Glacier Tongue Research Articles

Storstrømmen and L. Bistrup Bræ, North Greenland, Protected From Warm Atlantic Ocean Waters.

Storstrømmen and L. Bistrup Bræ are 20‐ and 10‐km wide, surge type glaciers in North Greenland in quiescent phase that terminate in the southernmost floating ice tongue in East Greenland. Novel multi‐beam echo sounding data collected in August 2020 indicate a seabed at 350–400 m depth along a relatively uniform ice shelf front, 100 m deeper than expected, but surrounded by shallower terrain (<100 m) over a 30‐km wide region that blocks the access of warm, salty, subsurface Atlantic Intermediate Water (AIW) at +1.6°C. Conductivity temperature depth data reveal waters in front of the glaciers at −1.8°C not connected to AIW in the outer fjord, Dove Bugt. The recent grounding line retreat of the glaciers is attributed to glacier thinning at its ablation rate, with little influence of ocean waters, which illustrates the fundamental importance of knowing the bathymetry of glacial fjords.

Simulating ice-shelf extent using damage mechanics

Abstract Inaccurate representations of iceberg calving from ice shelves are a large source of uncertainty in mass-loss projections from the Antarctic ice sheet. Here, we address this limitation by implementing and testing a continuum damage-mechanics model in a continental scale ice-sheet model. The damage-mechanics formulation, based on a linear stability analysis and subsequent long-wavelength approximation of crevasses that evolve in a viscous medium, links damage evolution to climate forcing and the large-scale stresses within an ice shelf. We incorporate this model into the BISICLES ice-sheet model and test it by applying it to idealized (1) ice tongues, for which we present analytical solutions and (2) buttressed ice-shelf geometries. Our simulations show that the model reproduces the large disparity in lengths of ice shelves with geometries and melt rates broadly similar to those of four Antarctic ice shelves: Erebus Glacier Tongue (length ~ 13 km), the unembayed portion of Drygalski Ice Tongue (~ 65 km), the Amery Ice Shelf (~ 350 km) and the Ross Ice Shelf (~ 500 km). These results demonstrate that our simple continuum model holds promise for constraining realistic ice-shelf extents in large-scale ice-sheet models in a computationally tractable manner.

The Experimental Research of Ablation of Adishi Glacier Tongue

he study of glacier mass balance is gaining significant interest worldwide against modern global climate change. Using Drone gave good results in glacier mass balance studies. There is a sizeable difference between the values ​​obtained by the modern and previous used methods, which is primarily because of the shortcomings of the old one. We believe the results obtained cause the revision of the old data of the mass balance ablation component. Therefore, it became essential to restore the observation series and determine the compatibility of the data obtained by the old, traditional methods with the data obtained using new, modern technologies. The study aimed to compare the results obtained by standard ablation studies (ablation stake) and modern aerial photogrammetric methods based on actual observations in the Adishi glacier tongue.

Crevasse advection increases glacier calving

Abstract Iceberg calving, the process where icebergs detach from glaciers, remains poorly understood. Moreover, few parameterizations of the calving process can easily be integrated into numerical models to accurately capture observations, resulting in large uncertainties in projected sea level rise. Recent efforts have focused on estimating crevasse depths assuming tensile failure occurs when crevasses fully penetrate the glacier thickness. However, these approaches often ignore the role of advecting crevasses on calculations of crevasse depth. Here, we examine a more general crevasse depth calving model that includes crevasse advection. We apply this model to idealized prograde and retrograde bed geometries as well as a prograde geometry with a sill. Neglecting crevasse advection results in steady glacier advance and ice tongue formation for all ice temperatures, sliding law coefficients and constant slope bed geometries considered. In contrast, crevasse advection suppresses ice tongue formation and increases calving rates, leading to glacier retreat. Furthermore, crevasse advection allows a grounded calving front to stabilize on top of sills. These results suggest that crevasse advection can radically alter calving rates and hint that future parameterizations of fracture and failure need to more carefully consider the lifecycle of crevasses and the role this plays in the calving process.

Modeling the Deformation Regime of Thwaites Glacier, West Antarctica, Using a Simple Flow Relation for Ice Anisotropy (ESTAR)

AbstractIce deformation dominates the evolution of ice shelf flow and the slow‐moving regions in the interior of ice sheets. However, deformation may be poorly represented in large‐scale ice sheet models that use the Glen flow relation, due to its questionable applicability to the steady‐state flow of anisotropic ice that prevails in ice sheets, having been derived from secondary creep rates of isotropic ice. We assess the deformation regimes of Thwaites Glacier, West Antarctica, using the Glen and “Empirical Scalar Tertiary Anisotropy Regime”, (ESTAR) flow relations, the latter being derived from steady‐state deformation rates of anisotropic ice. For grounded ice, the character of the flow relation determines the contribution of deformation to overall flow, with ESTAR producing greater bed‐parallel shear deformation than the standard Glen flow relation. The ESTAR experiments show larger basal shear stress maxima than the standard Glen experiment because ESTAR treats the responses to simple shear stresses and compression stresses differently, reducing the role of lateral and longitudinal stresses in momentum balance. On the Thwaites Glacier Tongue, ESTAR provides the best match to observed speeds by accounting for the differing effects of stresses on ice flow. Our results highlight the importance of the numerical description of anisotropy, particularly: In regions of transition from deformation‐dominated to sliding‐dominated flow; in the approach to the grounding line, and across ice shelves. Given the importance of these locations in determining mass flux into the ocean, our results have implications for projections of sea level change from Antarctic ice loss.

Energy mass balance and flow modeling of early Holocene glaciers in the Queshque valley, Cordillera Blanca, Peru

Our limited knowledge of the timing and pattern of early Holocene climate variability in the tropical Andes hinders our ability to evaluate any potential linkages between low and high latitude oceanic-atmospheric dynamics. There is mounting evidence that glaciers in the Peruvian Andes stabilized at times during the early Holocene, suggesting there were periods of colder and/or wetter conditions that interrupted an overall pattern of a warmer and drier climate. Evaluating the global significance of these glacial fluctuations requires that the nature of these apparent cooling events and any shifts in the hydrologic cycle be better quantified. Here we apply a physically-based glacier model to reconstruct and interpret early Holocene-aged paleoglaciers in steady-state with a range of tropical climatic conditions in the Queshque valley of the Cordillera Blanca, Peru (9.8°S). This model uses a LiDAR-based digital elevation model (DEM) and hourly meteorological data as inputs to calculate the fully-distributed surface energy and mass balance (SEMB). This approach allows us to better capture the diurnal range of environmental variability at a spatial resolution that has not been achieved with previous paleo-glacier modeling efforts in this region. A 3-D rendition of paleoglaciers was then developed based on a flow model that responds to the SEMB input, accounting for the valley topography. The model was validated by simulating glaciers that match both field and satellite observations of modern glacier area limits, as well as ice thicknesses that are consistent with recently measured ground-penetrating radar (GPR) profiles of the main glacier tongue. After adjusting for changes in early Holocene global radiation values, and using regional paleoclimate records as constraints for the model, the maximum early Holocene ice limit in the valley dating to ∼10.8 (±0.1) ka was reconstructed using a 3.0 °C cooling and a 25% increase in precipitation relative to today. An up-valley ice extent dating to ∼9.4 (±0.3) ka was reconstructed using a 2.8 °C cooling and no change in precipitation amounts relative to today. These results suggest that conditions were cold and wet enough to maintain glaciers at times during the early Holocene, even though a shift to drier conditions combined with modestly warmer temperatures drove a phase of ice retreat from at least ∼10.8 to ∼9.4 ka. These results suggest that southern tropical temperatures and the hydrologic cycle rapidly reorganized during the early Holocene in conjunction with diminishing ice sheets and shifting environmental conditions in the high latitudes.

Specific Effects of the 1988 Earthquake on Topography and Glaciation of the Tsambagarav Ridge (Mongolian Altai) Based on Remote Sensing and Field Data

Strong earthquakes could serve as a trigger for glacier detachment and associated ice–rock avalanches. The 1988 Tsambagarav earthquake (M = 6.4) initiated collapse of part of the glacier tongue and a further ice–rock avalanche with an abnormal 5 km long path in Zuslan valley, Tsambagarav ridge (Mongolian Altai). Early documentation of surface effects in 1988, remote sensing and field data gathered 16 and 30 years after this event allowed for the assessment of the seismic impact on a reduction of “damaged” glacier under conditions of global warming as well as estimating topography changes in this arid and seismically active area. Because of the earthquake, the glacier immediately lost 10.4 % of its area (0.1 km2 of tongue surface). Additionally, 56% of its area was lost during 1988–2015, shrinking much faster than neighboring glaciers of similar size and exposition. Collapse of snow–ice cornice in the accumulation zone could play a key role in rapid acceleration of the detached ice block and abnormally long path of the ice–rock avalanche. A large amount of debris material provided more than 16 years of ice melting. Downstream, the valley avalanche debris cover repeats the topography of underlying Pleistocene moraines, which should be considered in regional paleogeographical reconstructions.

Weakening of the pinning point buttressing Thwaites Glacier, West Antarctica

Abstract. The Thwaites Eastern Ice Shelf buttresses a significant portion of Thwaites Glacier through contact with a pinning point 40 km offshore of the present grounding line. Predicting future rates of Thwaites Glacier’s contribution to sea-level rise depends on the evolution of this pinning point and the resultant change in the ice-shelf stress field since the breakup of the Thwaites Western Glacier Tongue in 2009. Here we use Landsat-8 feature tracking of ice velocity in combination with ice-sheet model perturbation experiments to show how past changes in flow velocity have been governed in large part by changes in lateral shear and pinning point interactions with the Thwaites Western Glacier Tongue. We then use recent satellite altimetry data from ICESat-2 to show that Thwaites Glacier’s grounding line has continued to retreat rapidly; in particular, the grounded area of the pinning point is greatly reduced from earlier mappings in 2014, and grounded ice elevations are continuing to decrease. This loss has created two pinned areas with ice flow now funneled between them. If current rates of surface lowering persist, the Thwaites Eastern Ice Shelf will unpin from the seafloor in less than a decade, despite our finding from airborne radar data that the seafloor underneath the pinning point is about 200 m shallower than previously reported. Advection of relatively thin and mechanically damaged ice onto the remaining portions of the pinning point and feedback mechanisms involving basal melting may further accelerate the unpinning. As a result, ice discharge will likely increase up to 10 % along a 45 km stretch of the grounding line that is currently buttressed by the Thwaites Eastern Ice Shelf.

Retreat of Northern Hemisphere Marine‐Terminating Glaciers, 2000–2020

AbstractWe 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.

Late Glacial deglaciation of the Zackenberg area, NE Greenland

The Greenland Ice Sheet (GrIS) is a key component of the global climate system. However, our current understanding of the spatio-temporal oscillations and landscape transformation of the GrIS margins since the last glacial cycle is still incomplete. The objective of this work is to study the deglaciation of the Zackenberg Valley (74°N, 20°E), NE Greenland, and the origin of the derived landforms. Based on extensive fieldwork and high-detail geomorphological mapping we identified the different types of landforms, from which those of glacial and paraglacial origin were used to understand the processes driving regional environmental evolution. We applied cosmic-ray exposure (CRE) dating to 32 samples taken from erosive and depositional glacial landforms distributed across the valley. Geomorphological evidence shows that >800-m-thick Late Quaternary glacier filled the valleys and fjords and covered mountain summits. In subsequent phases, as ice thickness decreased, the glacier was limited to the interior of the valley, leaving several lateral moraines. The deglaciation of the Zackenberg Valley that started by ~13.7–12.5 ka also accelerated slope paraglacial processes. Many blocks from lateral moraines were remobilized and fell, reaching the valley floor where they covered the thinning glacier tongue; transforming it into a debris-covered glacier that subsequently melted gradually. By ca. 10.5 ka, the last remnants of glacial ice disappeared from the Zackenberg Valley floor, a chronology of deglaciation that is similar to that observed in other sites across NE Greenland. The results of this work must be considered in similar studies, reinforcing the need to support CRE ages of the different geomorphological phases with paleoclimatic data from other sedimentary records.

Kinematics of an Alpine rock glacier from multi-temporal UAV surveys and GNSS data

The quantification of rock glacier dynamics has gained increasing importance in recent years. In this study, the spatial and temporal flow patterns of perennially frozen debris in the active Gran Sometta rock glacier (Western Italian Alps) were investigated with repeated Unmanned Aerial Vehicle (UAV) surveys (2016–2019), Global Navigation Satellite System (GNSS) campaigns (2012−2020), geophysical prospections (2015) and ground surface temperature data (2014–2020).UAV data were used to generate maps of changes and elevation differences of the rock glacier surface by 3D point cloud comparison to evaluate surface lowering and accumulation processes. Horizontal velocities were quantified by an automatic image correlation technique and the results were then compared with horizontal surface velocities from GNSS measurements on selected points. The horizontal velocities estimated with the automatic method agree well with the GNSS velocities with an R2 = 0.99 and a RMSE lower than 0.07 m/y.Point cloud comparisons show surface lowering in the orographic left-hand side of the terminal part and in the central body of the rock glacier. The upper part exhibits almost absence of subsidence and any movement. This is explained by the lack of permafrost in this sector due to its overriding by the development of a small glacier during the Little Ice Age. As a result of the downslope movement, zones of surface rising occurred at the advancing front and at the moving ridge and furrow complexes. Surface velocity decreases from the orographic left to the right-hand side of the rock glacier tongue, where a thaw subsidence of up to 0.05 m/y was also observed. According to the GNSS measurements, the range of flow velocity of the rock glacier increased from 0.17–1.1 m/y in 2013 to 0.21–1.45 m/y in 2015 and then decreased until 2018 when the smallest surface velocity is detected. Since 2018, the creep velocities gradually started to increase again reaching values of 0.23 m/y up to a maximum of 1.9 m/y in the orographic left-hand side of the rock glacier tongue. This agrees with observations from other rock glaciers in the European Alps in recent decades.The complex Gran Sometta rock glacier dynamics can be explained by the heterogeneous distribution of permafrost and related subsurface perennially frozen ground which is thick enough (about 20–30 m) for permafrost creep to occur. Creep rates of the rock glacier permafrost depend also on the ground thermal regime: annual warmer surface conditions promote an acceleration of the creep rates within the rock glacier permafrost, whereas ground surface cooling causes a slight deceleration.

Parker Ice Tongue Collapse, Antarctica, Triggered by Loss of Stabilizing Land‐Fast Sea Ice

AbstractAfter a likely multi‐century period of intermittent calving, the full length of Parker Ice Tongue (18 km or 41 km2), calved in March 2020 co‐incident with repeated summer break‐outs of the surrounding land‐fast sea ice. A complete ice tongue collapse for these otherwise stable glaciological landmarks along the Victoria Land Coast is previously unrecorded. Prior to break‐off, we found an inverse correlation between fast ice extent and ice tongue velocities (R = −0.62; R2 = 0.39). The short summer period, characterized by decreased land‐fast sea ice extent, showed around 11% higher velocities compared to winter. Any previous events of comparable magnitude could not have occurred since at least ∼1,850, assuming continuous growth (∼193m yr−1) derived from the last 60 years of satellite observations. We highlight the vulnerability of the ice tongue once left exposed to oceanic processes, which poses questions about the fate of other ice tongues if land‐fast sea ice decreases more broadly in the future.

12 Years of Area Variation by the Drygalski Ice Tongue as Measured With COSMO-SkyMed

One important coastal polynya around Antarctica is Terra Nova Bay (TNB) polynya. Its formation and persistence are due to the combined effect of katabatic winds, regional ice conditions, and the Drygalski Ice Tongue (DIT). The combined effect of these elements arranges a delicate balance in Terra Nova Bay. To evaluate the interacting elements, we focused our attention on the Drygalski Ice Tongue, one of the largest ice tongues in Antarctica. The DIT is a 70-kilometre long tongue which juts out like a pier from the icy land into northern McMurdo Sound of Antarctica's Ross Dependency. The analysis presented in this paper was carried out using Cosmo-SkyMed (CSK) images. The methodology developed for the analysis consists of three steps: 1) DIT shape enhancement, 2) fuzzy Bayesian segmentation which associates the analysed set of pixels with a binary label field, and 3) parametric representation of the shape of the DIT to compare the area of the ice tongue as it was 12 years ago and as it is today. The computed mean value of the DIT surface velocity was 703 m y <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . To estimate the closeness of the derived parameter to the true value, using a statistical approach, the 95% confidence interval for the true mean value is 703±4.236 m y <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Our analysis confirms a trend of advance. That is, after the significant calving events of the tongue in 2005 and 2006, the DIT has resumed its normal growth, and its length has increased by 12% in the last 12 years.

A nested high-resolution unstructured grid 3-D ocean-sea ice-ice shelf setup for numerical investigations of the Petermann ice shelf and fjord

Three-dimensional numerical simulation of circulation in fjords hosting marine-terminating ice shelves is challenging because of the complexity of processes involved in such environments. This often requires a comprehensive model setup. The following elements are needed: bathymetry (usually unknown beneath the glacier tongue), ice shelf draft (impacting water column thickness), oceanographic state (including tidal elevation, salinity, temperature and velocity of the water masses), sea ice and atmospheric forcing. Moreover, a high spatial resolution is needed, at least locally, which may be augmented with a coarser and computationally cheaper (nested) model that provides sufficiently realistic conditions at the boundaries. Here, we describe procedures to systematically create such a setup that uses the Finite Volume Community Ocean Model (FVCOM) for the Petermann Fjord, Northwest Greenland. The first simulations are validated against temperature and salinity observations from the Petermann Fjord in September 2019. We provide•Complete bathymetry, ice-draft and water column thickness datasets of the Petermann Fjord, with an improved representation of the topography underneath the glacier tongue.•Boundary conditions for ocean, atmosphere and sea ice derived from a suite of high-resolution regional models that can be used to initialize and run the regional ocean model with realistic geophysical settings.

Mid-Holocene thinning of David Glacier, Antarctica: chronology and controls

Abstract. Quantitative satellite observations only provide an assessment of ice sheet mass loss over the last four decades. To assess long-term drivers of ice sheet change, geological records are needed. Here we present the first millennial-scale reconstruction of David Glacier, the largest East Antarctic outlet glacier in Victoria Land. To reconstruct changes in ice thickness, we use surface exposure ages of glacial erratics deposited on nunataks adjacent to fast-flowing sections of David Glacier. We then use numerical modelling experiments to determine the drivers of glacial thinning. Thinning profiles derived from 45 10Be and 3He surface exposure ages show David Glacier experienced rapid thinning of up to 2 m/yr during the mid-Holocene (∼ 6.5 ka). Thinning slowed at 6 ka, suggesting the initial formation of the Drygalski Ice Tongue at this time. Our work, along with ice thinning records from adjacent glaciers, shows simultaneous glacier thinning in this sector of the Transantarctic Mountains occurred 4–7 kyr after the peak period of ice thinning indicated in a suite of published ice sheet models. The timing and rapidity of the reconstructed thinning at David Glacier is similar to reconstructions in the Amundsen and Weddell embayments. To identify the drivers of glacier thinning along the David Glacier, we use a glacier flowline model designed for calving glaciers and compare modelled results against our geological data. We show that glacier thinning and marine-based grounding-line retreat are controlled by either enhanced sub-ice-shelf melting, reduced lateral buttressing or a combination of the two, leading to marine ice sheet instability. Such rapid glacier thinning events during the mid-Holocene are not fully captured in continental- or catchment-scale numerical modelling reconstructions. Together, our chronology and modelling identify and constrain the drivers of a ∼ 2000-year period of dynamic glacier thinning in the recent geological past.

Elemental and Sr–Nd isotopic compositions of surface clay-size sediments in the front end of major ice shelves around Antarctica and indications for provenance

Geochemical observations of continental outcrops and marine sediments provide insights into the nature of bedrock and erosion in Antarctica. The accelerated melt of ice shelves has increased the flux of terrigenous materials to the marginal seas and has changed the sedimentological environment. We characterized the provenance of the surficial fine-grained detrital sediments in the front ends of major ice shelves in the Amundsen Sea, Ross Sea, and Prydz Bay sections through comparisons to proximal rocks using the elemental compositions, as well as the Sr and Nd isotopic compositions, of the clay-size (<2 μm) fraction, rock samples, and bulk sediments and some residual fractions. The results show that the 87Sr/86Sr ratios of clay-size sediments range from 0.70962 to 0.71662, 0.70936 to 0.71960, and 0.72569 to 0.73492, and that the εNd values range from −2.1 to +0.8, −10.9 to −1.43, and −22.3 to −17. 3 in the Amundsen Sea, Ross Sea, and Prydz Bay, respectively, while those of rocks range from 0.70350 to 0.77080 and from 5.1 to −22.4 in all three areas. Elemental compositions of both bulk sediments and the clay-size fraction in the Western Ross Sea have similar patterns to those in Prydz Bay, showing that most major elements (e.g., Mg, Ca, and Fe) are lower than in the upper continental crust (UCC). Chondrite-normalized rare earth elements (REEs) are highest in the bulk sediments of the Amundsen Sea and lowest in the clay-size fraction of all three sections. We concluded that clay-size sediments in the Western Amundsen Sea shelf are largely from the Eastern Amundsen Sea coastal area, while some sediments reflect contributions from the Hobbs Coast due to gravitational currents. The Western Ross Sea can receive sediments from both East Antarctica and West Antarctica, while the clay-size fractions of western sites in Transect R (a latitudinal transect near Inexpressible Island, East Antarctica) in this study are mainly from southeastern Victoria Land, transported northward by bottom currents. The contribution from coarse ice-rafted debris (IRD) is minor, possibly because of blocking by the Drygalski Ice Tongue. The clay-size sediments of Eastern Prydz Bay are largely contributed by Princess Elisabeth Land and western Wilhelm II Land, while those in Western Prydz Bay are possibly derived from the Lambert-Amery Ice Shelf system and the western coast. These very fine-grained sediments are generally affected by oceanic currents (e.g., Antarctic Coastal Currents and Antarctic Bottom Currents). Most residual fraction show very similar εNd values to the bulk of clay-size sediments. It indicates that authigenic phases are generally sourced from similar regions to the silicate component of clay-size sediments.

Two decades of dynamic change and progressive destabilization on the Thwaites Eastern Ice Shelf

Abstract. The Thwaites Eastern Ice Shelf (TEIS) buttresses the eastern grounded portion of Thwaites Glacier through contact with a pinning point at its seaward limit. Loss of this ice shelf will promote further acceleration of Thwaites Glacier. Understanding the dynamic controls and structural integrity of the TEIS is therefore important to estimating Thwaites' future sea-level contribution. We present a ∼ 20-year record of change on the TEIS that reveals the dynamic controls governing the ice shelf's past behaviour and ongoing evolution. We derived ice velocities from MODIS and Sentinel-1 image data using feature tracking and speckle tracking, respectively, and we combined these records with ITS_LIVE and GOLIVE velocity products from Landsat-7 and Landsat-8. In addition, we estimated surface lowering and basal melt rates using the Reference Elevation Model of Antarctica (REMA) DEM in comparison to ICESat and ICESat-2 altimetry. Early in the record, TEIS flow dynamics were strongly controlled by the neighbouring Thwaites Western Ice Tongue (TWIT). Flow patterns on the TEIS changed following the disintegration of the TWIT around 2008, with a new divergence in ice flow developing around the pinning point at its seaward limit. Simultaneously, the TEIS developed new rifting that extends from the shear zone upstream of the ice rise and increased strain concentration within this shear zone. As these horizontal changes occurred, sustained thinning driven by basal melt reduced ice thickness, particularly near the grounding line and in the shear zone area upstream of the pinning point. This evidence of weakening at a rapid pace suggests that the TEIS is likely to fully destabilize in the next few decades, leading to further acceleration of Thwaites Glacier.

Calving cycle of Ninnis Glacier over the last 60 years

Iceberg calving is one of the contributing ways for the mass loss from ice shelves in Antarctica; the calving cycles of most ice shelves are quite long. The mechanism of iceberg calving has not been well investigated because of the limited temporal coverage of satellite observations. The Ninnis Glacier (NG) has experienced three calving events in the past 60 years (1980–1982, 1998, and 2018), which provides an opportunity to capture the long-term pattern of calving for the first time. In our study, ARGON, Landsat, and ERS-2 images were combined with field measurements to characterize the evolution of the Ninnis Glacier, especially for the last calving event, which can be studied using high-quality observations. The time span was over 23 years for the 1st calving cycle and the average time span was ∼18.5 years for each of the last two cycles. Supported by satellite-derived strain rates and ocean currents obtained from a numerical ocean model, we conclude that the calving of the Ninnis Glacier Tongue (NGT) was driven by the continuous impact of ocean current on the advected glacier tongue. According to a comparison among the three calving events, the calving area of the last calving event was ∼750 km2, only half of the 1st calving event, with breaking locations closer to the grounding line. We attribute these changes to the reduction of fast ice in this region, which can be related to the local oceanographic conditions.

Ablation Patterns of the Debris Covered Tongue of Halong Glacier Revealed by Short Term Unmanned Aerial Vehicle Surveys

Debris-covered glaciers are an important glacier type and have attracted more and more attention. This study presents the results of ablation patterns of debris-covered tongue of the Halong Glacier in the northeastern Tibetan Plateau, by using two repeated unmanned aerial vehicle (UAV) surveys performed on August 11 and September 15, 2019. The results show that the tongue of Halong Glacier has experienced strong ablation during the surveyed period, with an overall ice loss amount to 4.17 × 105 metric tons. Among all the briefly classified surface types, supraglacial debris has the largest area (80.9%) and also mass losses (58.6%) comparing to others. However, ice cliffs show the strongest and the most significant ablation rates (averagely 1.36 and 1.22 m w.e. for supraglacial and lateral ice cliffs, respectively), followed by clean ice regions (1.01 m w.e.). The backwastes of ice cliffs also resulted in up to 7.8 m horizontal back-off at different parts of Halong Glacier, lead to fast terminal retreat and narrowing down of the glacier tongue, and may result in the break off of Halong Glacier tongue into separated parts in the future. The surface ablation rates show a clear negative exponential relationship with the measured debris thicknesses, well in accordance with previous studies. Regions in cutting and flushing by supraglacial and lateral rivers have the largest surface elevation decreases but are not significant due to their limited area and the relatively lower quality of UAV digital surface models (DSMs) in those covered regions.

The Concept of Steady State, Cyclicity and Debris Unloading of Debris-Covered Glaciers

It can easily be expected that debris-covered glaciers show a different response on external forcing compared to clean-surface glaciers. The supra-glacial debris cover acts as an additional transfer layer for the energy exchange between atmosphere and ice. The related glacier reaction is the integral of local effects, which changes strongly between enhanced melt for thin debris layers and considerably reduced melt for thicker debris. Therefore, a realistic feedback study can only be performed, if both the ice flow and the debris-influenced melt is treated with a high degree of detail. We couple a full Stokes representation of ice dynamics and the most complete description of energy transfer through the debris layer, in order to describe the long-term glacier reaction in the coupled system. With this setup, we can show that steady-state conditions are highly unlikely for glaciers, in case debris is not unloaded from the surface. For continuous and complete debris removal from the lowermost glacier tongue, however, a balance of the debris budget and the glacier conditions are possible. Depending on displacement and removal processes, our results demonstrate that debris-covered glaciers have an inherent tendency to switch to an oscillating state. Then, glacier mass balance and debris balance are out of phase, such that glacier advance periods end with the separation of the heavily debris-loaded lowermost glacier tongue, at time scales of decades to centuries. As these oscillations are inherent and happen without any variations in climatic forcing, it is difficult to interpret modern observations on the fluctuation of debris-covered glaciers on the basis of a changing climate only.