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Recent surges of Dan Zhur (Donjek) Glacier have formed lakes at the glacier terminus that have drained catastrophically, resulting in hazards to people and infrastructure downstream. Here we use air photos and satellite imagery to describe lake formation, and the timing of filling and draining, since the 1930s. Between the 1930s and late 1980s, lakes were typically small (1 [km.sup.2]) and drain rapidly through or under the glacier by breaking a terminal ice dam. For the past two surges, since 2001, the lakes formed during or immediately after a surge in an increasingly larger basin between the Neoglacial maximum moraine and an increasingly smaller maximum terminus extent. Most recently, the 2012-2014 surge created a lake that drained in summer 2017, refilled, and drained again in both summer 2018 and summer 2019. The 2019 lake was 2.2 [km.sup.2], the largest on record, and drained entirely within 2 days. While a lake is unlikely to form again before the next expected surge in the mid-2020s, future surges of Dan Zhur Glacier are still likely to create terminal lakes, necessitating continued monitoring for surge activity and lake formation. Key words: glacier surge, remote sensing, glacier lake outburst flood, Yukon, St. Elias Mountains. Résumé: Les surges récentes du glacier Dan Zhùr (Donjek) ont formé des lacs au front du glacier qui se sont vidangés de manière catastrophique, posant un danger pour les humains et les infrastructures en aval. Nous utilisons des photographies aériennes et l'imagerie satellitaire pour décrire la formation des lacs et le moment de leur remplissage et de leur vidange depuis les années 1930. Entre cette décennie et la fin des années 1980, les lacs étaient typiquement petits (1 [km.sup.2]) et se vidangent rapidement à travers ou sous le glacier en brisant une obturation glaciaire frontale. Pour les deux dernières surges, depuis 2001, les lacs se sont formés durant ou immédiatement après une surge dans un bassin de plus en plus grand entre la moraine du maximum néoglaciaire et l'étendue maximumde plusen plus restreintedufront.Lasurgela plus récentede 2012-2014 aentraîné la formation d'un lac qui s'est vidangé à l'été 2017, puis s'est rempli et vidangé à nouveau aux étés 2018 et 2019. Le lac de 2019 faisait 2,2 [km.sup.2], soit le plus grand lac observé, et s'est complètement vidangé en deux jours. S'il est peu probable qu'un autre lac se forme avant la prochaine surge prévue au milieu de la décennie 2020, les surges futures du glacier Dan Zhùr sont toujours susceptibles de créer des lacs frontaux, nécessitant ainsi une surveillance continue de l'activité de surge et de la formation de lacs. [Traduit par la Rédaction] Mots-clés : surge glaciaire, télédétection, débâcle de lac glaciaire, Yukon, St. Elias Mountains., Introduction Glacier-dammed lakes pose a threat to downstream infrastructure and communities because glacial lake outburst floods can occur rapidly with little or no warning. Surge-type glaciers pose a unique set [...]

This study provides the first comprehensive reconstruction of the dynamics of Iceberg Glacier, located on western Axel Heiberg Island, and reveals detailed observations of a complete surge for the first time in the Canadian Arctic. Historical aerial photographs, declassified intelligence satellite photographs, optical satellite imagery and synthetic aperture radar data were used to quantify changes in terminus position, ice velocity and glacier thickness since the 1950s. A surge initiated at the terminus in 1981 and terminated in 2003, suggesting a 22-year active phase. High surface velocities, reaching ~2300 m a−1 in 1991, were accompanied by a maximum terminus advance of >7 km and a large transfer of mass down-glacier, causing significant median trunk-wide surface elevation changes attaining >3 ± 1 m a−1. We suggest that the retreat from a pinning point, flotation of the terminus, the removal of sea-ice from the ice front, and an increase in subglacial meltwater availability from relatively high air temperatures in 1981 likely contributed to surge initiation. The ensuing quiescent period has seen a continual decrease in surface flow rates to an average centreline velocity of 11.5 m a−1 in 2020–21, a gradual steepening of the glacier surface and a > 2.5 km terminus retreat. [ABSTRACT FROM AUTHOR]

Marine-terminating glaciers cover more than one-fourth of the total glacierized area in the Northern Hemisphere outside the Greenland ice sheet. It is therefore crucial to ensure an adequate representation of these glaciers when projecting large-scale glacier mass changes. We investigate how the introduction of marine frontal processes in the modeling chain influences the results of mass change projections, compared to projections neglecting such processes. We find that including frontal processes reduces the projected glacier mass loss, since incorporating frontal ablation in the model's mass-balance calibration results in a decrease in marine-terminating glaciers' sensitivity to atmospheric temperatures. We also find that retrograde bed slopes lead to increased frontal ablation as the atmosphere warms, while frontal ablation decreases if bed slopes are prograde. These opposing effects have the potential to partly cancel each other when considering large glacier ensembles. Although we do not account for potential future changes in oceanic climate yet, any effect of these would be moderated by around half of today's marine-terminating glaciers becoming land-terminating in the course of the 21st century. While we find a significant influence of ice flow parameters on our results, boundary conditions remain the largest source of uncertainty in our projections. [ABSTRACT FROM AUTHOR]

In Greenland, 87% of the glacierized area terminates in the ocean, but mass lost at the ice‐ocean interface, or frontal ablation, has not yet been fully quantified. Using measurements and models we calculate frontal ablation of Greenland's 213 outlet and 537 peripheral glaciers and find a total frontal ablation of 481.8 ± 24.0 for 2000–2010 and 510.2 ± 18.6 Gt a−1 for 2010–2020. Ice discharge accounted for ∼90% of frontal ablation during both periods, while mass loss due to terminus retreat comprised the remainder. Only 16 glaciers were responsible for the majority (>50%) of frontal ablation from 2010 to 2020. These estimates, along with the climatic‐basal balance, allow for a more complete accounting of Greenland Ice Sheet and peripheral glacier mass balance. In total, Greenland accounted for ∼90% of Northern Hemisphere frontal ablation for 2000–2010 and 2010–2020. Plain Language Summary: We estimate the mass of ice lost from all Greenland glaciers that entered the ocean during each of the last two decades. This ice loss at the front of these marine‐terminating glaciers is called frontal ablation and is approximately equal to the mass of icebergs entering the ocean. Frontal ablation is important because 87% of glacier area in Greenland ends in the ocean, through 750 outlets, and previous work has only approximated frontal ablation. This study quantifies it for the first time, helping to close the mass budget for the Greenland Ice Sheet and better partition its mass balance into components. We find that Greenland accounts for ∼90% of all Northern Hemisphere frontal ablation and, of that contribution, just 17 glaciers for 2000–2010 and 16 glaciers for 2010–2020 account for more than half of total Greenland frontal ablation. Key Points: Frontal ablation of the Greenland Ice Sheet averaged 510.2 ± 18.6 Gt a−1 for 2010–2020, ∼90% of which came from ice dischargeThe frontal ablation we measured is larger than the total mass loss from the ice sheet, indicating a positive climatic‐basal balanceOnly 16 glaciers account for 50% of the total frontal ablation from the Greenland Ice Sheet [ABSTRACT FROM AUTHOR]

Knowledge of frontal ablation from marine-terminating glaciers (i.e., mass lost at the calving face) is critical for constraining glacier mass balance, improving projections of mass change, and identifying the processes that govern frontal mass loss. Here, we discuss the challenges involved in computing frontal ablation and the unique issues pertaining to both glaciers and ice sheets. Frontal ablation estimates require numerous datasets, including glacier terminus area change, thickness, surface velocity, density, and climatic mass balance. Observations and models of these variables have improved over the past decade, but significant gaps and regional discrepancies remain, and better quantification of temporal variability in frontal ablation is needed. Despite major advances in satellite-derived large-scale datasets, large uncertainties remain with respect to ice thickness, depth-averaged velocities, and the bulk density of glacier ice close to calving termini or grounding lines. We suggest ways in which we can move toward globally complete frontal ablation estimates, highlighting areas where we need improved datasets and increased collaboration. [ABSTRACT FROM AUTHOR]

Glacier velocity measurements are essential to understand ice flow mechanics, monitor natural hazards, and make accurate projections of future sea-level rise. Despite these important applications, the method most commonly used to derive glacier velocity maps, feature tracking, relies on empirical parameter choices that rarely account for glacier physics or uncertainty. Here we test two statistics- and physics-based metrics to evaluate velocity maps derived from optical satellite images of Kaskawulsh Glacier, Yukon, Canada, using a range of existing feature-tracking workflows. Based on inter-comparisons with ground truth data, velocity maps with metrics falling within our recommended ranges contain fewer erroneous measurements and more spatially correlated noise than velocity maps with metrics that deviate from those ranges. Thus, these metric ranges are suitable for refining feature-tracking workflows and evaluating the resulting velocity products. We have released an open-source software package for computing and visualizing these metrics, the GLAcier Feature Tracking testkit (GLAFT). [ABSTRACT FROM AUTHOR]

Using historical and recent aerial photography and structure from motion (SfM) multiview stereo (MVS) techniques, we reconstruct the 1959 and 2018 ice surface topography and determine the geodetic mass balance of Bowman Glacier, a small mountain glacier on northern Ellesmere Island. This is combined with optical satellite imagery to reconstruct the evolution in extent of the glacier over six decades, and ground-penetrating radar measurements of ice thickness to estimate the remaining ice volume. Between 1959 and 2020, Bowman Glacier lost 78% of its extent (reducing from 2.75 to 0.61 km2), while average annual area loss rates have nearly tripled in the past two decades. Over the 1959–2018 period, glacier-wide ice-thickness change averaged −22.7 ± 4.7 m, corresponding to a mean specific annual mass balance of −347.0 ± 71.4 mm w.e. a−1. Projecting rates of area and volume change into the future indicates that the glacier will likely entirely disappear between 2030 and 2060. This study demonstrates the potential of SfM-MVS processing to generate elevation products from 1950/60s historical aerial photographs, and to extend observations of ice elevation and glacier volume change for the Canadian Arctic, prior to the satellite record. [ABSTRACT FROM AUTHOR]

Links between proglacial lakes and glacier dynamics are poorly understood but are necessary to predict how mountain glaciers will react to a warmer, wetter climate, where such lakes are expected to increase both in number and volume. Here, we examine a long-term (~120 year) record of terminus retreat, thinning and surface velocities from in-situ and remote sensing observations at the terminus of Kaskawulsh Glacier, Yukon, Canada, and determine the impact of a local proglacial hydrological reorganisation on glacier dynamics. After an initial deceleration during the late 1990s, terminus velocities increased at a rate of 3 m a−2 from 2000–12, while proglacial Slims Lake area increased simultaneously. The rapid drainage of the lake in May 2016 substantially altered the velocity profile, decreasing annual velocities by 48% within 3 km of the terminus between 2015 and 2021, at an average rate of ~ 12.5 m a−2. A key cause of the rapid drop in glacier motion was a reduction in flotation of the lower part of the glacier terminus after lake drainage. This has important implications for glacier dynamics and provides one of the first assessments of the impacts of a rapid proglacial lake drainage event on local terminus velocities. [ABSTRACT FROM AUTHOR]

Supraglacial drainage networks play an integral role in both glacier dynamics and run-off timing, and mapping them provides insight into their role in glacial systems. Here we present a reproducible approach for semi-automated mapping of supraglacial hydrologic features, which complements existing work in automated and manual mapping by providing clear definitions for identification of features. This framework uses a digital terrain model (DTM) to identify potential flow routes on the glacier surface, which are then classified using a set of standardized rules based on the DTM and an orthomosaic. We found that the normalized difference water index calculated from digital imagery was influenced by image brightness and introduce a new approach using average RGB values to correct for this. Using this framework we mapped supraglacial drainage networks at Nàłùdäy and Thores Glacier, Canada. The framework was easier to implement with high-resolution (0.5 m) imagery and DTMs, compared to data with lower resolution (10 m), due to the increased detail in topography and feature boundaries at high-resolution. Lower-resolution data captured larger streams (>2 pixels wide), however, indicating that the framework can still be used at this resolution. Mapping supraglacial hydrology using standardized methods opens possibilities for investigating many questions relating to changes in supraglacial hydrology over time. [ABSTRACT FROM AUTHOR]

Glacier velocity measurements are essential to understand ice flow mechanics, monitor natural hazards, and make accurate projections of future sea-level rise. Despite these important applications, the method most commonly used to derive glacier velocity maps, feature tracking, relies on empirical parameter choices that rarely account for glacier physics or uncertainty. Here we test two statistics- and physics-based metrics to assess velocity maps from a range of existing feature-tracking workflows at Kaskawulsh Glacier, Canada. Based on inter-comparisons with ground-truth data, velocity maps with metrics falling within our recommended ranges contain fewer erroneous measurements and more spatially correlated noise than velocity maps with metrics that deviate from those ranges. Thus, these metric ranges are suitable for refining feature-tracking workflows and evaluating the resulting velocity products. We have released an open-source software package for computing and visualizing these metrics, the GLAcier Feature Tracking testkit (GLAFT). [ABSTRACT FROM AUTHOR]

Speckle tracking of ALOS PALSAR fine beam data from 2007-2011 are used to determine the surface motion of major ice masses on Baffin Island and Bylot Island in the southern Canadian Arctic Archipelago. Glacier velocities are low overall, with peaks of ~100 m [a.sup.-1] and means of ~20-60 m [a.sup.-1] common along the main trunk of many outlet glaciers. Peak velocities on Penny and Bylot Island ice caps tend to occur near the mid-sections of their primary outlet glaciers, while the fastest velocities on all other glaciers usually occur near their termini due to relatively large accumulation areas draining through narrow outlets. Estimates of ice thickness at the fronts of tidewater-terminating glaciers are combined with the velocity measurements to determine a regional dynamic discharge rate of between ~17 Mt [a.sup.-1] and ~108 Mt [a.sup.-1], with a mid-point estimate of ~55 Mt [a.sup.-1], revising downward previous approximations. These velocities can be used as inputs for glacier flow models, and provide a baseline dataset against which future changes in ice dynamics can be detected. Le suivi speckle des donnees de faisceau fin ALOS PALSAR, prelevees entre 2007 et 2011, est utilise pour determiner le mouvement de surface des grandes masses de glace sur les iles de Baffin et de Bylot dans le sud de l'archipel arctique canadien. Les vitesses des glaciers sont generalement faibles avec des pointes d'environ 100 m [a.sup.-1] et des moyennes de 20 a 60 m [a.sup.-1] frequentes le long du tronc principal de nombreux glaciers emissaires. Les vitesses de pointe des champs de glace de Penny et de Bylot tendent a etre pres des sections medianes de leurs glaciers emissaires primaires, alors que les vitesses les plus rapides sur tous les autres glaciers sont enregistrees pres du front en raison des aires d'accumulation relativement grandes qui s'ecoulent a travers d'etroites decharges. Des estimations de l'epaisseur de la glace aux fronts de glaciers de maree sont combinees aux mesures de vitesse afin de determiner un taux regional de decharge dynamique entre ~17 Mt [a.sup.-1] et ~108 Mt [a.sup.-1], avec une estimation du taux median de ~55 [a.sup.-1], ce qui revise les estimations anterieures a la baisse. Ces vitesses peuvent etre utilisees comme donnees d'entree pour les modeles d'ecoulement glaciaire et ainsi fournir un ensemble de donnees de reference pour detecter les futurs changements dans la dynamique glaciaire. [Traduit par la Redaction], Introduction The southern Canadian Arctic Archipelago (CAA), consisting of Baffin Island and Bylot Island (Fig. 1), contain ~40 000 [km.sup.2] of glacier ice (Gardner et al. 2012). Recent studies indicate [...]

Terrestrial photographic imagery combined with structure-from-motion (SfM) provides a relatively easy-to-implement method for monitoring environmental systems, even in remote and rough terrain. However, the collection of in-situ positioning data and the identification of control points required for georeferencing in SfM processing is the primary roadblock to using SfM in difficult-to-access locations; it is also the primary bottleneck for using SfM in a time series. We describe a novel, computationally efficient, and semi-automated approach for georeferencing unreferenced point clouds (UPC) derived from terrestrial overlapping photos to a reference dataset (e.g., DEM or aerial point cloud; hereafter RPC) in order to address this problem. The approach utilizes a Discrete Global Grid System (DGGS), which allows us to capitalize on easily collected rough information about camera deployment to coarsely register the UPC using the RPC. The DGGS also provides a hierarchical set of grids which supports a hierarchical modified iterative closest point algorithm with natural correspondence between the UPC and RPC. The approach requires minimal interaction in a user-friendly interface, while allowing for user adjustment of parameters and inspection of results. We illustrate the approach with two case studies: a close-range (<1 km) vertical glacier calving front reconstructed from two cameras at Fountain Glacier, Nunavut and a long-range (>3 km) scene of relatively flat glacier ice reconstructed from four cameras overlooking Nàłùdäy (Lowell Glacier), Yukon, Canada. We assessed the accuracy of the georeferencing by comparing the UPC to the RPC, as well as surveyed control points; the consistency of the registration was assessed using the difference between successive registered surfaces in the time series. The accuracy of the registration is roughly equal to the ground sampling distance and is consistent across time steps. These results demonstrate the promise of the approach for easy-to-implement georeferencing of point clouds from terrestrial imagery with acceptable accuracy, opening the door for new possibilities in remote monitoring for change-detection, such as monitoring calving rates, glacier surges, or other seasonal changes at remote field locations. [ABSTRACT FROM AUTHOR]

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The once-contiguous Ellesmere Ice Shelf, first reported in writing by European explorers in 1876, and now almost completely disintegrated, has rolling, wave-like surface topography, the origin of which we investigate using a viscous buckling instability analysis. We show that rolls can develop during a winter season (~ 100 d) if sea-ice pressure (depth-integrated horizontal stress applied to the seaward front of the Ellesmere Ice Shelf) is sufficiently large (1 MPa m) and ice thickness sufficiently low (1–10 m). Roll wavelength initially depends only on sea-ice pressure, but evolves over time depending on amplitude growth rate. This implies that a thinner ice shelf, with its faster amplitude growth rate, will have a shorter wavelength compared to a thicker ice shelf when sea-ice pressure is equal. A drawback of the viscous buckling mechanism is that roll amplitude decays once sea-ice pressure is removed. However, non-Newtonian ice rheology, where effective viscosity, and thus roll change rate, depends on total applied stress may constrain roll decay rate to be much slower than growth rate and allow roll persistence from year to year. Whether the viscous-buckling mechanism we explore here ultimately can be confirmed as the origin of the Ellesmere Ice Shelf rolls remains for future research. [ABSTRACT FROM AUTHOR]

Knowledge of frontal ablation from marine-terminating glaciers (i.e., mass lost at the calving face) is critical for constraining glacier mass balance, improving projections of mass change, and identifying the processes that govern frontal mass loss. Here, we discuss the challenges involved in computing frontal ablation and the unique issues pertaining to both glaciers and ice sheets. Frontal ablation estimates require numerous datasets, including glacier terminus area change, thickness, surface velocity, density, and climatic mass balance. Observations and models of these variables have improved over the past decade, but significant gaps and regional discrepancies remain, and better quantification of temporal variability in frontal ablation is needed. Despite major advances in satellite-derived large-scale datasets, large uncertainties remain with respect to ice thickness, depth-averaged velocities, and the bulk density of glacier ice close to calving termini or grounding lines. We suggest ways in which we can move toward globally complete frontal ablation estimates, highlighting areas where we need improved datasets and increased collaboration. [ABSTRACT FROM AUTHOR]

Remote sensing data are a crucial tool for monitoring climatological changes and glacier response in areas inaccessible for in situ measurements. The Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) product provides temperature data for remote glaciated areas where air temperature measurements from weather stations are sparse or absent, such as the St. Elias Mountains (Yukon, Canada). However, MODIS LSTs in the St. Elias Mountains have been found in prior studies to show an offset from available weather station measurements, the source of which is unknown. Here, we show that the MODIS offset likely results from the occurrence of near-surface temperature inversions rather than from the MODIS sensor's large footprint size or from poorly constrained snow emissivity values used in LST calculations. We find that an offset in remote sensing temperatures is present not only in MODIS LST products but also in Advanced Spaceborne Thermal Emissions Radiometer (ASTER) and Landsat temperature products, both of which have a much smaller footprint (90–120 m) than MODIS (1 km). In all three datasets, the offset was most pronounced in the winter (mean offset >8 ∘C) and least pronounced in the spring and summer (mean offset <2 ∘C). We also find this enhanced seasonal offset in MODIS brightness temperatures, before the incorporation of snow surface emissivity into the LST calculation. Finally, we find the MODIS LST offset to be consistent in magnitude and seasonal distribution with modeled temperature inversions and to be most pronounced under conditions that facilitate near-surface inversions, namely low incoming solar radiation and wind speeds, at study sites Icefield Divide (60.68 ∘ N, 139.78 ∘ W; 2,603 m a.s.l) and Eclipse Icefield (60.84 ∘ N, 139.84 ∘ W; 3017 m a.s.l.). Although these results do not preclude errors in the MODIS sensor or LST algorithm, they demonstrate that efforts to convert MODIS LSTs to an air temperature measurement should focus on understanding near-surface physical processes. In the absence of a conversion from surface to air temperature based on physical principles, we apply a statistical conversion, enabling the use of mean annual MODIS LSTs to qualitatively and quantitatively examine temperatures in the St. Elias Mountains and their relationship to melt and mass balance. [ABSTRACT FROM AUTHOR]

The physical, sedimentological, mineralogical and elemental geochemical properties of sediment cores AMD1803‐02BC and 01PC from the Cape Norton Shaw Inlet were investigated to reconstruct glacial sediment discharges from southeastern Manson Icefield and document the impact of ice–ocean interactions on the sediment dynamics and opening of the North Water Polynya (NOW) in northwestern Baffin Bay since the last deglaciation. Laminated glaciomarine sediments rich in quartz and feldspar are observed prior to 11 cal. ka BP and were probably deposited by hyperpycnal currents triggered by the local retreat of the southern margin of the Innuitian Ice. Detrital proxies suggest that Early Holocene sediment dynamics were mainly influenced by sea ice and iceberg rafting and meltwater discharges related to the deglaciation of eastern Smith (~11 to 10.65 cal. ka BP) and Jones (~10.7 cal. ka BP) sounds. This also provides an upper limit to the timing of formation of the NOW. The high detrital carbonate contents during 8.8 to 6.6 cal. ka BP confirm that enhanced carbonate‐rich sediment export from Nares Strait to northern Baffin Bay occurred during and after the deglaciation of Kennedy Channel (8.8 to 8.2 cal. ka BP). Canadian Shield sediment inputs have dominated since 6.6 cal. ka BP, indicating that sedimentation is mainly influenced by Cape Norton Shaw glacier discharges. The lower level of sedimentation recorded in core 01PC during the Middle to Late Holocene suggests an accelerated landward retreat of the Cape Norton Shaw glaciers in response to warmer marine conditions. During the Neoglacial period, higher sedimentation rates and detrital proxies in the cores suggest increased glacial erosional processes, probably associated with the long‐term declines in boreal summer insolation and glacier growth. Finally, mineralogical and grain‐size data in core 02BC support the idea that increased Arctic atmospheric temperatures have had an important influence on the glacial dynamics during the industrial period. [ABSTRACT FROM AUTHOR]

Over the last two decades, Trinity, Wykeham, Ekblaw, and Cadogan glaciers have retreated and been identified as the primary sources of iceberg flux from Prince of Wales (POW) Icefield, Ellesmere Island, accounting for ∼83% of total solid ice discharge. In this study, we used a total of 167 Radarsat‐2 Synthetic Aperture Radar scene pairs collected between 2009 and 2019 to derive winter surface velocities of these four major basins of the POW Icefield. Over this period both Cadogan and Ekblaw glaciers underwent multiyear acceleration and deceleration limited to their lower parts, consistent with characteristics of "pulse‐type" glaciers. Trinity and Wykeham glaciers are currently the fastest flowing glaciers in the CAA and are grounded below sea level for ∼40 km up‐glacier from their termini. Both glaciers underwent multiyear repeating periods of velocity acceleration between 2009 and 2019 which coincided with significant thinning at their termini. As of 2017, Trinity and Wykeham were each within ∼10 m of flotation over their lowermost 4 km. We also identified a shift in flow since 2014 on Trinity and Wykeham, after which winter flow rates began to propagate up‐glacier and were nearly identical to flow rates observed throughout the rest of the year, indicating that seasonal variability in flow has become less apparent in recent years. Our findings suggest that Trinity and Wykeham glaciers have transitioned to a flow type dominated by dynamic thinning, which is strongly influenced by subglacial topography and may be susceptible to instability of the glacier front and large‐scale collapse. Plain Language Summary: Trinity, Wykeham, Ekblaw, and Cadogan are the four primary glacier basins on the Prince of Wales (POW) Icefield, and are responsible for the majority of solid ice discharged from it. Previous work on tidewater glacier dynamics in the Canadian Arctic has mainly focused on quantifying surface velocities and ice fluxes to the ocean, but less research has been undertaken on the physical processes controlling variability in glacier flow. Here, we use remote sensing methods to measure changes in ice flow, surface elevation, and terminus buoyancy to identify the processes controlling glacier motion in this region. We find that ice flow of the four major glaciers on the POW Icefield are strongly controlled by their underlying bed topography. Trinity and Wykeham glaciers are two of the fastest flowing glaciers in the Canadian Arctic and appear to have recently entered a period of instability at their termini, which, if they continue at the current rates of thinning, could lead to their large‐scale acceleration, retreat, and collapse. Key Points: We report updated winter velocity and surface elevation change estimates for four major basins on the Prince of Wales IcefieldFlow for Trinity, Wykeham, Cadogan, and Ekblaw glaciers is strongly influenced by bed topographyVelocity changes, surface thinning, and terminus buoyancy resulted in instability and near flotation of the termini of Trinity and Wykeham glaciers [ABSTRACT FROM AUTHOR]

We mapped the terminus position for every marine‐terminating glacier in the Northern Hemisphere for 2000, 2010, and 2020, including the Greenland Ice Sheet, to provide the first complete measure of their variability. In total, these 1,704 glaciers lost an average of 389.7 ± 1.6 km2 a−1 (total 7,527 ± 31 km2) from 2000 to 2020 with 123 glaciers becoming no longer marine‐terminating over this period. Overall, 85.3% of glaciers retreated, 2.5% advanced, and the remaining 12.3% did not change outside of uncertainty limits. Outlet glaciers of the Greenland Ice Sheet are responsible for 61.9% of total area loss, although their rate of retreat was 34% less in 2010–2020 than 2000–2010. Glaciers with the largest area loss terminate in ice shelves or ice tongues, are surge‐type, have an unstable basal geometry, or have an unusually wide calving margin. Plain Language Summary: North of the equator, 1,704 glaciers touched the ocean in 2000. Here, we present the first analysis to document the frontal position of every one of these glaciers in 2000, 2010, and 2020. We found that 85.3% retreated and are now reduced in area. Only 2.5% of glaciers advanced or increased in area. The remaining 12.3% did not change within uncertainty limits. Total area losses were 389.7 ± 1.6 km2 per year (total 7,527 ± 31 km2) over the 20‐year period. Glaciers flowing from the Greenland Ice Sheet accounted for over 60% of total area losses. We found wide variations in the response of glaciers to similar changes in air and ocean temperature and sea ice concentrations, showing that environmental conditions alone cannot explain why some glaciers retreated more than others. Instead, unique glacier characteristics are the most important factor in controlling the variability of terminus retreat. Glaciers with floating ice at their front (ice shelves or ice tongues), those that undergo periodic changes in their flow velocity (surges), those which have a weak connection to their beds, and glaciers that are unusually wide, experienced the largest area loss from 2000 to 2020. Key Points: There were 1704 marine‐terminating glaciers in 2000 in the Northern Hemisphere, of which 85.3% retreated and 2.5% advanced from 2000 to 2020Tidewater glaciers lost a total area of 7,527 ± 31 km2 from 2000 to 2020, with the Greenland Ice Sheet responsible for 61.9% of total lossesVariations in retreat are best explained by glacier characteristics: ice shelves/tongues, surging, basal geometry, and calving width [ABSTRACT FROM AUTHOR]

Substrate properties and hydrological conditions at the base of Haut Glacier d'Arolla, Switzerland, are nonuniform. The thickness and grain size of subglacial sediment vary on length scales less than one ice thickness, whereas hydrological conditions vary seasonally and on length scales of about one ice thickness transverse to ice motion. There is a close relationship between the annually averaged velocity field and this nonuniformity of bed conditions. Basal motion dominates in an area with large water-level variations in boreholes, whereas ice deformation contributes significantly to total movement elsewhere. Localized enhanced basal motion occurs primarily in summer, especially during a July 'spring event,' but this motion is barely discernible in annually averaged surface velocity measurements, because transverse coupling suppresses surface expression of the basal motion discontinuity. These results highlight the need to include representations of bed nonuniformity in models of glacier flow and to consider ice deformation and basal motion as interdependent processes.

In this study, we use aerial photographs, satellite imagery and field observations to quantify changes in the area, terminus length, snowline elevation and surface elevation of eight glaciers in the Alexandra Fiord region, eastern Ellesmere Island, between 1959 and 2019. Comparisons to written and pictorial descriptions from the British Arctic Expedition extend the record of change in terminus position and surface elevation to 1875 for Twin Glacier. Glacier area at Alexandra Fiord decreased by a total of 15.77 ± 0.65 km2 (11.77 ± 0.49%) between 1959 and 2019, the mean end of summer snowline increased in elevation by 360 ± 84 m (8 ± 2 m a−1) between 1974 and 2019, and the glaciers thinned at an average rate of 0.60 ± 0.06 m a−1 between 2001 and 2018. Annual rates of terminus retreat were ~3–5 times higher over the period 1974–2019 compared to 1875–1974, and rates of thinning were ~2–3 times higher over 2001–18 compared to 1875–2001. Our results are consistent with rates of change determined for other glaciers of similar size on Ellesmere Island, and with accelerated rates of ice loss coincident with regional increases in air temperature of ~1.5°C since the early 1980s. [ABSTRACT FROM AUTHOR]

In spring 2018, two firn cores (21 and 36 m in length) were extracted from the accumulation zone of Kaskawulsh Glacier, St. Elias Mountains, Yukon. The cores were analyzed for ice layer stratigraphy and density and compared against historical measurements made in 1964 and 2006. Deep meltwater percolation and refreezing events were evident in the cores, with a total ice content of 2.33±0.26 m in the 36 m core and liquid water discovered below a depth of 34.5 m. Together with the observed ice content, surface energy balance and firn modelling indicate that Kaskawulsh Glacier firn retained about 86 % of its meltwater in the years 2005–2017. For an average surface ablation of 0.38 m w.e. yr -1 over this period, an estimated 0.28 m w.e. yr -1 refroze in the firn, 0.05 m w.e. yr -1 was retained as liquid water, and 0.05 m w.e. yr -1 drained or ran off. The refrozen meltwater is associated with a surface lowering of 0.73±0.23 m between 2005 and 2017 (i.e., surface drawdown that has no associated mass loss). The firn has become denser and more ice-rich since the 1960s and contains a perennial firn aquifer (PFA), which may have developed over the past decade. This illustrates how firn may be evolving in response to climate change in the St. Elias Mountains, provides firn density information required for geodetic mass balance calculations, and is the first documented PFA in the Yukon–Alaska region. [ABSTRACT FROM AUTHOR]

Glacier surficial melt rates are commonly modelled using surface energy balance (SEB) models, with outputs applied to extend point-based mass-balance measurements to regional scales, assess water resource availability, examine supraglacial hydrology and to investigate the relationship between surface melt and ice dynamics. We present an improved SEB model that addresses the primary limitations of existing models by: (1) deriving high-resolution (30 m) surface albedo from Landsat 8 imagery, (2) calculating shadows cast onto the glacier surface by high-relief topography to model incident shortwave radiation, (3) developing an algorithm to map debris sufficiently thick to insulate the glacier surface and (4) presenting a formulation of the SEB model coupled to a subsurface heat conduction model. We drive the model with 6 years of in situ meteorological data from Kaskawulsh Glacier and Nàłùdäy (Lowell) Glacier in the St. Elias Mountains, Yukon, Canada, and validate outputs against in situ measurements. Modelled seasonal melt agrees with observations within 9% across a range of elevations on both glaciers in years with high-quality in situ observations. We recommend applying the model to investigate the impacts of surface melt for individual glaciers when sufficient input data are available. [ABSTRACT FROM AUTHOR]

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Links between climate and glacier surges are poorly understood but are required to enable prediction of surges and mitigation of associated hazards. Here, we investigate the role of snow accumulation, rain, and temperature on surge periodicity, area changes, and timing of surge initiation since the 1930s at Donjek Glacier, Yukon, Canada. Snow accumulation measured in three ice cores collected at Eclipse Icefield indicates that a cumulative accumulation of 15.5 ± 1.46 or 16.6 ± 2.0 m w.e. occurred in the ten to twelve years between each of its last eight surges, depending on ice motion spatiotemporal offset corrections. Although we find consistent snow accumulation between surges, the transient snow line has risen 10.3 m decade-1 vertically since the 1950s, and Burwash Landing weather station records indicate a 0.5°C decade-1 increase in mean annual air temperature since the 1960s. Changes in surface mass balance are accompanied by a consistent surge interval but decreasing surge extent. The three recent surge events initiated in years with the rainiest summers on record. These findings highlight a complex interplay between external (i.e., climate) and internal glacier processes that control surging at Donjek Glacier, with climate having a more direct influence on surge extent than on recurrence interval. [ABSTRACT FROM AUTHOR]

We would like to thank Dr. Ommanney for his insightful discussion regarding our paper (Van Wychen et al. 2015). Dr. Ommanney correctly points out that the glacier identification scheme provided [...]

Donjek Glacier has an unusually short and regular surge cycle, with eight surges identified since 1935 from aerial photographs and satellite imagery with a ~12 year repeat interval and ~2 year active phase. Recent surges occurred during a period of long-term negative mass balance and cumulative terminus retreat of 2.5 km since 1874. In contrast to previous work, we find that the constriction where the valley narrows and bedrock lithology changes, 21 km from the terminus, represents the upper limit of surging, with negligible surface speed or elevation change up-glacier from this location. This positions the entire surge-type portion of the glacier in the ablation zone. The constriction geometry does not act as the dynamic balance line, which we consistently find at 8 km from the glacier terminus. During the 2012–2014 surge event, the average lowering rate in the lowest 21 km of the glacier was 9.6 m a−1, while during quiescence it was 1.0 m a−1. Due to reservoir zone refilling, the ablation zone has a positive geodetic balance in years immediately following a surge event. An active surge phase can result in a strongly negative geodetic mass balance over the surge-type portion of the glacier. [ABSTRACT FROM AUTHOR]

A total of eight floating glacier tongues have shrunk in area by >85% from the Yelverton Bay region of Northern Ellesmere Island since 1959, with unusually large losses since 2005. To better understand the causes of these losses, this study undertakes the first examination of ice tongue changes in this region, including an assessment of changes in surrounding marine ice (i.e. sea ice, sikussak and mélange), and atmospheric and oceanographic forcings. From 1959 to 2017, the total ice tongue area decreased by 49.07 km2, with the majority of this loss occurring from 2005 to 2009 (34.68 km2). The loss of ice tongues since 2005 occurred when open water replaced multi-year landfast sea ice (MLSI) and first-year sea ice in the regions adjacent to the ice tongues. These changes were accompanied by an increase in mean annual mid-depth (i.e. 100 and 200 m) ocean temperatures from −0.29°C from 1999 to 2005 to 0.67°C from 2006 to 2012. Despite the recent return of ocean temperatures to below pre-2006 levels, atmospheric summer temperatures have continued to rise (+0.15°C decade−1 between 1948 and 2016), with open water continuing to occur. Without the sustained presence of MLSI in this region the ice tongues are unable to stabilize, making it unlikely that they will re-form in the current climate. [ABSTRACT FROM AUTHOR]

Previous studies reported that Good Friday Glacier had been actively surging in the 1950–60s, 1990s and again in 2000–15. Based on observations of terminus position change from air photos and satellite imagery, we fill the gaps between previous studies and conclude that the glacier has been advancing continuously since 1959. Ice surface velocities extracted from optical and synthetic aperture radar satellite images show higher flow rates than on most other marine-terminating glaciers in the region. This behaviour contrasts with the regional trend of glacier retreat over this period. Possible explanations involve a delayed response to positive mass-balance conditions of the Little Ice Age, or a dynamic instability. There is, however, insufficient evidence to attribute this behaviour to classical glacier surging as suggested in previous studies. Based on present-day ice velocity and glacier geometry patterns in the terminus region, we reconstruct the evolution of ice motion throughout the advance, and suggest that what has previously been interpreted as a surge, may instead have been a localised response to small-scale perturbations in bedrock topography. [ABSTRACT FROM AUTHOR]

Since the 1960s, warming air and sea surface temperatures have led to decreasing sea ice extent and longer periods of open water in the Canadian Arctic Archipelago (CAA), together with changes in glacier discharge patterns. An important question, therefore, is whether there is a relationship between changing sea ice conditions, glacier dynamics, and iceberg production in this region. Using synthetic aperture radar (SAR) (Radarsat-1, Radarsat-2, and ALOS PALSAR) and optical (Landsat 7 and 8) satellite imagery, iceberg plume events and sea ice break-up/freeze-up dates between 1997 and 2015 are investigated for 40 tidewater glaciers around the Prince of Wales (POW) Icefield, Ellesmere Island. Results show a clear relationship between the presence of sea ice and the production of icebergs, with ~49% of total iceberg plume events occurring during the 3–4 month long summer open water season and ~51% of events when sea ice was present the remaining 8–9 months of the year. There is no clear evidence of recent increases in iceberg production on a regional basis, but on a local, individual glacier scale there has been a connection between periods of increased iceberg plume events and: (a) acceleration in the surface velocity of Trinity and Wykeham glaciers; (b) increase in terminus retreat rates for glaciers which have not accelerated in flow speed over the past ~5–10 years. Comparisons with ocean temperature, surface air temperature from NCEP-NCAR reanalysis, and tidal data showed no clear relationship with iceberg plume events. [ABSTRACT FROM AUTHOR]