Ice Discharge Research Articles

Minimalist approach to left atrial appendage occlusion through three-dimensional intracardiac echocardiography: Procedural steps and single-center experience

Atrial fibrillation (AF), the most prevalent arrhythmia worldwide, is associated with an annual risk of ischemic stroke of up to 5%, with approximately 98% of thrombi originating from the left atrial appendage (LAA). Direct oral anticoagulants or vitamin K antagonists remain the gold standard for preventing systemic thromboembolism in patients with AF. LAA occlusion (LAAO) is an alternative stroke prevention technique for patients with AF who cannot tolerate long-term anticoagulation. Percutaneous LAAO typically employs a multimodal imaging approach with contrast fluoroscopy and transesophageal echocardiography (TEE) for visualization, device selection and deployment, and complication monitoring. TEE is also used for follow-up imaging. However, it necessitates general anesthesia or deep sedation and is associated with a high risk of adverse events. Three-dimensional intracardiac echocardiography (3D ICE) has been proposed as a viable alternative to TEE for LAAO procedures. In this review, we summarized the current evidence regarding ICE guidance in LAAO procedures. We also presented a case series of consecutive patients presenting at our center who underwent LAAO, which was performed by a single operator using a minimalist approach of moderate sedation, 3D ICE, and same-day discharge. We also presented a summary of the minimalist approach to LAAO using 3D ICE and detailed procedural steps of 3D ICE catheter manipulation for successful LAAO intervention.

Partitioning the drivers of Antarctic glacier mass balance (2003–2020) using satellite observations and a regional climate model

We investigate the mass changes of Antarctic glaciers from 2003 to 2020, partitioning them into the contributions of surface mass balance (SMB) and ice discharge, using high-resolution ice mass change estimates derived from the combination of two different types of satellite observations (gravimetry and altimetry) and outputs from a regional climate model. Our analysis indicates that changes in ice discharge have played a dominant role in ongoing ice mass trends and their accelerations, especially in glaciers near the Amundsen and Bellingshausen Seas in West Antarctica. In particular, mass losses of the Thwaites and Pine Island Glaciers have been mostly (>90%) controlled by ice discharge, while the contribution of SMB has been relatively minor. In East Antarctica, SMB accounts for significant portions (>50%) of ice mass imbalances of glaciers in e.g., Dronning Maud Land and Wilkes Land. Ice discharge has also played a notable role in overall mass gain in the region. While our ice discharge estimates agree well with previous estimates from satellite imagery in West Antarctica, notable differences are found in glaciers of East Antarctica and the Antarctic Peninsula. This highlights the need for more observations and improved numerical models to refine these estimates.

Melting of Totten Glacier, East Antarctica since the Last Glacial Maximum Revealed by Beryllium Isotope Ratios of Marine Sediment

The Totten Glacier of East Antarctica drains a basin containing about 3.5 m of sea-level rise equivalent ice. While Totten Glacier is generally considered somewhat more stable than the rapidly retreating sectors of the West Antarctic Ice Sheet, such as the Amundsen Sea Embayment and Getz Ice Shelf, our understanding of its intricate dynamics and interaction with the Southern Ocean since the Last Glacial Maximum remains incomplete. Situated in a precarious position grounded below sea level, Totten Glacier represents a critical yet enigmatic component of East Antarctic ice dynamics. Its susceptibility to marine ice sheet instability raises concerns, as positive feedback from grounding line retreat could trigger irreversible ice discharge or even complete collapse. A meticulous reconstruction of the retreat history of the Totten Glacier is imperative to comprehensively grasp the glacier's response to present and future climate changes.Here, we present a beryllium isotope ratio analysis of marine sediment from the continental slope proximal to Totten Glacier, the first such record from this location, supplemented by grain size data from the same sediment core. The results, when evaluated together with nearby proxy records, reveal that the initial deglaciation of the Totten Glacier sector of the East Antarctic Ice Sheet began at ca. 21 ka BP. The rapid deglaciation from ca. 7 ka BP that followed is determined to be caused by the intrusion of modified Circumpolar Deep Water, leading to melting at the grounding zone of the Totten Glacier and causing instability of the glacier system. The southward shift and intensification of Antarctic easterlies may be one of the causes of this intrusion. These results contribute to the body of knowledge regarding the dynamical response of marine-terminating glaciers to climate variability during the last deglaciation.

Meltwater Orientations Modify Seismic Anisotropy in Temperate Ice

AbstractSeismology is increasingly used to infer the magnitude and direction of glacial ice flow. However, the effects of interstitial meltwater on seismic properties remain poorly constrained. Here, we extend previous studies on seismic anisotropy in temperate ices to consider the role of melt preferred orientation (MPO). We used the ELLE numerical toolbox to simulate microstructural shear deformation of temperate ice with variable MPO strength and orientation, and calculated the effective seismic properties of these numerical ice‐melt aggregates. Our models demonstrate that even 3.5% melt volume is sufficient to rotate fast directions by up to 90°, to increase Vp anisotropy by up to +110%, and to modify Vs anisotropy by −9 to +36%. These effects are especially prominent at strain rates ≥3.17 × 10−12 s−1. MPO may thus obscure the geophysical signatures of temperate ice flow in regions of rapid ice discharge, and is therefore pivotal for understanding ice mass loss.

The deglacial history of 79N glacier and the Northeast Greenland Ice Stream

The Northeast Greenland Ice Stream (NEGIS) is the main artery for ice discharge from the northeast sector of the Greenland Ice Sheet (GrIS) to the North Atlantic. Understanding the past, present and future stability of the NEGIS with respect to atmospheric and oceanic forcing is of global importance as it drains around 17% of the GrIS and has a sea-level equivalent of 1.6 m. This paper reconstructs the deglacial and Holocene history of Nioghalvfjerdsbræ (or 79N Glacier); a major outlet of the NEGIS.At high elevation (>900 m asl) autochthonous blockfield, a lack of glacially moulded bedrock and pre LGM exposure ages point to a complex exposure/burial history extending back over half a million years. However, post Marine Isotope Stage 12, enhanced glacial erosion led to fjord incision and plateaux abandonment. Between 900 and 600 m asl the terrain is largely unmodified by glacial scour but post LGM erratics indicate the advection of cold-based ice through the fjord. In contrast, below ∼600 m asl Nioghalvfjerdsfjorden exhibits a geomorphological signal indicative of a warm-based ice stream operating during the last glacial cycle. Dated ice marginal landforms and terrain along the fjord walls show initial thinning rates were slow between ∼23 and 10 ka, but post-10 ka it is evident that Nioghalvfjerdsfjorden deglaciated extremely quickly with complete fjord deglaciation below ∼500 m asl between 10.0 and 8.5 ka.Both increasing air and ocean temperatures were pivotal in driving surface lowering and submarine melt during deglaciation, but the final withdrawal of ice through Nioghalvfjerdsfjorden was facilitated by the action of marine ice sheet instability. Our estimates show that thinning and retreat rates reached a maximum of 5.29 ma−1 and 613 ma−1, respectively, as the ice margin withdrew westwards. This would place the Early Holocene disintegration of this outlet of the NEGIS at the upper bounds of contemporary thinning and retreat rates seen both in Greenland and Antarctica. Combined with recent evidence of ice stream shutdown during the Holocene, as well as predictions of changing ice flow dynamics within downstream sections of the NEGIS catchment, this suggests that significant re-organisation of the terminal zone of the ice stream is imminent over the next century.

Diverse behaviors of marine ice sheets in response to temporal variability of the atmospheric and basal conditions

Abstract The observed retreat of the grounding line of the present-day ice sheets and the simulated grounding-line retreat of ice sheets under changing climate conditions are often interpreted as indications of marine ice-sheet instability (MISI), driven by a positive feedback between the ice discharge and conditions at the grounding line. However, the arguments that support this feedback are valid only for steady-state conditions. Here, we assess how unconfined marine ice sheets may behave if atmospheric conditions and basal conditions evolve with time. We find that the behavior of grounding lines can exhibit a range from unstoppable advance and retreat to irregular oscillation irrespective of the stability of the corresponding steady-state configurations obtained with time-invariant conditions. Our results show that numerical simulations with a parameterization of the ice flux through the grounding line used in large-scale ice-sheet models produce markedly different results from simulations without the parameterization. Our analysis demonstrates that the grounding-line migration can be driven by the temporal variability in the atmospheric and basal conditions and not by MISI, which assumes unchanging conditions. Instead, the grounding-line advance or retreat is determined by interactions between ice flow, basal processes and environmental conditions throughout the length of a marine ice sheet in addition to the circumstances at its grounding line.

Drivers of Seasonal Land‐Ice‐Flow Variability in the Antarctic Peninsula

AbstractLand‐ice flow in Antarctica has experienced multi‐annual acceleration in response to increased rates of ice thinning, ice‐shelf collapse and grounding‐line retreat. Superimposed upon this trend, recent observations have revealed that land‐ice flow in the Antarctic Peninsula exhibits seasonal velocity variability with distinct summertime speed‐ups. The mechanism, or mechanisms, responsible for driving this seasonality are unconstrained at present, yet detailed, process‐based understanding of such forcing will be important for accurately estimating Antarctica's future contributions to sea level. Here, we perform time‐series analysis on an array of remotely sensed, modeled and reanalysis data sets to examine the influence of potential drivers of ice‐flow seasonality in the Antarctic Peninsula. We show that both meltwater presence and ocean temperature act as statistically significant precursors to summertime ice‐flow acceleration, although each elicits an ice‐velocity response after a distinct lag, with the former prompting a more immediate response. Furthermore, we find that the timing and magnitude of these local drivers are influenced by large‐scale climate phenomena, namely the Amundsen Sea Low and the El Niño Southern Oscillation, with the latter initiating an anomalous wintertime ice‐flow acceleration event in 2016. This hitherto unidentified link between seasonal ice flow and large‐scale climatic forcing may have important implications for ice discharge at and beyond the Antarctic Peninsula in the future, depending upon how the magnitude, frequency and duration of such climate phenomena evolve in a warming world.

Heinrich event ice discharge and the fate of the Atlantic Meridional Overturning Circulation.

During Heinrich events, great armadas of icebergs episodically flooded the North Atlantic Ocean and weakened overturning circulation. The ice discharges of these episodes constrain the sensitivity of overturning circulation to iceberg melting. We reconstructed these ice discharges to be as high as 0.13 sverdrup (Sv) (1 Sv = 1 million cubic meters per second) during Heinrich event 4 and to average 0.029 Sv over all episodes. The present-day Greenland Ice Sheet calving of icebergs is comparable to that of a mid-range Heinrich event. As the future Greenland Ice Sheet recedes from marine-terminating outlets, its iceberg calving likely will not persist long enough for icebergs alone to cause catastrophic disruption to the Atlantic overturning circulation, although the accelerating Greenland runoff and continued global warming remain threats to the circulation stability.

Heterogeneous impacts of ocean thermal forcing on ice discharge from Greenland's peripheral tidewater glaciers over 2000–2021

The Greenland Ice Sheet is losing mass at increasing rates. Substantial amounts of this mass loss occur by ice discharge which is influenced by ocean thermal forcing. The ice sheet is surrounded by thousands of peripheral, dynamically decoupled glaciers. The mass loss from these glaciers is disproportionately high considering their negligible share in Greenland’ overall ice mass. We study the relevance of ocean thermal forcing for ice discharge evolution in the context of this contrasting behaviour. Our estimate of ice discharge from the peripheral tidewater glaciers yields a rather stable Greenland-wide mean of 5.40 ± 3.54 Gt a−1 over 2000–2021. The evolutions of ice discharge and ocean thermal forcing are heterogeneous around Greenland. We observe a significant sector-wide increase of ice discharge in the East and a significant sector-wide decrease in the Northeast. Ocean thermal forcing shows significant increases along the northern/eastern coast, while otherwise unchanged conditions or decreases prevail. For East Greenland, this implies a clear influence of ocean thermal forcing on ice discharge. Similarly, we find clear influences at peripheral tidewater glaciers with thick termini that are similar to ice sheet outlet glaciers. At the peripheral glaciers in Northeast Greenland ice discharge evolution opposes ocean thermal forcing for unknown reasons.

Geometric amplification and suppression of ice-shelf basal melt in West Antarctica

Abstract. Glaciers along the Amundsen Sea coastline in West Antarctica are dynamically adjusting to a change in ice-shelf mass balance that triggered their retreat and speed-up prior to the satellite era. In recent decades, the ice shelves have continued to thin, albeit at a decelerating rate, whilst ice discharge across the grounding lines has been observed to have increased by up to 100 % since the early 1990s. Here, the ongoing evolution of ice-shelf mass balance components is assessed in a high-resolution coupled ice–ocean model that includes the Pine Island, Thwaites, Crosson, and Dotson ice shelves. For a range of idealized ocean-forcing scenarios, the combined evolution of ice-shelf geometry and basal-melt rates is simulated over a 200-year period. For all ice-shelf cavities, a reconfiguration of the 3D ocean circulation in response to changes in cavity geometry is found to cause significant and sustained changes in basal-melt rate, ranging from a 75 % decrease up to a 75 % increase near the grounding lines, irrespective of the far-field forcing. These previously unexplored feedbacks between changes in ice-shelf geometry, ocean circulation, and basal melting have a demonstrable impact on the net ice-shelf mass balance, including grounding-line discharge, at multi-decadal timescales. They should be considered in future projections of Antarctic mass loss alongside changes in ice-shelf melt due to anthropogenic trends in the ocean temperature and salinity.

Estimating the Mass Balance of the Antarctic Ice Sheet from 1985 to 2021 using the Input-Output Method

Accurate monitoring of the Antarctic ice sheet's mass balance is crucial for comprehending its contribution to global sea level rise and climate patterns. We estimate the Antarctic mass balance from 1985 to 2021 using an input-output method. Ice discharge is estimated using an automated algorithm, while surface mass balance data is obtained from the RACMO2.3p2 model. We find a recent decrease in the rate of ice mass loss in Antarctica, consistent with the recent studies. The West Antarctic Ice Sheet (WAIS) drives the increase in ice discharge and mass loss, influenced by rising ocean temperatures and extreme weather events. The East Antarctic Ice Sheet (EAIS) shows stability in mass balance, except for an increase in discharge and mass loss in the Cp-D sub-region. The Antarctic Peninsula experiences increased ice discharge and mass loss, while the islands maintain a positive mass balance. The findings contribute to our understanding of the ongoing changes in ice mass and highlight the sensitivity of different regions to external factors.

Ocean response to a century of observation-based freshwater forcing around Greenland in EC-Earth3

The acceleration of Greenland ice sheet melting over the past decades is raising concern regarding the impacts on ocean circulation in the North Atlantic and Arctic regions. Global climate models struggle to assess these impacts as they do not include a realistic amount of meltwater from the Greenland ice sheet. Using an extended observation-based dataset of runoff and solid ice discharge for the recent historical period (1920–2019), we force the EC-Earth3 climate model following the ensemble approach and protocol of a previous study using a different model. We observe a slight increase of the ensemble mean AMOC with a large spread in the response: 0.20±0.81\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.20\\pm 0.81$$\\end{document} Sv for the maximum AMOC at 45∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}N. We notice that members with a strong initial AMOC state (18 Sv) show a strengthening of the AMOC, while members with intermediate AMOC strength between 16 and 18 Sv are not affected by the freshwater forcing. Weaker initial AMOC members respond with a mean weakening of -0.21±0.58\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${-}\\,0.21\\pm 0.58$$\\end{document} Sv of the AMOC. The AMOC ensemble spread is reduced by half in the freshwater forced ensemble at the end of the experiment and the negative trend is stronger compared to historical simulations without the forcing. This suggests the possible ability of the freshwater to constrain AMOC variability on multi-decadal time-scales. Comparison of the two ten member ensembles with ORAS5 reanalysis shows a reduction of the surface temperature and salinity biases in the freshwater forced ensemble in key regions such as the North Atlantic subpolar gyre and the Beaufort Gyre relative to the historical simulations. Recent trends are also more aligned with reanalysis in those regions. Arctic Ocean Atlantic Water subsurface core temperature is also closer to reanalysis but still strongly biassed. Members with a high AMOC initial state display an unrealistic Atlantic water layer core depth.

A marine record of Patagonian ice sheet changes over the past 140,000 years

Terrestrial glacial records from the Patagonian Andes and New Zealand Alps document quasi-synchronous Southern Hemisphere-wide glacier advances during the late Quaternary. However, these records are inherently incomplete. Here, we provide a continuous marine record of western-central Patagonian ice sheet (PIS) extent over a complete glacial-interglacial cycle back into the penultimate glacial (~140 ka). Sediment core MR16-09 PC03, located at 46°S and ~150 km offshore Chile, received high terrestrial sediment and meltwater input when the central PIS extended westward. We use biomarkers, foraminiferal oxygen isotopes, and major elemental data to reconstruct terrestrial sediment and freshwater input related to PIS variations. Our sediment record documents three intervals of general PIS marginal fluctuations, during Marine Isotope Stage (MIS) 6 (140 to 135 ka), MIS 4 (~70 to 60 ka), and late MIS 3 to MIS 2 (~40 to 18 ka). These higher terrigenous input intervals occurred during sea-level low stands, when the western PIS covered most of the Chilean fjords, which today retain glaciofluvial sediments. During these intervals, high-amplitude phases of enhanced sediment supply occur at millennial timescales, reflecting increased ice discharge most likely due to a growing PIS. We assign the late MIS 3 to MIS 2 phases and, by inference, older advances to Antarctic cold stages. We conclude that the increased sediment/meltwater release during Southern Hemisphere millennial-scale cold phases was likely related to higher precipitation caused by enhanced westerly winds at the northwestern margin of the PIS. Our records complement terrestrial archives and provide evidence for PIS climate sensitivity.

A New Eulerian Iceberg Module for Climate Studies

AbstractIcebergs modulate the effective location of freshwater input from ice sheets into the ocean and therefore play an important role for the climate, especially during times of increased ice discharge (e.g., Heinrich events). None of the models participating in the Paleo Modeling Intercomparison Project simulations of the Last Glacial Maximum or the last deglaciation included icebergs. Here, we present a newly developed dynamic/thermodynamic iceberg module that was specifically designed to be incorporated in climate models used for long‐term climate simulations with interactive ice sheets. In contrast to the widely used Lagrangian iceberg models, it is formulated in an Eulerian framework. This simplifies coupling to ocean models and enhances computational efficiency for glacial climates. In a set of sensitivity experiments, where the module was implemented into an Earth System Model, we validate the model for present‐day climate conditions and test its sensitivity to key parameters. Further, we investigate the effect of iceberg hosing on the Atlantic meridional overturning circulation (AMOC) as compared to traditional freshwater hosing. Varying the hosing rate slowly in time yields a good approximation of the hysteresis curve of the AMOC. We find that the sensitivity of the AMOC to iceberg hosing is stronger than to freshwater hosing in the same ocean point, but weaker as compared to a latitude belt forcing in the North Atlantic. This emphasizes the 2e necessity to include interactive icebergs in long‐term coupled climate simulations to realistically represent melt patterns and the response of the AMOC to freshwater input from melting ice sheets.

Progressive unanchoring of Antarctic ice shelves since 1973

Mass loss of the Antarctic Ice Sheet has been driven primarily by the thinning of the floating ice shelves that fringe the ice sheet1, reducing their buttressing potential and causing land ice to accelerate into the ocean2. Observations of ice-shelf thickness change by satellite altimetry stretch back only to 1992 (refs. 1,3–5) and previous information about thinning remains unquantified. However, extending the record of ice-shelf thickness change is possible by proxy, by measuring the change in area of the surface expression of pinning points—local bathymetric highs on which ice shelves are anchored6. Here we measure pinning-point change over three epochs spanning the periods 1973–1989, 1989–2000 and 2000−2022, and thus by proxy infer changes to ice-shelf thickness back to 1973–1989. We show that only small localized pockets of ice shelves were thinning between 1973 and 1989, located primarily in the Amundsen Sea Embayment and the Wilkes Land coastline. Ice-shelf thinning spreads rapidly into the 1990s and 2000s and is best characterized by the proportion of pinning points reducing in extent. Only 15% of pinning points reduced from 1973 to 1989, before increasing to 25% from 1989 to 2000 and 37% from 2000 to 2022. A continuation of this trend would further reduce the buttressing potential of ice shelves, enhancing ice discharge and accelerating the contribution of Antarctica to sea-level rise.

Disentangling the drivers of future Antarctic ice loss with a historically calibrated ice-sheet model

Abstract. We use an observationally calibrated ice-sheet model to investigate the future trajectory of the Antarctic ice sheet related to uncertainties in the future balance between sub-shelf melting and ice discharge, on the one hand, and the surface mass balance, on the other. Our ensemble of simulations, forced by a panel of climate models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), suggests that the ocean will be the primary driver of short-term Antarctic mass loss, initiating ice loss in West Antarctica already during this century. The atmosphere initially plays a mitigating role through increased snowfall, leading to an Antarctic contribution to global mean sea-level rise by 2100 of 6 (−8 to 15) cm under a low-emission scenario and 5.5 (−10 to 16) cm under a very high-emission scenario. However, under the very high-emission pathway, the influence of the atmosphere shifts beyond the end of the century, becoming an amplifying driver of mass loss as the ice sheet's surface mass balance decreases. We show that this transition occurs when Antarctic near-surface warming exceeds a critical threshold of +7.5 ∘C, at which the increase in surface runoff outweighs the increase in snow accumulation, a signal that is amplified by the melt–elevation feedback. Therefore, under the very high-emission scenario, oceanic and atmospheric drivers are projected to result in a complete collapse of the West Antarctic ice sheet along with significant grounding-line retreat in the marine basins of the East Antarctic ice sheet, leading to a median global mean sea-level rise of 2.75 (6.95) m by 2300 (3000). Under a more sustainable socio-economic pathway, we find that the Antarctic ice sheet may still contribute to a median global mean sea-level rise of 0.62 (1.85) m by 2300 (3000). However, the rate of sea-level rise is significantly reduced as mass loss is likely to remain confined to the Amundsen Sea Embayment, where present-day climate conditions seem sufficient to commit to a continuous retreat of Thwaites Glacier.

Graphics-processing-unit-accelerated ice flow solver for unstructured meshes using the Shallow-Shelf Approximation (FastIceFlo v1.0.1)

Abstract. Ice-sheet flow models capable of accurately projecting their future mass balance constitute tools to improve flood risk assessment and assist sea-level rise mitigation associated with enhanced ice discharge. Some processes that need to be captured, such as grounding-line migration, require high spatial resolution (under the kilometer scale). Conventional ice flow models mainly execute on central processing units (CPUs), which feature limited parallel processing capabilities and peak memory bandwidth. This may hinder model scalability and result in long run times, requiring significant computational resources. As an alternative, graphics processing units (GPUs) are ideally suited for high spatial resolution, as the calculations can be performed concurrently by thousands of threads, processing most of the computational domain simultaneously. In this study, we combine a GPU-based approach with the pseudo-transient (PT) method, an accelerated iterative and matrix-free solution strategy, and investigate its performance for finite elements and unstructured meshes with application to two-dimensional (2-D) models of real glaciers at a regional scale. For both the Jakobshavn and Pine Island glacier models, the number of nonlinear PT iterations required to converge a given number of vertices (N) scales in the order of 𝒪(N1.2) or better. We further compare the performance of the PT CUDA C implementation with a standard finite-element CPU-based implementation using the price-to-performance metric. The price of a single Tesla V100 GPU is 1.5 times that of two Intel Xeon Gold 6140 CPUs. We expect a minimum speedup of at least 1.5 times to justify the Tesla V100 GPU price to performance. Our developments result in a GPU-based implementation that achieves this goal with a speedup beyond 1.5 times. This study represents a first step toward leveraging GPU processing power, enabling more accurate polar ice discharge predictions. The insights gained will benefit efforts to diminish spatial resolution constraints at higher computing performance. The higher computing performance will allow for ensembles of ice-sheet flow simulations to be run at the continental scale and higher resolution, a previously challenging task. The advances will further enable the quantification of model sensitivity to changes in upcoming climate forcings. These findings will significantly benefit process-oriented sea-level-projection studies over the coming decades.

Configuration and timing of the postglacial marine incursion along the eastern Ungava Peninsula (Nunavik, Canada): implications for deglaciation models

ABSTRACTThe final deglaciation stages of the Laurentide Ice Sheet in northern Quebec–Labrador were marked by the incursion of the d'Iberville Sea into the coastal areas of Ungava Bay. Remote and field mapping of raised marine strandlines along the Ungava Peninsula east coast show that the maximum marine limit decreases from south (165 m) to north (100 m), reflecting differential uplift linked to an ice mass remnant located to the southwest of the bay. Reconstruction of the uppermost marine limit in two east–west transects extending into the Peninsula interior reveals distinct sea‐level stands with limited extents towards the west, indicating that the early stages of the marine incursion occurred in contact with the ice margin. The reconstruction also identifies the maximum eastward extent of ice‐dammed lakes that occupied the Arnaud River and Aux‐Feuilles River valleys. 10Be surface exposure dating of marine strandlines yielded consistent 10Be ages that indicate a rapid deglaciation of Ungava Bay between 8900 ± 200 a (north) and 7900 ± 200 a (south). These results provide constraints on the position of the Labrador Sector eastern and western ice margins during the late deglaciation, in addition to the timing of ice and meltwater discharges from the Ungava Bay region.

Under cover ice transport processes under equilibrium and non-equilibrium conditions

The transport of ice particles beneath a stable ice cover (referred to as cover load transport), and the processes leading to ice deposition and erosion are highly dynamic and complex. Based on the strong analogous relationship between bed load and cover load transport processes, knowledge from bed load transport research can be used when developing experiments for cover load transport processes. This paper presents a detailed experimental study of cover load transport rate in a laboratory flume with simulated ice particles and a simulated ice cover. The effect of the particle shape and size on cover load transport rate is quantified using the buoyant velocity and its velocity coefficient. The critical dimensionless flow strength is identified as an important parameter which determines the incipient motion of the under cover ice particles, and was calculated using hydraulic measurements obtained through laboratory experiments. A new cover load transport formula is introduced based on regression analysis of experimental data for a wide range of dimensionless flow strength conditions. The impact of incoming ice supply rate on the under cover ice transport rate was also investigated by altering the ice supply rate to the flume. A control volume analysis of ice discharge underneath the ice cover is used as the approach to quantify the dimensionless ice transport rate at different sections of the flume. The concept of equilibrium ice transport rate is introduced based on the experimental observations with variable incoming ice supply rates. The experimental observations on the impact of incoming ice supply were supported by an analysis of field data from Hequ Reach, China (Sun et al., 1986; Wang, 1992). The introduction of a cover load transport formula developed exclusively with simulated ice particles that have similar physical properties to ice forming in a natural environment, and the discussion on equilibrium ice transport rate are important contributions to advance the present understanding on under cover ice transport processes.

Empirical projection of global sea level in 2050 driven by Antarctic and Greenland ice mass variations

Global mean sea level rise, driven by ice mass loss in Antarctic and Greenland Ice Sheets (AIS and GrIS), is a significant consequence of global warming. Although various ice sheet models have attempted to predict the ice mass balance and subsequent sea level changes, non-trivial disagreements between models exist. In this study, we employ an empirical approach to estimate the future (2050) ice mass changes for both ice sheets, assuming their historical patterns of ice dynamics would persist in the coming decades. To achieve this, we estimate decadal-scale ice discharge variations by subtracting the surface mass balance (SMB) from the observed ice mass changes and extrapolate linear trend and acceleration components of ice discharges up to 2050. We also consider future SMB data from Coupled Model Intercomparison Project phase 6 models to estimate net ice mass balance. Our estimates suggest that from 2021 to 2050, the global sea level rise due to AIS and GrIS ranges between 6–19 mm and 15–31 mm, respectively. Additionally, we investigate regional sea level variability resulting from geoid changes induced by ice mass changes in both regions, highlighting that heterogeneous sea level changes may cause more pronounced sea level rises in lower latitude regions, where major cities are located.