factors influencing accuracy and precision of glacier area, elevation change (from altimetry and DEM differencing), and velocity products derived from satellite data, along with measures for calculating them. A tiered list of recommendations is provided (sorted for effort from Level 0 to 3) as a guide for analysts to apply what is possible given the datasets used and available to them. The more simple measures to describe product quality (Levels 0 and 1) can often easily be applied and should thus always be reported. Medium efforts (Level 2) require additional work but provide a more realistic assessment of product precision. Real accuracy assessment (Level 3) requires independent and coincidently acquired reference data with high accuracy. However, these are rarely available and their transformation into an unbiased source of information is challenging. This overview is based on the experiences and lessons learned in the ESA project Glaciers_cci rather than a review of the literature.
... This is especially true for marine terminating glaciers, where measurements of ice motion can be combined with ice thickness information to determine frontal ablation rates, and hence the contribution of glaciers and ice sheets to global sea level rise. In light of this, in recent years there has been a concerted effort to map glacier dynamics at the regional scale for many of the glaciated regions of the world: Antarctica [1], the Greenland Ice Sheet [2,3], the majority of Alaskan glaciers [4,5], the Canadian High Arctic [6][7][8] and South America [9][10][11]. ...
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Glacier dynamics play an important role in the mass balance of many glaciers, ice caps and ice sheets. In this study we exploit Radarsat-2 (RS-2) Wide Fine (WF) data to determine the surface speed of Svalbard glaciers in the winters of 2012/2013 and 2013/2014 using Synthetic Aperture RADAR (SAR) offset and speckle tracking. The RS-2 WF mode combines the advantages of the large spatial coverage of the Wide mode (150 × 150 km) and the high pixel resolution (9 m) of the Fine mode and thus has a major potential for glacier velocity monitoring from space through offset and speckle tracking. Faster flowing glaciers (1.95 m·d−1–2.55 m·d−1) that are studied in detail are Nathorstbreen, Kronebreen, Kongsbreen and Monacobreen. Using our Radarsat-2 WF dataset, we compare the performance of two SAR tracking algorithms, namely the GAMMA Remote Sensing Software and a custom written MATLAB script (GRAY method) that has primarily been used in the Canadian Arctic. Both algorithms provide comparable results, especially for the faster flowing glaciers and the termini of slower tidewater glaciers. A comparison of the WF data to RS-2 Ultrafine and Wide mode data reveals the superiority of RS-2 WF data over the Wide mode data.
... This idea is supported by the well-developed englacial drainage network, hydrofracturing process, and presence of pressurized water beneath the Waldemarbreen [32,55]. Similar ice bed lubrication has been suggested, for example, by Kraaijenbrink et al. [44] for a debris-covered Himalayan glacier and by Melkonian et al. [78] for glaciers in Alaska. All these mentioned examples showed the peak velocities during the summer related to higher liquid water input from increased melt rates and rainwater. ...
Article
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Unmanned Aerial Vehicles (UAVs) are being increasingly used in glaciology demonstrating their potential for the generation of high-resolution digital elevation models (DEMs) that can be further used for the evaluation of glacial processes in detail. Such investigations are especially important for the evaluation of surface changes of small valley glaciers, which are not well-represented in lower-resolution satellite-derived products. In this study, we performed two UAV surveys at the end of the ablation season in 2019 and 2021 on Waldemarbreen, a High-Arctic glacier in NW Svalbard. We derived the mean annual glacier surface velocity of 5.3 m. The estimated mean glacier surface elevation change from 2019 to 2021 was −1.46 m a−1 which corresponds to the geodetic mass balance (MB) of −1.33 m w.e. a−1. The glaciological MB for the same period was −1.61 m w.e. a−1. Our survey includes all Waldemarbreen and demonstrates the efficiency of high-resolution DEMs produced from UAV photogrammetry for the reconstruction of changes in glacier surface elevation and velocity. We suggest that glaciological and geodetic MB methods should be used complementary to each other.
... Many of the glaciers have been identified as surge type based on direct observa- tions or from their looped moraines Herreid and Truffer, 2016). Furthermore, glacier elevation change in this region is heterogeneous ( Muskett et al., 2003;Berthier et al., 2010;Melkonian et al., 2014), providing another indication of complicated responses. Gaining a better understanding of causes of glacier mass redistribution is necessary in order to separate surging and seasonal variation from longer-term trends. ...
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The measurement of glacier velocity fields using repeat satellite imagery has become a standard method of cryospheric research. However, the reliable discovery of important glacier velocity variations on a large scale is still problematic because time series span different time intervals and are partly populated with erroneous velocity estimates. In this study we build upon existing glacier velocity products from the GoLIVE dataset (https://nsidc.org/data/golive, last access: 26 February 2019) and compile a multi-temporal stack of velocity data over the Saint Elias Mountains and vicinity. Each layer has a time separation of 32 days, making it possible to observe details such as within-season velocity change over an area of roughly 150 000 km2. Our methodology is robust as it is based upon a fuzzy voting scheme applied in a discrete parameter space and thus is able to filter multiple outliers. The multi-temporal data stack is then smoothed to facilitate interpretation. This results in a spatiotemporal dataset in which one can identify short-term glacier dynamics on a regional scale. The goal is not to improve accuracy or precision but to enhance extraction of the timing and location of ice flow events such as glacier surges. Our implementation is fully automatic and the approach is independent of geographical area or satellite system used. We demonstrate this automatic method on a large glacier area in Alaska and Canada. Within the Saint Elias and Kluane mountain ranges, several surges and their propagation characteristics are identified and tracked through time, as well as more complicated dynamics in the Wrangell Mountains.
... This is especially true for marine terminating glaciers, where measurements of ice motion can be combined with ice thickness information to determine frontal ablation rates, and hence the contribution of glaciers and ice sheets to global sea level rise. In light of this, in recent years there has been a concerted effort to map glacier dynamics at the regional scale for many of the glaciated regions of the world: Antarctica [1], the Greenland Ice Sheet [2,3], the majority of Alaskan glaciers [4,5] the Canadian High Arctic [6][7][8] and South America [9][10][11]. ...
Article
Glacier dynamics play an important role in the mass balance of many glaciers, ice caps and ice sheets. In this study we exploit Radarsat-2 (RS-2) Wide Fine (WF) data to determine the surface speed of Svalbard glaciers in the winters of 2012/2013 and 2013/2014 using Synthetic Aperture RADAR (SAR) offset and speckle tracking. The RS-2 WF mode combines the advantages of the large spatial coverage of the Wide mode (150 x 150 km) and the high pixel resolution (9m) of the Fine mode and thus has a major potential for glacier velocity monitoring from space through offset and speckle tracking. Faster flowing glaciers (1.95 m d-1 - 2.55 m d-1) which are studied in detail are Nathorstbreen, Kronebreen, Kongsbreen and Monacobreen. Using our Radarsat-2 WF dataset, we compare the performance of two SAR tracking algorithms, namely the GAMMA Remote Sensing Software and a custom written MATLAB script (GRAY method) that has primarily been used in the Canadian Arctic. Both algorithms provide comparable results, especially for the faster flowing glaciers and the termini of slower tidewater glaciers. A comparison of the WF data to RS-2 Ultrafine and Wide mode data reveals the superiority of RS-2 WF data over the Wide mode data.
... The previous minimum mass balance was −1.34 m in 1997. The trend of increasing negative balance has been observed for Taku Glacier, Alaska and globally [4,[19][20][21]. The rate of TSL rise during July 2018 was the highest observed to date, indicating the maximum identified ablation rate. ...
Article
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The Juneau Icefield Research Program (JIRP) has been examining the glaciers of the Juneau Icefield since 1946. The height of the transient snowline (TSL) at the end of the summer represents the annual equilibrium line altitude (ELA) for the glacier, where ablation equals accumulation. On Taku Glacier the ELA has been observed annually from 1946 to 2018. Since 1998 multiple annual observations of the TSL in satellite imagery identify both the migration rate of the TSL and ELA. The mean ELA has risen 85 ± 10 m from the 1946–1985 period to the 1986–2018 period. In 2018 the TSL was observed at: 900 m on 5 July; 975 m on 21 July; 1075 m on 30 July; 1400 m on 16 September; and 1425 m on 1 October. This is the first time since 1946 that the TSL has reached or exceeded 1250 m on Taku Glacier. The 500 m TSL rise from 5 July to 30 July, 8.0. md−1, is the fastest rate of rise observed. This combined with the observed balance gradient in this region yields an ablation rate of 40–43 mmd−1, nearly double the average ablation rate. On 22 July a snow pit was completed at 1405 m with 0.93 m w.e. (water equivalent), that subsequently lost all snow cover, prior to 16 September. This is one of eight snow pits completed in July providing field data to verify the ablation rate. The result of the record ELA and rapid ablation is the largest negative annual balance of Taku Glacier since records began in 1946.
... Icefield (JIF) is located in the northern Coast Mountains and crosses the border from southeast Alaska to British Columbia. At 3,830 km 2 , the JIF is the fifth largest icefield in the Western Hemisphere(Melkonian et al., 2014). Icefield elevations range from sea level in the southwest, where the city of Juneau, Alaska is located, to ~2,500 m a.s.l.(Roth et al., 2018). ...
Article
Understanding glacial erosion rates is important because debris eroded by a glacier can impact glacier flow speeds, protect tidewater glaciers from rapid retreat, and impact the productivity of marine ecosystems. Traditionally, glacial erosion models rely on a rock’s inherent “erodibility”, typically presented as a constant, to predict how much debris will be eroded by the glacier. However, the erodibility of bedrock varies spatially as a function of its fracture density, fracture orientation, and lithology, so the notion of applying a constant erodibility term to a whole field site does not fully capture the actual bedrock dynamics of the system. In this work, I present a novel approach to quantify bedrock fracture density and orientation through the generation of a 3D Structure from Motion (SfM) model and the application of a series of machine learning algorithms. To test this approach, I quantified the fracture density of a glacial bedrock nunatak in the Juneau Icefield of Southeast (SE) Alaska. The spatial variation in fracture density across this nunatak was found to be highly variable. Bedrock in the SE region of this field site showed a relatively high fracture density (>20% fractured), whereas the central region of this field site showed a relatively low fracture density (0-10% fractured). Fracture orientations were shown to have a bimodal distribution, with the most common fracture orientations being approximately 0 and ± 90 degrees. This fracture density methodology and associated results can applied across the Juneau Icefield and other glacier systems to improve glacial bedrock erosion models.
... As an important model parameter, glacier centerline can be used to determine the change of glacier length (Leclercq et al., 2012a;Nuth et al., 2013), analyze the velocity field (Heid and Kääb, 2012;Melkonian et al., 2017), estimate the glacier ice volume (Li et al., 2012;Linsbauer et al., 2012), and develop one-dimensional glacier model (Oerlemans, 1997;Sugiyama et al., 35 2007). Meanwhile, the length of the longest glacier centerline is one of the key determinants of glacier geometry and an important parameter of glacier inventory (Leclercq et al., 2012b;Paul et al., 2009). ...
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Glacier centerlines are crucial input for many glaciological applications. From the morphological perspective, we proposed a new automatic method to derive glacier centerlines, which is based on the Euclidean allocation and the terrain characteristics of glacier surface. In the algorithm, all glaciers are logically classified as three types including simple glacier, simple compound glacier and complex glacier, with corresponding process ranges from simple to complex. The process for extracting centerlines of glaciers introduces auxiliary reference lines, and follows the setting of not passing through bare rock. The program of automatic extraction of glacier centerline was implemented in Python and only required glacier boundary and digital elevation model (DEM) as input. Application of this method to 48571 glaciers in the second Chinese glacier inventory automatically yielded the corresponding glacier centerlines with an average computing time of 20.96 s, a success rate of 100 % and a comprehensive accuracy of 94.34 %. A comparison of the longest length of glaciers to the corresponding glaciers in the Randolph Glacier Inventory v6.0 revealed that our results were more superior. Meanwhile, our final product provided more information about glacier length, such as the average length, the largest length, the lengths in the accumulation and ablation regions of each glacier.
... As an important model parameter, the glacier centerline can be used to determine the change of glacier length (Leclercq et al., 2012a;Nuth et al., 2013), analyze the velocity field (Heid and Kääb, 2012;Melkonian et al., 2017), estimate the glacier ice volume (Li et al., 2012;Linsbauer et al., 2012) and develop one-dimensional glacier models (Oerlemans, 1997;Sugiyama et al., 2007). Meanwhile, the length of the longest glacier centerline is one of the key determinants of glacier geometry and an important parameter of a glacier inventory (Paul et al., 2009;Leclercq et al., 2012b). ...
Article
Full-text available
Glacier centerlines are crucial input for many glaciological applications. From the morphological perspective, we proposed a new automatic method to derive glacier centerlines, which is based on the Euclidean allocation and the terrain characteristics of glacier surface. In the algorithm, all glaciers are logically classified as three types including simple glacier, simple compound glacier, and complex glacier, with corresponding process ranges from simple to complex. The process for extracting centerlines of glaciers introduces auxiliary reference lines and follows the setting of not passing through bare rock. The program of automatic extraction of glacier centerlines was implemented in Python and only required the glacier boundary and digital elevation model (DEM) as input. Application of this method to 48 571 glaciers in the second Chinese glacier inventory automatically yielded the corresponding glacier centerlines with an average computing time of 20.96 s, a success rate of 100 % and a comprehensive accuracy of 94.34 %. A comparison of the longest length of glaciers to the corresponding glaciers in the Randolph Glacier Inventory v6.0 revealed that our results were superior. Meanwhile, our final product provides more information about glacier length, such as the average length, and the longest length, the lengths in the accumulation and ablation regions of each glacier.
... These have primarily relied on geodetic approaches (e.g., digital elevation model differencing) that determine bulk volume loss between two known dates. Despite sourcing imagery from different satellite sensors and covering different time spans, all studies calculated negative glacier-wide mass balance rates over the investigated periods between 1962 and 2016 (Berthier et al., 2010(Berthier et al., , 2018Larsen et al., 2007;Melkonian et al., 2014). A recent study has also modeled future glacier mass balance for the icefield under different climate scenarios, projecting a volume loss of 58%-68% of the icefield by 2100 (Ziemen et al., 2016). ...
Article
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With a unique biogeophysical signature relative to other freshwater sources, meltwater from glaciers plays a crucial role in the hydrological and ecological regime of high latitude coastal areas. Today, as glaciers worldwide exhibit persistent negative mass balance, glacier runoff is changing in both magnitude and timing, with potential downstream impacts on infrastructure, ecosystems, and ecosystem resources. However, runoff trends may be difficult to detect in coastal systems with large precipitation variability. Here, we use the coupled energy balance and water routing model SnowModel-HydroFlow to examine changes in timing and magnitude of runoff from the western Juneau Icefield in Southeast Alaska between 1980 and 2016. We find that under sustained glacier mass loss (−0.57 ± 0.12 m w. e. a−1), several hydrological variables related to runoff show increasing trends. This includes annual and spring glacier ice melt volumes (+10% and +16% decade−1) which, because of higher proportions of precipitation, translate to smaller increases in glacier runoff (+3% and +7% decade−1) and total watershed runoff (+1.4% and +3% decade−1). These results suggest that the western Juneau Icefield watersheds are still in an increasing glacier runoff period prior to reaching “peak water.” In terms of timing, we find that maximum glacier ice melt is occurring earlier (2.5 days decade−1), indicating a change in the source and quality of freshwater being delivered downstream in the early summer. Our findings highlight that even in maritime climates with large precipitation variability, high latitude coastal watersheds are experiencing hydrological regime change driven by ongoing glacier mass loss.
... This technique is well established for polar glaciers [e.g., Moon et al., 2012]. Details of our processing can be found in Willis et al. [2012] and Melkonian et al. [2014]. Uncertainties on motions from cross-correlated pairs of TerraSAR-X images are very small, averaging 7.8 m a À1 over bedrock with slopes of less than 2% (see the supporting information). ...
Article
The Matusevich Ice Shelf (MIS), located within the Severnaya Zemlya Archipelago in the Russian Arctic, rapidly broke apart between August 10th and September 7th 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 (DEMs) to track ice surface elevation change rates (dh/dt) between 1984-2014. Glacier speeds are measured by pixel-tracking from optical and RADAR imagery between 2010-2014, and InSAR in 1995 to compare pre- and post-collapse 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 2003-2009. 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 non-melt season glacier speeds are continuing to accelerate.
... This is especially true for marine terminating glaciers, where measurements of ice motion can be combined with ice thickness information to determine frontal ablation rates, and hence the contribution of glaciers and ice sheets to global sea level rise. In light of this, in recent years there has been a concerted effort to map glacier dynamics at the regional scale for many of the glaciated regions of the world: Antarctica [1], the Greenland Ice Sheet [2,3], the majority of Alaskan glaciers [4,5] the Canadian High Arctic [6][7][8] and South America [9][10][11]. ...
Article
Glacier dynamics play an important role in the mass balance of many glaciers, ice caps and ice sheets. In this study we exploit Radarsat-2 (RS-2) Wide Fine (WF) data to determine the surface speed of Svalbard glaciers in the winters of 2012/2013 and 2013/2014 using Synthetic Aperture RADAR (SAR) offset and speckle tracking. The RS-2 WF mode combines the advantages of the large spatial coverage of the Wide mode (150 x 150 km) and the high pixel resolution (9m) of the Fine mode and thus has a major potential for glacier velocity monitoring from space through offset and speckle tracking. Faster flowing glaciers (1.95 md-1 - 2.55 md-1) which are studied in more detail are Nathorstbreen, Kronebreen, Kongsbreen and Monacobreen. Using our Radarsat-2 Wide Fine dataset, we compare the performance of two SAR tracking algorithms, namely the GAMMA Remote Sensing Software and a custom written MATLAB script that has primarily been used in the Canadian Arctic. Both algorithms provide comparable results, especially for the faster flowing glaciers and the termini of slower tidewater glaciers. A comparison of the WF data to RS-2 Ultrafine and Wide mode data reveals the superiority of RS-2 WF data over the Wide mode data.
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We study the evolution of the Juneau Icefield, one of the largest icefields in North America (>3700 km 2 ), using the Parallel Ice Sheet Model (PISM). We test two climate datasets: 20 km Weather Research and Forecasting Model (WRF) output, and data from the Scenarios Network for Alaska Planning (SNAP), derived from spatial interpolation of observations. Good agreement between simulated and observed surface mass balance was achieved only after substantially adjusting WRF precipitation to account for unresolved orographic effects, while SNAP's climate pattern is incompatible with observations of surface mass balance. Using the WRF data forced with the RCP6.0 emission scenario, the model projects a decrease in ice volume by 58–68% and a 57–63% area loss by 2099 compared with 2010. If the modeled 2070–99 climate is held constant beyond 2099, the icefield is eliminated by 2200. With constant 1971–2010 climate, the icefield stabilizes at 86% of its present-day volume. Experiments started from an ice-free state indicate that steady-state volumes are largely independent of the initial ice volume when forced by identical scenarios of climate stabilization. Despite large projected volume losses, the complex high-mountain topography makes the Juneau Icefield less susceptible to climate warming than low-lying Alaskan icefields.
Chapter
Southern Alaska is one of the most glaciated alpine regions on Earth. Volume losses in the region have been a key contributor to global sea level in the last century. This chapter examines the Juneau Icefield as an example of what has been occurring in the region. The icefield has a temperate maritime climate dominated by the passage of frequent cyclonic systems, with precipitation enhanced by orographic processes. Mendenhall Glacier is the most visited and photographed terminus in the region. Herbert Glacier is in the next valley north of Mendenhall Glacier. Eagle Glacier is in the next valley north of Herbert Glacier. Eagle Glacier has experienced a significant and sustained retreat since 1948. The Antler Glacier is an outlet glacier of the Juneau Icefield. It is actually a distributary glacier of the Bucher Glacier. The East and West Twin Glaciers are receding up separate fjords, though they are fed from a joint accumulation zone.
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The large Juneau and Stikine icefields (Alaska, JIF and SIF) lost mass rapidly in the second part of the 20th century. Laser altimetry, gravimetry and sparse field measurements suggest continuing mass loss in the early 21st century. However, two recent studies based on time series of SRTM and ASTER digital elevation models (DEMs) indicate a slowdown in mass loss after 2000. Here, the ASTER-based geodetic mass balance is recalculated, carefully avoiding the use of the SRTM DEM because of the unknown penetration depth of the C-Band radar signal. We find strongly negative mass balances from 2000 to 2016 (−0.68 ± 0.15 m w.e. a⁻¹ for JIF and −0.83 ± 0.12 m w.e. a⁻¹ for SIF), in agreement with laser altimetry, confirming that mass losses are continuing at unabated rates for both icefields. The SRTM DEM should be avoided or used very cautiously to estimate glacier volume change, especially in the North Hemisphere and over timescales of less than ~ 20 yrs.
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We reanalyzed mass balance records at Taku and Lemon Creek Glaciers to better understand the relative roles of hypsometry, local climate and dynamics as mass balance drivers. Over the 1946–2018 period, the cumulative mass balances diverged. Tidewater Taku Glacier advanced and gained mass at an average rate of +0.25 ± 0.28 m w.e. a –1 , contrasting with retreat and mass loss of −0.60 ± 0.15 m w.e. a ⁻¹ at land-terminating Lemon Creek Glacier. The uniform influence of regional climate is demonstrated by strong correlations among annual mass balance and climate data. Regional warming trends forced similar statistically significant decreases in surface mass balance after 1989: −0.83 m w.e. a –1 at Taku Glacier and −0.81 m w.e. a –1 at Lemon Creek Glacier. Divergence in cumulative mass balance arises from differences in glacier hypsometry and local climate. Since 2013 negative mass balance and glacier-wide thinning prevailed at Taku Glacier. These changes initiated terminus retreat, which could increase dramatically if calving begins. The future mass balance trajectory of Taku Glacier hinges on dynamics, likely ending the historic dichotomy between these glaciers.
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Glacier mass loss in Alaska has implications for global sea level rise, fresh water input into the Gulf of Alaska and terrestrial fresh water resources. We map all glaciers (>4000 km ² ) on the Kenai Peninsula, south central Alaska, for the years 1986, 1995, 2005 and 2016, using satellite images. Changes in surface elevation and volume are determined by differencing a digital elevation model (DEM) derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer stereo images in 2005 from the Interferometric Synthetic Aperture Radar DEM of 2014. The glacier area shrunk by 543 ± 123 km ² (12 ± 3%) between 1986 and 2016. The region-wide mass-balance rate between 2005 and 2014 was −0.94 ± 0.12 m w.e. a ⁻¹ (−3.84 ± 0.50 Gt a ⁻¹ ), which is almost twice as negative than found for earlier periods in previous studies indicating an acceleration in glacier mass loss in this region. Area-averaged mass changes were most negative for lake-terminating glaciers (−1.37 ± 0.13 m w.e. a ⁻¹ ), followed by land-terminating glaciers (−1.02 ± 0.13 m w.e. a ⁻¹ ) and tidewater glaciers (−0.45 ± 0.14 m w.e. a ⁻¹ ). Unambiguous attribution of the observed acceleration in mass loss over the last decades is hampered by the scarcity of observational data, especially at high elevation, and by large interannual variability.
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Ice-marginal lakes impact glacier mass balance, water resources, and ecosystem dynamics and can produce catastrophic glacial lake outburst floods (GLOFs) via sudden drainage. Multitemporal inventories of ice-marginal lakes are a critical first step in understanding the drivers of historic change, predicting future lake evolution, and assessing GLOF hazards. Here, we use Landsat-era satellite imagery and supervised classification to semi-automatically delineate lake outlines for four ∼5-year time periods between 1984 and 2019 in Alaska and northwest Canada. Overall, ice-marginal lakes in the region have grown in total number (+183 lakes, 38 % increase) and area (+483 km2, 59 % increase) between the time periods of 1984–1988 and 2016–2019. However, changes in lake numbers and area were notably unsteady and nonuniform. We demonstrate that lake area changes are connected to dam type (moraine, bedrock, ice, or supraglacial) and topological position (proglacial, detached, unconnected, ice, or supraglacial), with important differences in lake behavior between the sub-groups. In strong contrast to all other dam types, ice-dammed lakes decreased in number (six fewer, 9 % decrease) and area (−51 km2, 40 % decrease), while moraine-dammed lakes increased (56 more, 26 % and +479 km2, 87 % increase for number and area, respectively) at a faster rate than the average when considering all dam types together. Proglacial lakes experienced the largest area changes and rate of change out of any lake position throughout the period of study and moraine-dammed lakes which experienced the largest increases are associated with clean-ice glaciers (
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Since the mid-1980s, glaciers in the European Alps have shown widespread and accelerating mass losses. This article presents glacier-specific changes in surface elevation, volume and mass balance for all glaciers in the Swiss Alps from 1980 to 2010. Together with glacier outlines from the 1973 inventory, the DHM25 Level 1 digital elevation models (DEMs) for which the source data over glacierized areas were acquired from 1961 to 1991 are compared to the swissALTI3D DEMs from 2008 to 2011 combined with the new Swiss Glacier Inventory SGI2010. Due to the significant differences in acquisition dates of the source data used, mass changes are temporally homogenized to directly compare individual glaciers or glacierized catchments. Along with an in-depth accuracy assessment, results are validated against volume changes from independent photogrammetrically derived DEMs of single glaciers. Observed volume changes are largest between 2700 and 2800 m a.s.l. and remarkable even above 3500 m a.s.l. The mean geodetic mass balance is −0.62 ± 0.07 m w.e. yr−1 for the entire Swiss Alps over the reference period 1980–2010. For the main hydrological catchments, it ranges from −0.52 to −1.07 m w.e. yr−1. The overall volume loss calculated from the DEM differencing is −22.51 ± 1.76 km3.
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The recent evolution of Pamir-Karakoram-Himalaya (PKH) glaciers, widely acknowledged as valuable high-altitude as well as mid-latitude climatic indicators, remains poorly known. To estimate the region-wide glacier mass balance for 9 study sites spread from the Pamir to the Hengduan Shan (eastern Himalaya), we compared the 2000 Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) to recent (2008–2011) DEMs derived from SPOT5 stereo imagery. During the last decade, the region-wide glacier mass balances were contrasted with moderate mass losses in the eastern and central Himalaya (−0.22 ± 0.12 m w.e. yr−1 to −0.33 ± 0.14 m w.e. yr−1) and larger losses in the western Himalaya (−0.45 ± 0.13 m w.e. yr−1). Recently reported slight mass gain or balanced mass budget of glaciers in the central Karakoram is confirmed for a larger area (+0.10 ± 0.16 m w.e. yr−1) and also observed for glaciers in the western Pamir (+0.14 ± 0.13 m w.e. yr−1). Thus, the "Karakoram anomaly" should be renamed the "Pamir-Karakoram anomaly", at least for the last decade. The overall mass balance of PKH glaciers, −0.14 ± 0.08 m w.e. yr−1, is two to three times less negative than the global average for glaciers distinct from the Greenland and Antarctic ice sheets. Together with recent studies using ICESat and GRACE data, DEM differencing confirms a contrasted pattern of glacier mass change in the PKH during the first decade of the 21st century.
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The use of SAR interferometry is often impeded by decorrelation from thermal noise, temporal change, and baseline geometry. Power spectra of interferograms are typically the sum of a narrow-band component combined with broad-band noise. We describe a new adaptive filtering algorithm that dramatically lowers phase noise, improving both measurement accuracy and phase unwrapping, while demonstrating graceful degradation in regions of pure noise. The performance of the filter is demonstrated with SAR data from the ERS satellites over the Jakobshavns glacier of Greenland.
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The Patagonia Icefields are characterized by a large number of outlet glaciers calving into lakes and the ocean. In contrast to the recent intensive research activities on tidewater glaciers in other regions, very few observations have been made on calving glaciers in Patagonia. We analysed satellite images of Glaciar Upsala, the third largest freshwater calving glacier in the Southern Patagonia Icefield, to investigate changes in its front position, ice velocity and surface elevation from 2000 to 2011. Our analyses revealed a clear transition from a relatively stable phase to a rapidly retreating and fast-flowing condition in 2008. The glacier front receded by 2.9 km, and the ice velocity increased by 20–50%, over the 2008–11 period. We also found that the ice surface lowered at a rate of up to 39 m a–1 from 2006 to 2010. This magnitude and the rate of changes in the glacier front position, ice velocity and surface elevation are greater than previously reported for Glaciar Upsala, and comparable to recent observations of large tidewater glaciers in Greenland. Our data illustrate details of a rapidly retreating calving glacier in Patagonia that have been scarcely reported despite their importance to the mass budget of the Patagonia Icefields.
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We present a high-resolution Gravity Recovery and Climate Experiment (GRACE) mascon solution for Gulf of Alaska (GOA) glaciers and compare this with in situ glaciological, climate and other remote-sensing observations. Our GRACE solution yields a GOA glacier mass balance of –65 AE AE 11 Gt a –1 for the period December 2003 to December 2010, with summer balances driving the interannual variability. Between October/November 2003 and October 2009 we obtain a mass balance of –61 AE 11 Gt a –1 from GRACE, which compares well with –65 AE 12 Gt a –1 from ICESat based on hypsometric extrapolation of glacier elevation changes. We find that mean summer (June–August) air temperatures derived from both ground and lower-troposphere temperature records were good predictors of GRACE-derived summer mass balances, capturing 59% and 72% of the summer balance variability respectively. Large mass losses during 2009 were likely due to low early melt season surface albedos, measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) and likely associated with the 31 March 2009 eruption of Mount Redoubt, southwestern Alaska. GRACE data compared well with in situ measurements at Wolverine Glacier (maritime Alaska), but poorly with those at Gulkana Glacier (interior Alaska). We conclude that, although GOA mass estimates from GRACE are robust over the entire domain, further constraints on subregional and seasonal estimates are necessary to improve fidelity to ground observations.
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The annual surface mass balance records of the Lemon Creek Glacier and Taku Glacier observed by the Juneau Icefield Research Program are the longest continuous glacier annual mass balance data sets in North America. Annual surface mass balance ( B a) measured on Taku Glacier averaged +0.40 m a-1 from 1946-1985, and -0.08 m a-1 from 1986-2011. The recent annual mass balance decline has resulted in the cessation of the long-term thickening of the glacier. Mean B a on Lemon Creek Glacier has declined from -0.30 m a-1 for the 1953-1985 period to -0.60 m a-1 during the 1986-2011 period. The cumulative change in annual surface mass balance is -26.6 m water equivalent, a 29 m of ice thinning over the 55 yr. Snow-pit measurements spanning the accumulation zone, and probing transects above the transient snow line (TSL) on Taku Glacier, indicate a consistent surface mass balance gradient from year to year. Observations of the rate of TSL rise on Lemon Creek Glacier and Taku Glacier indicate a comparatively consistent migration rate of 3.8 to 4.1 m d-1. The relationship between TSL on Lemon Creek Glacier and Taku Glacier to other Juneau Icefield glaciers (Norris, Mendenhall, Herbert, and Eagle) is strong, with correlations exceeding 0.82 in all cases.
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Regional glacier mass balances can be measured by subtracting multi-temporal digital elevation models (DEMs). However, DEMs are often biased with altitude and it remains unclear whether the elevation differences observed on the ice-free terrain can be used to correct biases on ice-covered areas. We investigate such altitude-related biases using DEMs from three different sensors: SPOT-5, SRTM C-band and SRTM X-band. The bias due to different original DEM resolutions can be corrected using a relationship between curvature and elevation difference, calculated on ice-free terrain. The impact of C-band radar penetration into snow and ice can be evaluated for a specific region by comparing SRTM C-band and SRTM X-band DEMs. In our test area (Karakoram), the resolution-related bias has a minor influence on the region-wide elevation change. Conversely, not accounting for C-Band penetration would seriously biases the mean glacier mass balance.
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A time-series composed of 156 ASTER derived Digital Elevation Models (DEMs) and a radar-penetration-bias corrected version of the Shuttle Radar Topography Mission (SRTM) DEM is used to derive ice surface height and volume changes at the Southern Patagonian Ice Field (SPI) in southern South America. The observations, made between February 2000 and March 2012, indicate that the ice field is rapidly losing volume at many of the largest outlet glaciers, and in most cases thinning extends to the highest elevations of the ice field. Mass loss is occurring at a rate of -20.0 ± 1.2 Gt a-1, which, when summed with mass-loss at the adjacent Northern Patagonian Ice Field results in a combined rate of -24.4 ± 1.4 Gt a-1, equivalent to +0.067 ± 0.004 mm a-1 of sea level rise. Our decade-long mass loss rates are substantially higher than those derived during the last three decades of the 20th century, but are in good agreement with recent GRACE observations. Our volume loss estimate is sensitive to constraints applied to the amount of thickening in the accumulation zone. New field measurements and a continued DEM time-series will be required to refine our estimates.
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We produce the first icefield-wide volume change rate and glacier velocity estimates for the Cordillera Darwin Icefield (CDI), a 2605 km2 temperate icefield in southern Chile (69.6° W, 54.6° S). Velocities are measured from optical and radar imagery between 2001-2011. Thirty-six digital elevation models (DEMs) from ASTER and the SRTM DEM are stacked and a weighted linear regression is applied to elevations on a pixel-by-pixel basis to estimate volume change rates. The CDI lost mass at an average rate of -3.9 ± 1.5 Gt yr-1 between 2000 and 2011, equivalent to a sea level rise (SLR) of 0.01 ± 0.004 mm yr-1 and an area-averaged thinning rate of -1.5 ± 0.6 m w.e.(water equivalent) yr-1. Thinning is widespread, with concentrations near the front of two northern glaciers (Marinelli, Darwin) and one western (CDI-08) glacier. Thickening is apparent in the south, most notably over the advancing Garibaldi Glacier. The northeastern part of the CDI has an average thinning rate of -1.9 ± 0.7 m w.e. yr-1, while the southwestern part has an average thinning rate of -1.0 ± 0.4 m w.e. yr-1. Velocities are obtained over many of the CDI glaciers for the first time. We provide a repeat speed time series at the Marinelli Glacier. There we measure maximum front speeds of 7.5 ± 0.2 m day-1 in 2001, 9.5 ± 0.6 m day-1 in 2003 and 10 ± 0.3 m day-1 in 2011. The maintenance of high front speeds from 2001 to 2011 supports the hypothesis that Marinelli is in the retreat phase of the tidewater cycle, with dynamic thinning governed by the fjord bathymetry.
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The Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud, and land Elevation Satellite (ICESat) provides a globally-distributed data set well suited for evaluating the vertical accuracy of Shuttle Radar Topography Mission (SRTM) digital elevation models (DEMs). The horizontal error (2.4 ± 7.3 m) and vertical error (0.04 ± 0.13 m per degree of incidence angle) for the ICESat data used are small compared to those for SRTM. Using GLAS echo waveforms we document differences between the SRTM C-band phase center and the highest, centroid, and lowest elevations within ICESat laser footprints in the western United States. In areas of low relief and sparse tree cover, the mean and standard deviation of elevation differences between the ICESat centroid and SRTM are -0.60 ± 3.46 m. The differences are -5.61 ± 15.68 m in high relief, sparse tree cover areas, and -3.53 ± 8.04 m in flat areas with dense tree cover. The largest differences occur in rugged, densely-vegetated regions.
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We use Interferometric Synthetic Aperture Radar (InSAR) data to derive continuous maps for three orthogonal components of the co-seismic surface displacement field due to the 1999 Mw7.1 Hector Mine earthquake in southern California. Vertical and horizontal displacements are both predominantly antisymmetric with respect to the fault plane, consistent with predictions of linear elastic models of deformation for a strike-slip fault. Some deviations from symmetry apparent in the surface displacement data may result from complexity in the fault geometry.
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A simple model using once-daily upper-air values in the NCEP NCAR reanalysis database estimates seasonal mass balance at two glaciers in southern Alaska, one in western Canada, and one in Washington substantially better than any of several seasonally averaged, large-scale climate indices commonly used. Whereas sea level pressure and sea surface temperature in the Pacific exert a strong influence on the climate in the region, temperature and moisture flux at 850 mb have a more direct effect on mass balance processes---accumulation and ablation---because their temporal variability better matches that of those processes. The 40-yr record of 850-mb temperature shows winter warming after 1976 and summer warming after 1988 throughout the region; mass balance records reflect the summer warming at all four glaciers but winter warming only at the southern two. The only pronounced long-term change in the moisture regime is a decrease of precipitation in the south and an increase in the north. Interannual variations in the location of the moisture flux, however, apparently account for the strong negative correlation between the Alaska glaciers and the other two.
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Current heuristic laws that relate the motion of glaciers due to sliding along the bed to the subglacial water pressure fail to reproduce variations in sliding speed on timescales of specific hydrologic events, such as lake drainage, rainfall, or surging. This may be due to the importance of subglacial cavity evolution and shifts in the glacier stress field, both of which are not accounted for in typical sliding laws. We use multiple time series of surface motion over a 66-day period at Breiðamerkurjökull, Iceland, to infer changes in bed separation and longitudinal force budget. We observe multiple, distinct periods of increased surface motion and uplift corresponding to periods of rainfall and/or increased temperatures. We find consistent hysteresis and lags between motion and variations in both the bed separation and longitudinal stress gradient that we attribute to the redistribution of normal stresses at the bed during cavity growth. Increases in the longitudinal stress gradient suggest a downglacier stress transfer during increased basal motion that is consistent with increased drainage system efficiency toward the terminus. Our results suggest that the transient evolution of the subglacial drainage system and shifts in the glacier stress field are important controls on basal motion.
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Glaciers distinct from the Greenland and Antarctic Ice Sheets are losing large amounts of water to the world’s oceans. However, estimates of their contribution to sea level rise disagree. We provide a consensus estimate by standardizing existing, and creating new, mass-budget estimates from satellite gravimetry and altimetry and from local glaciological records. In many regions, local measurements are more negative than satellite-based estimates. All regions lost mass during 2003–2009, with the largest losses from Arctic Canada, Alaska, coastal Greenland, the southern Andes, and high-mountain Asia, but there was little loss from glaciers in Antarctica. Over this period, the global mass budget was –259 ± 28 gigatons per year, equivalent to the combined loss from both ice sheets and accounting for 29 ± 13% of the observed sea level rise.
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A large portion of the recent increase in the rate of mass loss from the Greenland ice sheet is from increased outlet glacier discharge along its southeastern margin. While previous investigations of the region's two largest glaciers suggest that acceleration is a dynamic response to thinning and retreat of the calving front, it is unknown whether this mechanism can explain regional acceleration and what forcing is responsible for initiating rapid thinning and retreat. We examine seasonal and interannual changes in ice-front position, surface elevation and flow speed for 32 glaciers along the southeastern coast between 2000 and 2006. While substantial seasonality in front position and speed is apparent, nearly all the observed glaciers show net retreat, thinning and acceleration, with speed-up corresponding to retreat. The ratio of retreat to the along-flow stress-coupling length is proportional to the relative increase in speed, consistent with typical ice-flow and sliding laws. This affirms that speed-up results from loss of resistive stress at the front during retreat, which leads to along-flow stress transfer. Large retreats were often preceded by the formation of a flat or reverse-sloped surface near the front, indicating that subsequent retreats were influenced by the reversed bed slope. Many retreats began with an increase in thinning rates near the front in the summer of 2003, a year of record high coastal-air and sea-surface temperatures. This anomaly was driven in part by recent warming, suggesting that episodes of speed-up and retreat may become more common in a warmer climate.
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Estimates of glacier mass balance using geodetic methods can differ significantly from estimates using direct glaciological field-based measurements. To determine if such differences are real or methodological, there is a need to improve uncertainty estimates in both methods. In this paper, we focus on the uncertainty of geodetic methods and describe a geostatistical technique that takes into account the spatial correlation of the elevation differences when calculating spatially averaged elevation changes. We apply this method to the western Svartisen ice cap, Norway, using elevation differences from the surrounding bedrock derived from stereophotogrammetry. We show that the uncertainty is not only dependent on the standard error of the individual elevation differences but is also dependent on the size of the averaging area and the scale of the spatial correlation. To assess if the geostatistical analysis made over bedrock is applicable to glacier surfaces, we use concurrent photogrammetrical and laser scanning data from bedrock and a range of glacier surfaces to evaluate the dependency of the geostatistical analysis on the surface type. The estimated geodetic mass balance, and its uncertainty, is -2.6±0.9 m w.e. for the period 1968-85, and -2.0±2.2 m w.e. for 1985-2002.
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Ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in Alaska, Greenland and Antarctica. Glaciers that calve in water flow much faster than those that terminate on land, as a result of enhanced basal ice motion where basal water pressure is high. However, a scarcity of subglacial observations in calving glaciers limits a mechanistic understanding. Here we present high-frequency measurements of ice speed and basal water pressures from Glaciar Perito Moreno, a fast-flowing calving glacier in Patagonia. We measured water pressure in boreholes drilled at a site where the glacier is 515+/-5m thick, and where more than 60% of the ice is below the level of proglacial lakes. We found that the mean basal water pressure was about 95% of the pressure imposed by the weight of the overlying ice. Moreover, changes in basal water pressure by a few per cent drove nearly 40% of the variations in ice flow speed. The ice speed was strongly correlated to air temperature, suggesting that glacier motion was modulated by water pressure changes as meltwater entered the system. We conclude that basal water pressure in calving glaciers is important for glacier dynamics, and closely connected to climate conditions.
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Over the past 50 years, retreating glaciers and ice caps contributed 0.5 mm yr-1to sea-level rise, and one third of this contribution is believed to come from ice masses bordering the Gulf of Alaska. However, these estimates of ice loss in Alaska are based on measurements of a limited number of glaciers that are extrapolated to constrain ice wastage in the many thousands of others. Uncertainties in these estimates arise, for example, from the complex pattern of decadal elevation changes at the scale of individual glaciers and mountain ranges. Here we combine a comprehensive glacier inventory with elevation changes derived from sequential digital elevation models. We find that between 1962 and 2006, Alaskan glaciers lost 41.9 ± 8.6 km 3yr-1 of water, and contributed 0.12 ± 0.02 mm yr-1 to a-level rise, 34% less than estimated earlier2,3. Reasons for our lower values include the higher spatial resolution of our glacier inventory as well as the reduction of ice thinning underneath debris and at the glacier margins, which were not resolved in earlier work. We suggest that estimates of mass loss from glaciers and ice caps in other mountain regions could be subject to similar revisions.
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Glaciers are among the best indicators of terrestrial climate variability, contribute importantly to water resources in many mountainous regions and are a major contributor to global sea level rise. In the Hindu Kush-Karakoram-Himalaya region (HKKH), a paucity of appropriate glacier data has prevented a comprehensive assessment of current regional mass balance. There is, however, indirect evidence of a complex pattern of glacial responses in reaction to heterogeneous climate change signals. Here we use satellite laser altimetry and a global elevation model to show widespread glacier wastage in the eastern, central and south-western parts of the HKKH during 2003-08. Maximal regional thinning rates were 0.66 ± 0.09 metres per year in the Jammu-Kashmir region. Conversely, in the Karakoram, glaciers thinned only slightly by a few centimetres per year. Contrary to expectations, regionally averaged thinning rates under debris-mantled ice were similar to those of clean ice despite insulation by debris covers. The 2003-08 specific mass balance for our entire HKKH study region was -0.21 ± 0.05 m yr(-1) water equivalent, significantly less negative than the estimated global average for glaciers and ice caps. This difference is mainly an effect of the balanced glacier mass budget in the Karakoram. The HKKH sea level contribution amounts to one per cent of the present-day sea level rise. Our 2003-08 mass budget of -12.8 ± 3.5 gigatonnes (Gt) per year is more negative than recent satellite-gravimetry-based estimates of -5 ± 3 Gt yr(-1) over 2003-10 (ref. 12). For the mountain catchments of the Indus and Ganges basins, the glacier imbalance contributed about 3.5% and about 2.0%, respectively, to the annual average river discharge, and up to 10% for the Upper Indus basin.
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1] The rapid wastage of mountain glaciers and their contribution to sea level rise require worldwide monitoring of their mass balance. In this paper, we show that changes in glacier thickness can be accurately measured from satellite images. We use SPOT image pairs to build Digital Elevation Models (DEMs) of the Mont Blanc area (French Alps) for different years. To register the DEMs, we adjust their longitude, latitude and altitude over motionless areas. The uncertainty of the thickness change measurement is greatly reduced by averaging over areas covering altitude intervals of 50 m. Comparisons with topographic profiles and a differential DEM from aerial photographs obtained on the Mer de Glace indicate an overall accuracy of 1 m for the thickness change measurement. Below 2100 m, satellite DEMs show an evolution of the thinning rate from 1 ± 0.4 m.
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Phased-Array Synthetic-Aperture Radar (PALSAR) is an L-band frequency (1.27 GHz) radar capable of continental-scale interferometric observations of ice sheet motion. Here, we show that PALSAR data yield excellent measurements of ice motion compared to C-band (5.6 GHz) radar data because of greater temporal coherence over snow and firn. We compare PALSAR velocities from year 2006 in Pine Island Bay, West Antarctica with those spanning years 1974 to 2007. Between 1996 and 2007, Pine Island Glacier sped up 42% and ungrounded over most of its ice plain. Smith Glacier accelerated 83% and ungrounded as well. Their largest speed up are recorded in 2007. Thwaites Glacier is not accelerating but widening with time and its eastern ice shelf doubled its speed. Total ice discharge from these glaciers increased 30% in 12 yr and the net mass loss increased 170% from 39 ± 15 Gt/yr to 105 ± 27 Gt/yr. Longer-term velocity changes suggest only a moderate loss in the 1970s. As the glaciers unground into the deeper, smoother beds inland, the mass loss from this region will grow considerably larger in years to come.
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This article was published in the journal, Annals of Glaciology [© International Glaciological Society]. The definitive version is available at http://www.igsoc.org/annals/ Sequential optical images of high spatial resolution were used for the first time to derive surface ice velocities of Glaciar Upsala, a fast-moving fresh-water calv-ing glacier in southern Patagonia. Cross-correlation methods applied to four Landsat ETM+ images acquired in 2000^01 yielded average velocities of around 1600 m a ^1 , similar to values measured in the field in November 1993. The derived velocities show almost no seasonal variation for the analyzed calving termini. During the period of satel-lite coverage, clear readvances were detected in the autumn^winter period, followed by recessions during summers. Between 24 April 1999 and 14 October 2001, the glacier front has been fluctuating seasonally within about 400 m, in contrast to the previous dramatic recession. During the last 2.5 years, Glaciar Upsala west terminus had a net advance of around 300 m. In addition, the available satellite images allowed us to determine recent calving speeds and confirm the improved calving-rate/water-depth relationship, recently proposed by incorporating new data from Patagonian glaciers.
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Digital elevation models of glaciated terrain pro-duced by the NASA/Jet Propulsion Laboratory (JPL) air-borne interferometric synthetic-aperture radar (InSAR) in-strument in Greenland and Alaska at the C-(5.6 cm wave-length) and L-band (24-cm) frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth, or penetration depth, δp. On cold polar firn at Greenland sum-mit, δp = 9±2m at C-and 14±4m at L-band. On the ex-posed ice surface of Jakobshavn Isbrae, west Greenland, δp = 1±2 m at C-and 3±3 m at L-band except on smooth, marginal ice where δp = 15±5 m. On colder marginal ice of northeast Greenland, δp reaches 60 to 120 m at L-band. On the temperate ice of Brady Glacier, Alaska, δp is 4±2 m at C-and 12±6 m at L-band, with little dependence on snow/ice conditions. The implications of the results on the scientific use of InSAR data over snow/ice terrain is discussed.
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Taku Glacier, Alaska, USA, is currently in the advance stage of the tidewater glacier cycle. We investigated the near-terminus dynamics by measuring surface velocities, surface elevation changes, ice thickness and ablation. Velocities vary on sub-daily, diurnal, seasonal and interannual timescales. Flowline modeling shows that the modeled surface velocities are sensitive to changes in till yield strength and thus effective basal pressures. The glacier bed deepens in the up-glacier direction and this imposes a minimum subglacial water pressure necessary for water to drain along the bed. In a simple model we impose water-pressure gradients based on phreatic surfaces of constant slopes to simulate the winter– summer transitions. This proves sufficient to explain an observed early-season switch from compressional to block flow. Velocities also vary between years. Changing basal conditions can result in lower horizontal velocities, which decrease the ice supply to the terminus and result in temporary surface lowering. But a decrease in ice flux to the terminus must lead to ice storage further upstream, and that ice mass will eventually reach the terminus. This can explain the observed episodic nature of terminus advance.
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1] Repeat satellite laser altimetry is critical for observing the rapidly changing mass balance of the Greenland Ice Sheet. However, sparse sampling and high surface slopes over rapidly thinning, coastal outlet glaciers may result in underestimation of mass loss. Here we supplement ICESat-derived surface elevation changes with differenced ASTER digital elevation models of outlet glaciers in southeastern Greenland, the region with the largest concentrated change in outlet glacier mass loss. We estimate a 2002 – 2005 regional volume-loss rate of 108 km 3 /yr. Our results are consistent with drainage-scale GRACE and mass-budget estimates when differences in observation periods are taken into account. The two largest glaciers, Kangerdlugssuaq and Helheim, account for only 28% of the mass loss, illustrating the combined importance of smaller glaciers and the need for complete observational coverage. Additionally, we find that rapid, concentrated thinning within the outlets represents a small contribution to the total volume change compared to dispersed inland thinning.
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The mass changes of the Gulf of Alaska (GoA) glaciers are computed from the Gravity Recovery and Climate Experiment (GRACE) inter-satellite range-rate data for the period April 2003– September 2007. Through the application of unique processing techniques and a surface mass concentration (mascon) parameterization, the mass variations in the GoA glacier regions have been estimated at high temporal (10 day) and spatial (2 Â Â 2 arc-degrees) resolution. The mascon solutions are directly estimated from a reduction of the GRACE K-band inter-satellite range-rate data and, unlike previous GRACE solutions for the GoA glaciers, do not exhibit contamination by leakage from mass change occurring outside the region of interest. The mascon solutions reveal considerable temporal and spatial variation within the GoA glacier region, with the largest negative mass balances observed in the St Elias Mountains including the Yakutat and Glacier Bay regions. The most rapid losses occurred during the 2004 melt season due to record temperatures in Alaska during that year. The total mass balance of the GoA glacier region was –84 AE 5 Gt a –1 contributing 0.23 AE 0.01 mm a –1 to global sea-level rise from April 2003 through March 2007. Highlighting the large seasonal and interannual variability of the GoA glaciers, the rate determined over the period April 2003–March 2006 is –102 AE 5 Gt a –1 , which includes the anomalously high temperatures of 2004 and does not include the large 2007 winter balance-year snowfall. The mascon solutions agree well with regional patterns of glacier mass loss determined from aircraft altimetry and in situ measurements.
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1] The digital elevation model (DEM) from the 2000 Shuttle Radar Topography Mission (SRTM) was differenced from a composite DEM based on air photos dating from 1948 to 1987 to determine glacier volume changes in southeast Alaska and adjoining Canada. SRTM accuracy was assessed at ±5 m through comparison with airborne laser altimetry and control locations measured with GPS. Glacier surface elevations lowered over 95% of the 14,580 km 2 glacier-covered area analyzed, with some glaciers thinning as much as 640 m. A combination of factors have contributed to this wastage, including calving retreats of tidewater and lacustrine glaciers and climate change. Many glaciers in this region are particularly sensitive to climate change, as they have large areas at low elevations. However, several tidewater glaciers that had historically undergone calving retreats are now expanding and appear to be in the advancing stage of the tidewater glacier cycle. The net average rate of ice loss is estimated at 16.7 ± 4.4 km 3 /yr, equivalent to a global sea level rise contribution of 0.04 ± 0.01 mm/yr. (2007), Glacier changes in southeast Alaska and northwest British Columbia and contribution to sea level rise, J. Geophys. Res., 112, F01007, doi:10.1029/2006JF000586.
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Temperature and precipitation records from 1949 to 1998 were examined for 25 stations throughout the State of Alaska. Mean, maxima, and minima temperatures, diurnal temperature range, and total precipitation were analyzed for linear trends using least squares regressions. Annual and seasonal mean temperature increases were found throughout the entire state, and the majority were found to be statistically significant at the 95% level or better. The highest increases were found in winter in the Interior region (2.2 °C) for the 50 year period of record. Decreases in annual and seasonal mean diurnal temperature range were also found, of which only about half were statistically significant. A state-wide decrease in annual mean diurnal temperature range was found to be 0.3 °C, with substantially higher decreases in the South/Southeastern region in winter. Increases were found in total precipitation for 3 of the 4 seasons throughout most of Alaska, while summer precipitation showed decreases at many stations. Few of the precipitation trends were found to be statistically significant, due to high interannual variability. Barrow, our only station in the Arctic region, shows statistically significant decreases in annual and winter total precipitation. These findings are largely in agreement with existing literature, although they do contradict some of the precipitation trends predicted by the CO2-doubling GCM’s.
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Observations of ice motion are critical for constraining ice sheet mass balance and contribution to sea level rise, as well as predicting future changes. A wealth of imagery now exists for measuring ice motion from space, but existing repeat-image feature-tracking (RIFT) algorithms require the selection of several location- and data-specific parameters and manual data editing and are therefore not efficient for processing large numbers of image pairs for differing regions. Here, we present the multiple-image/multiple-chip RIFT algorithm which does not involve any user-defined local/empirical parameters and has a higher matching success rate than conventional single-image single-chip correlation matching. We also present an efficient method for applying RIFT to null-value striped data, such as the Landsat-7 Enhanced Thematic Mapper Plus. This method offers the potential for fully automated processing of large data sets.
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Mendenhall Glacier is a dynamic maritime glacier in southeast Alaska that is undergoing substantial recession and thinning. The terminus has retreated 3 km during the 20th century and the lower part of the glacier has thinned 200 m or more since 1909. Glacier-wide volume loss between 1948 and 2000 is estimated at 5.5 km3. Wastage has been the strongest in the glacier's lower reaches, but the glacier has also thinned at higher elevations. The shrinkage of Mendenhall Glacier appears to be due primarily to surface melting and secondarily to lake calving. The change in the average rate of thinning on the lower glacier, <1 m a−1 between 1948 and 1982 and >2 m a−1 since 1982, agrees qualitatively with observed warming trends in the region. Mean annual temperatures in Juneau decreased slightly from 1947 to 1976; they then began to increase, leading to an overall warming of ∼1.6 °C since 1943. Lake calving losses have periodically been a small but significant fraction of glacier ablation. The portion of the terminus that ends in the lake is becoming increasingly vulnerable to calving because of a deep pro-glacial lake basin. If current climatic trends persist, the glacier will continue to shrink and the terminus will recede onto land at a position about 500 m inland within one to two decades. The glacier and the meltwaters that flow from it are integral components of the Mendenhall Valley hydrologic system. Approximately 13% of the recent average annual discharge of the Mendenhall River is attributable to glacier shrinkage. Glacier melt contributes 50% of the total river discharge in summer.
Article
Using radio-echo soundings and seismic reflections, we measured cross-sections of Taku Glacier, near Juneau, Alaska, to resolve inconsistencies in previous measurements and to understand better the glacier’s dynamics. The maximum thickness is about 1477 m and the minimum bed elevation is about 600 m below sea level, which establishes Taku Glacier as the thickest and deepest temperate glacier yet measured. Our data indicate that, during the 19th century, the terminus of Taku Glacier may have begun its rapid advance at a position where the ice bed was greater than 300 m below sea level and more than 25 km from the inland end of its submarine trough; this behavior is uncharacteristic of temperate tide-water glaciers. The glacier, which no longer calves, has eroded a sediment layer 100 m thick since 1890 at an average rate of about 3 m a−1 since 1948; this high erosion rate retards advance by entrenching the glacier into the terminal moraine. Calculations based on ice-deformation theory indicate significant basal ice motion near the terminus and high basal shear stress (140–220kPa) along much of its length. Estimated differences between ice flux and balance flux are consistent with observed thickening and positive net mass balance; these data indicate that ice volume is increasing and that further advance is likely.
Article
The maximum thickness is about 1477 m and the minimum bed elevation is about 600 m below sea level, which establishes Taku Galcier as the thickest and deepest temperate glacier yet measured. Data indicates that, during the 19th century, the terminus of Taku Galcier may have begun its rapid advance at a position where the ice bed was greater than 300 m below sea level and more than 35 km from the inland end of its submarine trough. The glacier has eroded a sediment layer 100 m thick since 1890 at an average rate of about 3 m a-1 since 1948. Calculations based on ice-deformation theory indicate significant basal ice motion near the terminus and high basal shear stress (140-220 kPa) along much of its length. Estimated differences between ice flux and balance flux are consistent with observed thickening and positive net mass balance; these data indicate that ice volume is increasing and that further advance is likely. -from Authors
Article
The Glacier Bay region of southeast Alaska, USA, and British Columbia, Canada, has undergone major glacier retreat since the Little Ice Age (LIA). We used airborne laser altimetry elevation data acquired between 1995 and 2011 to estimate the mass loss of the Glacier Bay region over four time periods (1995–2000, 2000–05, 2005–09, 2009–11). For each glacier, we extrapolated from center-line profiles to the entire glacier to estimate glacier-wide mass balance, and then averaged these results over the entire region using three difference methods (normalized elevation, area-weighted method and simple average). We found that there was large interannual variability of the mass loss since 1995 compared with the long-term (post-LIA) average. For the full period (1995–2011) the average mass loss was 3.93±0.89 Gt a–1 (0.6±0.1 m w.e.a–1), compared with 17.8 Gt a–1 for the post-LIA (1770–1948) rate. Our mass loss rate is consistent with GRACE gravity signal changes for the 2003–10 period. Our results also show that there is a lower bias due to center-line profiling than was previously found by a digital elevation model difference method.
Article
Taku Glacier, Alaska, which until around 1950 calved icebergs into tidewater, has advanced 7.3 km since 1890, although all other valley glaciers in the immediate area have retreated. This anomalous advance generally has been attributed directly to climate. Meanwhile, the Le Conte Glacier, similar in physical setting to Taku, retreated 4 km between 1887 and 1963 and has since maintained this retracted position. During the recession the Le Conte calved icebergs in tidewater as much as 300 m deep. We propose that these asynchronous advances and retreats reflect changes in iceberg calving rates with time. The Taku has a high AAR (0.83) and its present advance is expected to continue, but at a diminishing rate as the glacier spreads out in the Taku River valley. Despite its exceptionally high AAR of 0.90, calving losses now balance the high flow rate of the Le Conte Glacier. No sustained advance can be expected until a terminal moraine shoal is constructed to inhibit calving. [Key words: glacier, iceberg calving, Le Conte Glacier, Taku Glacier, Alaska.]
Article
Both lake-calving Yakutat Glacier (337 km2), Alaska, USA, and its parent icefield (810 km2) are experiencing strong thinning, and under current climate conditions will eventually disappear. Comparison of digital elevation models shows that Yakutat Glacier thinned at area-averaged rates of 4.76±0.06 m w.e.a–1 (2000–07) and 3.66±0.03 m w.e.a–1 (2007–10). Simultaneously, adjacent Yakutat Icefield land-terminating glaciers thinned at lower but still substantial rates (3.79 and 2.94 m w.e.a–1 respectively for the same time periods), indicating lake-calving dynamics helps drive increased mass loss. Yakutat Glacier terminates into Harlequin Lake and for over a decade sustained a ∼3 km long floating tongue, which started to disintegrate into large tabular icebergs in 2010. Such floating tongues are rarely seen on temperate tidewater glaciers. We hypothesize that this difference is likely due to the lack of submarine melting in the case of lake-calving glaciers. Floating-tongue ice losses were evaluated in terms of overall mass balance and contribution to sea-level rise. The post-Little Ice Age collapse of Yakutat Icefield was driven in part by tidewater calving retreats of adjacent glaciers, the lake-calving retreat of Yakutat Glacier, a warming climate and by the positive feedback mechanisms through surface lowering.
Article
E averaged 2.0 ± 0.1 m a1 (1940-2005) at L = 3 km (H = 350 m), and 1.5 ± 0.2 m a1 (1952-2005) at L = 1.5 km (H = 250 m). Detailed mapping over a 4 km2 area of the terminus revealed a deeply incised channel in line with a major outlet stream. Glaciofluvial processes must play the dominant role in the subglacial erosion and removal of these unlithified sediments. Citation: Motyka, R. J., M. Truffer, E. M. Kuriger, and A. K. Bucki (2006), Rapid erosion of soft sediments by tidewater glacier advance: Taku Glacier, Alaska, USA, Geophys. Res. Lett., 33, L24504, doi:10.1029/2006GL028467.
Article
Precise registration and orthorectification of remote sensing images are the basic processes for quantitative remote sensing applications, especially for multi-temporal image analysis. In this paper, we present an automated precise registration and orthorectification package (AROP) for Landsat and Landsat-like data processing. The Landsat and Landsat-like satellite images acquired from different sensors at different spatial resolutions and projections can be re-projected, co-registered, and orthorectified to the same projection, geographic extent, and spatial resolution using a common base image through a combined resampling strategy; this allows us to perform multi-temporal image analysis directly. This paper presents and tests the AROP package on Landsat and Landsat-like data. The package is now freely available from our research web site.
Article
Ice flow speeds were measured at Glaciar Soler in northern Patagonia during the middle of the melt season (November-December) in 1998 and compared to data from 1985. In 1998 the surface flow speed was greater at all survey points, yet the ice was about 40 m thinner; the greater melt rate in 1998 probably explains these differences because of the effect of melt rate on basal sliding speed. Multiday variations in surface speed were well correlated with daily variations in surface water input, which is the sum of melt rate and rainfall. Although the basal sliding speeds vary from place to place, we obtained similar linear relationships between basal sliding speed and surface water input. This result indicates the possibility of taking account of basal sliding as a function of surface water input.
Article
Our poor understanding of tidewater glacier dynamics remains the primary source of uncertainty in sea level rise projections. On the ice sheets, mass lost from tidewater calving exceeds the amount lost from surface melting. In Alaska, the magnitude of calving mass loss remains unconstrained, yet immense calving losses have been observed. With 20% of the global new-water sea level rise coming from Alaska, partitioning of mass loss sources in Alaska is needed to improve sea level rise projections. Here we present the first regionally comprehensive map of glacier flow velocities in Central Alaska. These data reveal that the majority of the regional downstream flux is constrained to only a few coastal glaciers. We find regional calving losses are 17.1 Gt a(-1), which is equivalent to 36% of the total annual mass change throughout Central Alaska.
Article
RO1_PAC V2.3, a Repeat Orbit Interferometry package that allows topographic and surface change researchers to apply Interferometric Synthetic Aperture Radar (InSAR) methods, is now freely available to the community InSAR is the synthesis of conventional SAR and interferometry techniques that have been developed over several decades in radio astronomy and radar remote sensing. In recent years, it has opened entirely new application areas for radar in the Earth system sciences, including topographic mapping and geodesy. RO1_PAC, developed primarily to work with European Remote Sensing (ERS) satellite radar data, currently supports ERS-1, ERS-2, and Japanese Earth Resources Satellite (JERS) radar data, and is configurable to work with “strip-mode” data from all existing satellite radar instruments. The first release of RO1_ PAC (V1.0) was made quietly in 2000, and roughly 30 groups in the academic and research community currently use it.
Based on a review of observations on different types of calving glaciers, a simple calving model is proposed. Glaciers that exist in a sufficiently cold climate can form floating ice shelves and ice tongues that typically do not extend beyond confinements such as lateral fjord walls or mountains, and ice rises. If the local climate exceeds the thermal limit of ice shelf viability, as is the case for temperate glaciers, no floating tongue can be maintained and the position of the terminus is determined by the thickness in excess of flotation. If the snout is sufficiently thick, a stable terminus position at the mouth of the confining fjord - usually marked by a terminal shoal - can be maintained. Further advance is not possible because of increasing sea-floor depth and diverging flow resulting from lack of lateral constraints. If a mass balance deficiency causes the terminal region to thin, retreat is initiated with the calving front retreating to where the thickness is slightly in excess of flotation. In that case, the calving rate is determined by glacier speed and thickness change at the glacier snout. Advance or retreat of the calving front is not driven by changes in the calving rate, but by flow-induced changes in the geometry of the terminal region. This model is essentially different from prior suggestions in which some empirical relation - most commonly the water-depth model - is used to calculate calving, rate and the rate of retreat or advance of the terminus.
Article
To assess the impact of sampling density on the determination of a glacier's annual mass balance, we used varying densities of measurements to determine annual mass balance on Columbia Glacier, Washington and Lemon Creek Glacier, Alaska. Mass balance was determined solely from field measurements. The density of the mass balance networks ranged from 1 to 375 points km-2. The smaller networks were subsamples of the highest measurement density network. The results on both glaciers indicate significant improvement in accuracy resulting from increasing the total number of measurements from 1 to 4 points km-2 on Lemon Creek Glacier and 12 to 46 points km-2 on Columbia Glacier. There was no significant improvement in accuracy on the smaller Columbia Glacier for utilizing more than 46 points km-2. On Lemon Creek Glacier there was little improvement in mass balance assessment for a network greater than 4 points km-2. On both glaciers this represented a network of 40 measurement sites.
Article
We use a satellite-based survey of glacier surface elevation changes, speeds and surface melt conditions between 2000 and 2011 to quantify mass loss from the Northern Patagonian Icefield (NPI), Chile. A history of ice elevation change is found by differencing ASTER Digital Elevation Models (DEMs) relative to a void-filled version of the DEM collected by the Shuttle Radar Topography Mission (SRTM) in February 2000. Thinning rates have accelerated at lower elevations, while above the Equilibrium Line Altitude (ELA) recent thinning rates are not significantly different from those observed in previous studies. A volumetric change of − 4.06 ± 0.11 km³/yr is found by summing surface elevation changes over all glaciers in the NPI. This is regarded as a lower bound because volume loss due to frontal retreat and sub-aqueous melting is not included. This volume change is converted to a mass loss of 3.40 ± 0.07 Gt/yr, taking into account density differences above and below the equilibrium line. We find that the NPI is providing at least 0.009 ± 0.0002 mm/yr to ongoing sea level change, in agreement with previous estimates.
Article
New ice-velocity measurements are obtained for the main trunk of Byrd Glacier, East Antarctica, using recently acquired Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery. The velocities are derived from the application of a cross-correlation technique to sequential images acquired in 2000 and 2001. Images were co-registered and ortho-rectified with the aid of a digital elevation model (DEM) generated from ASTER stereo imagery. This paper outlines the process of DEM generation, image co-registration and correction, and the application of the cross-correlation technique to obtain ice velocities. Comparison of the new velocity map with earlier measurements of velocity from 1978 indicates that the glacier has undergone a substantial deceleration between observations. Portions of the glacier flowing at speeds of ∼850 m a−1 in 1978/79 were flowing at ∼650 m a−1 in 2000/01. The cause of this change in ice dynamics is not known, but the observation shows that East Antarctic outlet glaciers can undergo substantial changes on relat ively short timescales.
Article
Mendenhall Glacier is a lake-calving glacier in southeastern Alaska, USA, that is experiencing substantial thinning and increasingly rapid recession. Long-term mass wastage linked to climatic trends is responsible for thinning of the lower glacier and leaving the terminus vulnerable to buoyancy-driven calving and accelerated retreat. Bedrock topography has played a major role in stabilizing the terminus between periods of rapid calving and retreat. Lake-terminating glaciers form a population distinct from both tidewater glaciers and polar ice tongues, with some similarities to both groups. Lacustrine termini experience fewer perturbations (e.g. tidal flexure, high subaqueous melt rates) and are therefore inherently more stable than tidewater termini. At Mendenhall, rapid thinning and simultaneous retreat into a deeper basin led to flotation conditions along approximately 50% of the calving front. This unstable terminus geometry lasted for approximately 2 years and culminated in large-scale calving and terminus collapse during summer 2004. Buoyancy-driven calving events and terminus break-up can result from small, rapidly applied perturbations in lake level.
Article
Offset fields between pairs of JERS-1 satellite SAR data acquired in winter with 44 days time interval were employed for the estimation of Arctic glacier motion over Svalbard, Novaya Zemlya and Franz-Josef Land. The displacement maps show that the ice caps are divided into a number of clearly defined fast-flowing units with displacement larger than about 6 m in 44 days (i.e. 50 m/year). The estimated error of the JERS-1 offset tracking derived displacement is on the order of 20 m/year. Occasionally, azimuth streaks related to auroral zone ionospheric disturbances were detected and dedicated processing steps were applied to minimize their influence on the estimated motion pattern. Our analysis demonstrated that offset tracking of L-band SAR images is a robust and direct estimation technique of glacier motion. The method is particularly useful when differential SAR interferometry is limited by loss of coherence, i.e. for rapid and incoherent flow and large acquisition time intervals between the two SAR images. The JERS-1 results, obtained using SAR data acquired by a satellite operated until 1998, raise expectations of L-band SAR data from the ALOS satellite launched in early 2006.
Article
The worldwide retreat of mountain glaciers has important consequences for the water, food, and power supply of large and densely populated areas in South and Central Asia. Successful mitigation of the hydrological impacts on societies as well as assessing glacier-related hazards require large-scale monitoring of glacier dynamics. However, detailed glaciological data from the Asian highlands are lacking, due to its size and difficult accessibility. We have applied a novel technique for precise orthorectification, co-registration, and sub-pixel correlation of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite imagery to derive surface velocities of Himalayan glaciers. Our approach allows for the correction of offsets due to attitude effects and sensor distortions, as well as elevation errors if a digital elevation model (DEM) from the Shuttle Radar Topography Mission (SRTM) was used for orthorectification. After post-processing, the error on the displacements is on the order of 2–4 m per correlation. Translated into annual velocities, this error is reduced (increased) when the correlated images are more (less) than a year apart. Through application of a filtering procedure and several quality tests, the consistency of the results is validated to provide confidence in the remotely sensed velocity measurements, despite the lack of ground control. This novel approach allows fast, easy, and economically viable acquisition of detailed glaciological data in areas of difficult access and provides a means for large-scale monitoring of glaciers in high mountainous terrain.
Article
For 11 days in February 2000, the Shuttle Radar Topography Mission (SRTM) successfully recorded by interferometric synthetic aperture radar (InSAR) data of the entire land mass of the earth between 60°N and 57°S. The data acquired in C- and X-bands are processed into the first global digital elevation models (DEMs) at 1 arc sec resolution, by NASA-JPL and German aerospace center (DLR), respectively. From the perspective of the SRTM-X system, we give in this paper an overview of the mission and the DEM production, as well as an evaluation of the DEM product quality. Special emphasis is on challenges and peculiarities of the processing that arose from the unique design of the SRTM system, which has been the first single-pass interferometer in space.
Article
Alaska's climate is changing and one of the most significant indications of this change has been the late 19th to early 21st century behavior of Alaskan glaciers. Weather station temperature data document that air temperatures throughout Alaska have been increasing for many decades. Since the mid-20th century, the average change is an increase of ∼ 2.0 °C. In order to determine the magnitude and pattern of response of glaciers to this regional climate change, a comprehensive analysis was made of the recent behavior of hundreds of glaciers located in the eleven Alaskan mountain ranges and three island areas that currently support glaciers. Data analyzed included maps, historical observations, thousands of ground-and-aerial photographs and satellite images, and vegetation proxy data. Results were synthesized to determine changes in length and area of individual glaciers. Alaskan ground photography dates from 1883, aerial photography dates from 1926, and satellite photography and imagery dates from the early 1960s. Unfortunately, very few Alaskan glaciers have any mass balance observations.In most areas analyzed, every glacier that descends below an elevation of ∼ 1500 m is currently thinning and/or retreating. Many glaciers have an uninterrupted history of continuous post-Little-Ice-Age retreat that spans more than 250 years. Others are characterized by multiple late 19th to early 21st century fluctuations. Today, retreating and/or thinning glaciers represent more than 98% of the glaciers examined. However, in the Coast Mountains, St. Elias Mountains, Chugach Mountains, and the Aleutian Range more than a dozen glaciers are currently advancing and thickening. Many currently advancing glaciers are or were formerly tidewater glaciers. Some of these glaciers have been expanding for more than two centuries. This presentation documents the post-Little-Ice-Age behavior and variability of the response of many Alaskan glaciers to changing regional climate.