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The ice streams flowing into the Amundsen Sea, West Antarctica, are losing mass due to changes in oceanic basal melting of their floating ice shelves. Rapid ice‐shelf melting is sustained by the delivery of warm Circumpolar Deep Water to the ice‐shelf cavities, which is first supplied to the continental shelf by an undercurrent that flows eastward along the shelf break. Temporal variability of this undercurrent controls ice‐shelf basal melt variability. Recent work shows that on decadal timescales the undercurrent variability opposes surface wind variability. Using a regional model, we show that undercurrent variability is induced by sea‐ice freshwater fluxes, particularly those north of the shelf break, which affect the cross‐shelf break density gradient. This sea‐ice variability is linked to tropical Pacific variability impacting atmospheric conditions over the Amundsen Sea. Ice‐shelf melting also feeds back onto the undercurrent by affecting the on‐shelf density, thereby influencing shelf‐break density gradient anomalies. Plain Language Summary: The glaciers that flow toward the Amundsen Sea, West Antarctica, are losing ice faster than most others about the continent. Once these glaciers reach the coast, they extend out onto the ocean surface, forming ice shelves. The rapid loss of ice is caused by changes in melting by relatively warm ocean waters beneath the floating ice shelves. In the Amundsen Sea, a deep ocean current is responsible for delivering warm water from the deep ocean to the ice shelves. We present model results that show that this deep current varies on decadal timescales as a consequence of systematic sea‐ice melt and formation patterns. A faster current drives more rapid ice shelf melting which, via a feedback process, further accelerates the current. Climate variability originating in the tropical Pacific Ocean is responsible for the variability in the sea‐ice, and is therefore also responsible for the effects on melting of the ice shelves. Key Points: In the Amundsen Sea decadal variability of an undercurrent flowing along the shelf break drives decadal variability in ice‐shelf basal meltSea‐ice freshwater fluxes and positive feedbacks from ice‐shelf basal melt drive the undercurrent variabilityTropical Pacific teleconnections induce atmospheric anomalies over the Amundsen Sea which drive the sea‐ice freshwater flux variability [ABSTRACT FROM AUTHOR]

Today, relatively warm Circumpolar Deep Water is melting Thwaites Glacier at the base of its ice shelf and at the grounding zone, contributing to significant ice retreat. Accelerating ice loss has been observed since the 1970s; however, it is unclear when this phase of significant melting initiated. We analyzed the marine sedimentary record to reconstruct Thwaites Glacier's history from the early Holocene to present. Marine geophysical surveys were carried out along the floating ice-shelf margin to identify core locations from various geomorphic settings. We use sedimentological data and physical properties to define sedimentary facies at seven core sites. Glaciomarine sediment deposits reveal that the grounded ice in the Amundsen Sea Embayment had already retreated to within ~45 km of the modern grounding zone prior to ca. 9,400 y ago. Sediments deposited within the past 100+ y record abrupt changes in environmental conditions. On seafloor highs, these shifts document ice-shelf thinning initiating at least as early as the 1940s. Sediments recovered from deep basins reflect a transition from ice proximal to slightly more distal conditions, suggesting ongoing grounding-zone retreat since the 1950s. The timing of ice-shelf unpinning from the seafloor for Thwaites Glacier coincides with similar records from neighboring Pine Island Glacier. Our work provides robust new evidence that glacier retreat in the Amundsen Sea was initiated in the mid-twentieth century, likely associated with climate variability. [ABSTRACT FROM AUTHOR]

We observe the evacuation of 11-year-old landfast sea ice in the Larsen B embayment on the East Antarctic Peninsula in January 2022, which was in part triggered by warm atmospheric conditions and strong offshore winds. This evacuation of sea ice was closely followed by major changes in the calving behaviour and dynamics of a subset of the ocean-terminating glaciers in the region. We show using satellite measurements that, following a decade of gradual slow-down, Hektoria, Green, and Crane glaciers sped up by approximately 20 %–50 % between February and the end of 2022, each increasing in speed by more than 100 ma-1. Circumstantially, this is attributable to their transition into tidewater glaciers following the loss of their ice shelves after the landfast sea ice evacuation. However, a question remains as to whether the landfast sea ice could have influenced the dynamics of these glaciers, or the stability of their ice shelves, through a buttressing effect akin to that of confined ice shelves on grounded ice streams. We show, with a series of diagnostic modelling experiments, that direct landfast sea ice buttressing had a negligible impact on the dynamics of the grounded ice streams. Furthermore, we suggest that the loss of landfast sea ice buttressing could have impacted the dynamics of the rheologically weak ice shelves, in turn diminishing their stability over time; however, the accompanying shifts in the distributions of resistive stress within the ice shelves would have been minor. This indicates that this loss of buttressing by landfast sea ice is likely to have been a secondary process in the ice shelf disaggregation compared to, for example, increased ocean swell or the drivers of the initial landfast sea ice disintegration. [ABSTRACT FROM AUTHOR]

The Antarctic Ice Sheet represents the largest source of uncertainty in future sea level rise projections, with a contribution to sea level by 2100 ranging from -5 to 43 cm of sea level equivalent under high carbon emission scenarios estimated by the recent Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). ISMIP6 highlighted the different behaviors of the East and West Antarctic ice sheets, as well as the possible role of increased surface mass balance in offsetting the dynamic ice loss in response to changing oceanic conditions in ice shelf cavities. However, the detailed contribution of individual glaciers, as well as the partitioning of uncertainty associated with this ensemble, have not yet been investigated. Here, we analyze the ISMIP6 results for high carbon emission scenarios, focusing on key glaciers around the Antarctic Ice Sheet, and we quantify their projected dynamic mass loss, defined here as mass loss through increased ice discharge into the ocean in response to changing oceanic conditions. We highlight glaciers contributing the most to sea level rise, as well as their vulnerability to changes in oceanic conditions. We then investigate the different sources of uncertainty and their relative role in projections, for the entire continent and for key individual glaciers. We show that, in addition to Thwaites and Pine Island glaciers in West Antarctica, Totten and Moscow University glaciers in East Antarctica present comparable future dynamic mass loss and high sensitivity to ice shelf basal melt. The overall uncertainty in additional dynamic mass loss in response to changing oceanic conditions, compared to a scenario with constant oceanic conditions, is dominated by the choice of ice sheet model, accounting for 52 % of the total uncertainty of the Antarctic dynamic mass loss in 2100. Its relative role for the most dynamic glaciers varies between 14 % for MacAyeal and Whillans ice streams and 56 % for Pine Island Glacier at the end of the century. The uncertainty associated with the choice of climate model increases over time and reaches 13 % of the uncertainty by 2100 for the Antarctic Ice Sheet but varies between 4 % for Thwaites Glacier and 53 % for Whillans Ice Stream. The uncertainty associated with the ice–climate interaction, which captures different treatments of oceanic forcings such as the choice of melt parameterization, its calibration, and simulated ice shelf geometries, accounts for 22 % of the uncertainty at the ice sheet scale but reaches 36 % and 39 % for Institute Ice Stream and Thwaites Glacier, respectively, by 2100. Overall, this study helps inform future research by highlighting the sectors of the ice sheet most vulnerable to oceanic warming over the 21st century and by quantifying the main sources of uncertainty. [ABSTRACT FROM AUTHOR]

Open ocean areas surrounded by sea ice and maintained by ocean heat, or sensible‐heat polynyas, are linked to key ice‐sheet processes, such as ice‐shelf basal melt and ice‐shelf fracture, when they occur near ice‐shelf fronts. However, the lack of detailed multi‐year records of polynya variability prevent assessing coupling between polynya and frontal dynamics. Here, we present the first multi‐decadal polynya area record (2000–2022) at Pine Island Glacier (PIG), West Antarctica, from thermal and optical satellite imagery. We found substantial interannual variability in polynya area, with consistencies in the timing of polynya opening, maximal extent, and closing. Furthermore, the largest polynya in our record (269 km2) occurred at PIG's western margin just 68 days before iceberg B‐27 calved, suggesting that polynya size and position may influence rifting dynamics. Our new data set provides a pathway to assess coevolving polynya and frontal dynamics, demonstrating the importance of building long‐term, year‐round polynya variability records. Plain Language Summary: Persistent sensible‐heat polynyas are areas of open ocean surrounded by sea ice maintained by ocean heat. These surface features occur near ice‐shelf fronts at the coastal margins of Antarctica and therefore have the potential to impact ice‐shelf stability through heat transfer processes. However, our understanding of long‐term polynya variability remains limited due to the lack of multi‐year records documenting polynya evolution. Here, we use satellite imagery to measure polynya area near Pine Island Glacier (PIG) and build the first multi‐decadal record in Antarctica. We observed a large amount of year‐to‐year area variability from 2000 to 2022, with the largest polynya in our record (269 km2) occurring at the western edge of PIG just 68 days before a large iceberg calved from PIG. This correspondence suggests that polynya size and position may influence iceberg calving. Our new data set provides a pathway to assess potentially coupled ice and ocean processes, demonstrating the importance of building long‐term, year‐round polynya variability records. Key Points: We generated a 22 years record of polynya area at Pine Island Glacier from satellite thermal and optical imageryOur data set shows high variability in sensible‐heat polynya area (0–322 km2) from 2000 to 2022Large, marginal, and persistent sensible‐heat polynyas may reduce ice‐shelf buttressing and contribute to rift initiation and propagation [ABSTRACT FROM AUTHOR]

Soil and sediment mixing and associated permafrost processes are not widely studied or understood in the McMurdo Dry Valleys of Antarctica. In this study, we investigate the stability and depositional history of near-surface permafrost sediments to ∼ 3 m depth in the Pearse and lower Wright valleys using measured cosmogenic 10 Be and 26 Al depth profiles. In Pearse Valley, we estimate a minimum depositional age of ∼ 74 ka for the active layer and paleoactive-layer sediments (< 0.65 m). Combined depth profile modelling of 10 Be and 26 Al gives a depositional age for near-surface (< 1.65 m) permafrost in Pearse Valley of 180 +20/-40 ka, implying that the deposition of permafrost sediments predates MIS 5 advances of Taylor Glacier. Deeper permafrost sediments (> 2.09 m) in Pearse Valley are thus inferred to have a depositional age of > 180 ka. At a coastal, lower-elevation site in neighbouring lower Wright Valley, 10 Be and 26 Al depth profiles from a second permafrost core exhibit near-constant concentrations with depth and indicate the sediments are either vertically mixed after deposition or sufficiently young so that post-depositional nuclide production is negligible relative to inheritance. 26Al/10Be concentration ratios for both depth profiles range between 4.0 and 5.2 and are all lower than the nominal surface production rate ratio of 6.75, indicating that prior to deposition, these sediments experienced complex, yet similar, exposure–burial histories. Assuming a single-cycle exposure–burial scenario, the observed 26Al/10Be ratios are equivalent to a total minimum exposure–burial history of ∼ 1.2 Myr. In proximity to the depth profile core site, we measured cosmogenic 10 Be and 26 Al in three granite cobbles from thin, patchy drift (Taylor 2 Drift) in Pearse Valley to constrain the timing of retreat of Taylor Glacier. Assuming simple continuous exposure, our minimum, zero-erosion exposure ages suggest Taylor Glacier partially retreated from Pearse Valley no later than 65–74 ka. The timing of retreat after 65 ka and until the Last Glacial Maximum (LGM) when Taylor Glacier was at a minimum position remains unresolved. The surface cobble ages and permafrost processes reveal Taylor Glacier advances during MIS 5 were non-erosive or mildly erosive, preserving the underlying permafrost sediments and peppering boulders and cobbles upon an older, relict surface. Our results are consistent with U/Th ages from central Taylor Valley and suggest changes in moisture delivery over Taylor Dome during MIS 5e, 5c, and 5a appear to be associated with the extent of the Ross Ice Shelf and sea ice in the Ross Sea. These data provide further evidence of antiphase behaviour through retreat of a peripheral lobe of Taylor Glacier in Pearse Valley, a region that was glaciated during MIS 5. We suggest a causal relationship of cold-based glacier advance and retreat that is controlled by an increase in moisture availability during retreat of sea ice and perhaps the Ross Ice Shelf, as well as, conversely, a decrease during times of sea ice and Ross Ice Shelf expansion in the Ross Sea. [ABSTRACT FROM AUTHOR]

Antarctica's floating ice shelves modulate discharge of grounded ice into the ocean by providing a backstress. Ice shelf thinning and grounding line retreat have reduced this backstress, driving rapid drawdown of key unstable areas of the Antarctic Ice Sheet, leading to sea-level rise. If ice shelf loss continues, it may initiate irreversible glacier retreat through the marine ice sheet instability. Identification of areas undergoing significant change requires knowledge of spatial and temporal patterns in recent ice shelf loss. We used 26 years (1992–2017) of satellite-derived Antarctic ice shelf thickness, flow, and basal melt rates to construct a time-dependent dataset of ice shelf thickness and basal melt on a 3 km grid every 3 months. We used a novel data fusion approach, state-of-the-art satellite-derived velocities, and a new surface mass balance model. Our data revealed an overall pattern of thinning all around Antarctica, with a thinning slowdown starting around 2008 widespread across the Amundsen, Bellingshausen, and Wilkes sectors. We attribute this slowdown partly to modulation in external ocean forcing, altered in West Antarctica by negative feedbacks between ice shelf thinning rates and grounded ice flow, and sub-ice-shelf cavity geometry and basal melting. In agreement with earlier studies, the highest rates of ice shelf thinning are found for those ice shelves located in the Amundsen and Bellingshausen sectors. Our study reveals that over the 1992–2017 observational period the Amundsen and Bellingshausen ice shelves experienced a slight reduction in rates of basal melting, suggesting that high rates of thinning are largely a response to changes in ocean conditions that predate our satellite altimetry record, with shorter-term variability only resulting in small deviations from the long-term trend. Our work demonstrates that causal inference drawn from ice shelf thinning and basal melt rates must take into account complex feedbacks between thinning and ice advection and between ice shelf draft and basal melt rates. [ABSTRACT FROM AUTHOR]

The seafloor depths under the Cook Ice Shelf and Ninnis Glacier Tongue have not been directly measured, despite their importance for understanding ocean circulation and ice shelf change. We model the bathymetry underneath the floating ice and surrounding ocean using airborne gravity data. Our model is constrained by few ship‐based seafloor measurements near the ice front and by ice‐base measurements over areas of grounded ice from radar data. Localized basins (∼1,400 m deep) are found beneath both ice shelves. The shallowest modeled bathymetry (∼200 m) represents the offshore extension of Cape Freshfield. Near the grounding line, seafloor depths are found to be deeper than the observed depth of the modified Circumpolar Deep water in the region (<350 m), key factor for basal melt analyses. From transit flight gravity anomalies, we suggest the relocation of the mapped edge of the continental shelf and a narrowing of the Cook Shelf Depression. Plain Language Summary: The knowledge of how deep the ocean floor is under the floating ice shelves that connect to grounded ice sheets, is crucial for understanding how ocean water circulates and interacts with the overlying ice. We present a new bathymetric model of the seafloor beneath two ice shelves located in East Antarctica: Cook Ice Shelf and Ninnis Glacier Tongue. Both ice shelves are inaccessible to ships due to heavy sea ice conditions, so the data used in our model were collected from airborne surveys. Our bathymetry model shows new information on the depth and shape of the seafloor that will help understanding the ocean circulation in the area and how this might impact ice thickness changes. Key Points: High resolution bathymetry model of Cook Ice Shelf, Ninnis Glacier Tongue, and surrounding open ocean from airborne gravity inversionNew bathymetry model improves the understanding of water pathways between the ice shelves and the continental shelf edgeTransit flight gravity anomalies suggest relocation of the edge of the continental shelf northwards of the currently attributed position [ABSTRACT FROM AUTHOR]

Thwaites Ice Shelf (TWIS), the floating extension of Thwaites Glacier, West Antarctica, is changing rapidly and may completely disintegrate in the near future. Any buttressing that the ice shelf provides to the upstream grounded Thwaites glacier will then be lost. Previously, it has been argued that this could lead to onset of dynamical instability and the rapid demise of the entire glacier. Here we provide the first systematic quantitative assessment of how strongly the upstream ice is buttressed by TWIS and how its collapse affects future projections. By modeling the stresses acting along the current grounding line, we show that they deviate insignificantly from the stresses after ice shelf collapse. Using three ice‐flow models, we furthermore model the transient evolution of Thwaites Glacier and find that a complete disintegration of the ice shelf will not substantially impact future mass loss over the next 50 years. Plain Language Summary: As ice is lost, global sea levels rise. Currently glaciers and ice sheets contribute about half of observed sea level rise (SLR). Some ice is lost through surface melting, but sea levels can also be impacted by ice that flows directly into the ocean. This loss of ice, from the grounded interior into the surrounding ocean, is particularly important for the Antarctic Ice Sheet. There the grounded sections of the ice sheet are sometimes surrounded by floating seaward extensions referred to as ice shelves. These ice shelves can restrict, that is, buttress, upstream flow. Here we estimate the buttressing provided to one of the largest and most dynamical glaciers of the Antarctic Ice Sheet, the Thwaites Glacier. The glacier is currently abutted by an ice shelf, which has become progressively smaller and weaker over the last few decades. It has been suggested that a disintegration of the ice shelf might lead to greatly enhanced ice flow. However, we find that the ice shelf does not significantly buttress upstream flow, and removing the ice shelf has little effect on SLR over the next 50 years. Key Points: Thwaites Ice Shelf provides limited buttressing to the grounded upstream Thwaites GlacierRemoval of the ice shelf changes the contribution of the West Antarctic Ice Sheet to sea level rise by at most a few millimeters over 50 yearsObservations and modeling suggest that Thwaites Glacier is currently unsteady and stable [ABSTRACT FROM AUTHOR]

The fastest projected rates of sea level rise appear in models which include "the marine ice cliff instability (MICI)," a hypothesized but mostly unobserved process defined by rapid fracture and wastage of terminal ice cliffs that outpaces viscous relaxation and ice‐shelf formation. Crane Glacier's response to the Larsen B Ice Shelf collapse has been invoked as evidence of MICI in the observational record. Using available remote sensing data, we analyze Crane's retreat, arrest, and regrowth over the last two decades. Much of Crane's terminus retreat occurred in floating, not grounded ice. Retreat accelerated by at least 54% during the 2 years following ice shelf collapse. This is inconsistent with contemporaneous regional forcing that promoted ice shelf growth during this period, but consistent with a geometrically controlled positive feedback. We infer a maximum possible cliff height of 111 m, which according to process models, could enable cliff calving assuming damaged ice. Plain Language Summary: The behavior of Antarctic glaciers will largely determine the pace and magnitude of future sea level rise. But the projections made by ice sheet models are uncertain, in part due to the uncertain response of Antarctica to the future loss of its floating ice shelves. It has been hypothesized that ice shelf breakup could trigger a self‐sustaining mechanism of ice loss whereby ice cliffs collapse under their own weight. This idea is controversial because it has not been unambiguously observed in modern glacier systems. We show that after the loss of its ice shelf, Crane Glacier experienced a 2‐year period of accelerating ice loss. While not an unambiguous validation of the instability hypothesis, what we do observe is consistent with a positive feedback and unforced retreat. Models of ice cliff failure that assume glacier ice has pre‐existing weaknesses are consistent with the behavior we observe at Crane, suggesting that if ice was sufficiently damaged at the Crane terminus, retreat by cliff failure was theoretically possible. Key Points: Retreat of Crane Glacier's terminus after the collapse of the Larsen B Ice Shelf accelerated between November 2002 and 20046.74 km of the 10.30 km retreat occurred in floating ice, with ice cliff failure possible during retreat assuming damaged iceCrane's retreat into a narrow fjord and sea‐ice growth re‐established buttressing stresses, slowed calving, and reversed terminus retreat [ABSTRACT FROM AUTHOR]

We present a 700 km airborne electromagnetic survey of late‐spring fast ice and sub‐ice platelet layer (SIPL) thickness distributions from McMurdo Sound to Cape Adare, providing a first‐time inventory of fast ice thickness close to its annual maximum. The overall mode of the consolidated ice (including snow) thickness was 1.9 m, less than its mean of 2.6 ± 1.0 m. Our survey was partitioned into level and rough ice, and SIPL thickness was estimated under level ice. Although level ice, with a mode of 2.0 m and mean of 2.0 ± 0.6 m, was prevalent, rough ice occupied 41% of the transect by length, 50% by volume, and had a mode of 3.3 m and mean of 3.2 ± 1.2 m. The thickest 10% of rough ice was almost 6 m on average, inclusive of a 2 km segment thicker than 8 m in Moubray Bay. The thickest ice occurred predominantly along the northwestern Ross Sea, due to compaction against the coast. The adjacent pack ice was thinner (by ∼1 m) than the first‐year fast ice. In Silverfish Bay, offshore Hells Gate Ice Shelf, New Harbor, and Granite Harbor, the SIPL transect volume was a significant fraction (0.30) of the consolidated ice volume. The thickest 10% of SIPLs averaged nearly 3 m thick, and near Hells Gate Ice Shelf the SIPL was almost 10 m thick, implying vigorous heat loss to the ocean (∼90 W m−2). We conclude that polynya‐induced ice deformation and interaction with continental ice influence fast ice thickness in the western Ross Sea. Plain Language Summary: Sea ice held stationary by the coast is named fast ice. Around Antarctica, it is often observed near floating land ice and grounded icebergs. Fast ice is a critical habitat for marine life and known to be a sensitive indicator to changing weather and oceanic processes. Despite its importance, satellite measurements of its area are recent, and little is known about its thickness. Using a specialized instrument on a fixed wing aircraft, we measured the 2017 November fast ice thickness in the western Ross Sea. Most of the fast ice surveyed was 2 m thick on average and had formed in sheltered embayments. Where floating glaciers are in contact with fast ice, a 3–10 m thick layer of loose ice crystals was sometimes observed beneath, which contributed to its thickness. However, most of the thicker ice was rough ice, formed when ferocious offshore winds pushed ice from the coast. It then drifted with the prevailing ocean currents until eventually becoming frozen in place against coastal topography. This rough ice, 3.3 m thick on average and comprising half the surveyed volume, was significantly thicker than adjacent freely moving pack ice. Our survey provides a baseline from which future change can be measured. Key Points: Annual maximum fast ice and sub‐ice platelet layer thickness distributions are surveyed by airborne electromagnetics over 700 km of Ross SeaDeformed against the coast, 50% of transect volume was rough first‐year fast ice (mode 3.3 m thick), that was thicker than nearby pack iceSub‐ice platelet layers over 0.5 m thick underlay level ice over large parts of four embayments, revealing land ice‐fast ice interaction [ABSTRACT FROM AUTHOR]

Satellite data has revealed that warm ocean tides are causing the Thwaites Glacier in West Antarctica, also known as the "doomsday glacier," to melt at a faster rate than previously thought. The intrusion of seawater beneath the glacier is melting it from below, making it more vulnerable to collapse. The Thwaites Glacier is already responsible for 4% of global sea level rise, and its disintegration could lead to a rise of over 3 meters in global ocean levels. The study, led by glaciologist Eric Rignot, used radar data collected by satellites to analyze the movements and changes in surface elevation of the glacier. The findings suggest that current projections for the glacier's retreat have been too conservative. [Extracted from the article]

Coupled ice sheet‐ocean models are beginning to be used to study the response of ice sheets to ocean warming. Initializing an ice‐ocean model is challenging and can introduce nonphysical transients, and the extent to which such transients can affect model projections is unclear. We use a synchronously‐coupled ice‐ocean model to investigate evolution of Pope, Smith and Kohler Glaciers, West Antarctica, over the next half‐century. Two methods of initialization are used: In one, the ice‐sheet model is constrained with observed velocities in its initial state; in another, the model is constrained with both velocities and grounded thinning rates over a 4‐year period. Each method is applied to two basal sliding laws. For each resulting initialization, two climate scenarios are considered: one where ocean conditions during the initialization period persist indefinitely, and one where the ocean is in a permanent "warm" state. At first, model runs initialized with thinning data exhibit volume loss rates much closer to observed values than those initialized with velocity only, but after 1–2 decades, the forcing primarily determines rates of volume loss and grounding line retreat. Such behavior is seen for both basal sliding laws, although volume loss rates differ quantitatively. Under the "warm" scenario, a grounding line retreat of ∼30 km is simulated for Smith and Kohler, although variation in total retreat due to initialization is nearly as large as that due to forcing. Furthermore it is questionable whether retreat will continue due to narrowing of submarine troughs and limiting of heat transport by bathymetric obstacles. Plain Language Summary: In a number of locations along the Antarctic coastline, particularly in West Antarctica, warm waters which originate in the midlatitudes exist on the continental shelf, protected from cold air and freezing conditions at the surface. These warm waters are able to circulate underneath ice shelves—The vast floating extensions of fast‐flowing outlet glaciers—Leading to high levels of melt and thinning. The thinning, in turn, drives speedup and thinning of the glaciers, contributing to rising sea levels. To study this process, interactive models of ice dynamics and ocean circulation are used. As in other areas of Earth System modeling, such as weather prediction, the way in which these models are initialized impacts model predictions, and due to the complexity of interactions between ice and ocean, these impacts are poorly understood. Here, we use a novel model of ice‐ocean interaction and consider different ways of initializing the model using remotely‐observed data, applied to a very fast‐thinning system of West Antarctic glaciers. We find that 10–20 years predictions of ice‐ocean evolution depend strongly on initialization, but longer‐term predictions depend more on evolving climate. Key Points: Different initialization strategies, sliding laws, and climate forcings are applied to a coupled ice sheet‐ocean modelGrounded ice loss rates are determined by model initialization in the first 1–2 decades, but controlled by forcing over longer time scalesDespite the influence of forcing, overall grounding line retreat is affected as strongly by initialization as by forcing for 50‐year simulations [ABSTRACT FROM AUTHOR]

Tidal modulation of ice streams and their adjacent ice shelves is a real‐world experiment to understand ice‐dynamic processes. We observe the dynamics of Priestley Glacier, Antarctica, using Terrestrial Radar Interferometry (TRI) and GNSS. Ocean tides are predominantly diurnal but horizontal GNSS displacements also oscillate semi‐diurnally. The oscillations are strongest in the ice shelf and tidal signatures decay near‐linearly in the TRI data over >10 km upstream of the grounding line. Tidal flexing is observed >6 km upstream of the grounding line including cm‐scale uplift. Tidal grounding line migration is small and <40% of the ice thickness. The frequency doubling of horizontal displacements relative to the ocean tides is consistent with variable ice‐shelf buttressing demonstrated with a visco‐elastic Maxwell model. Taken together, this supports previously hypothesized flexural ice softening in the grounding‐zone through tides and offers new observational constraints for the role of ice rheology in ice‐shelf buttressing. Plain Language Summary: Temperatures in Antarctica's interior are below zero, so that ice continuously accumulates through snowfall. This mass gain is balanced by ice transport from the interior toward the coast. There, floating ice shelves form on the ocean and ice is eventually lost through iceberg calving and ocean induced melting. Small changes in any of these processes impact global ocean circulation and mean sea level. The speed of the ice varies with ocean tides. We use a specialized instrument that detects this variability and tidal flexing area‐wide and every few hours. We find that although low‐ and high‐tide only occur once a day in this area, ice flow is largest twice a day. This can be explained because ice softens when it is flexed by the ocean tides so that it can flow episodically faster. This is important because the deformation of ice shelves determines to an extent the stability of the entire ice sheet. Key Points: Terrestrial radar interferometry and GNSS identify dynamic changes at Priestley Glacier, Antarctica, forced by diurnal ocean tidesHorizontal displacements vary semi‐diurnally, are strongest in the shelf and decay near‐linearly >10 km upstream of the grounding lineGrounding line migration is small and flexural softening in ice‐shelf shear margins can explain the semi‐diurnal ice flow component [ABSTRACT FROM AUTHOR]

A major uncertainty in Antarctica's contribution to future sea‐level rise is the ice sheet response timescales to ocean warming. Totten Glacier drains a region containing 3.9 m global sea level equivalent and has been losing mass over recent decades. We use an ice sheet model coupled to an ice‐shelf cavity combined ocean box and plume model to investigate Totten's response to variable ocean forcing. Totten's grounding line is stable for a limited range of ocean temperatures near current observations (i.e., −0.95°C to −0.75°C), with topography influencing the discharge periodicity. For increases of ≥0.2°C in temperatures beyond this range, grounding line retreat occurs. Variable ocean forcing can reduce retreat relative to constant forcing, and different variability amplitudes can cause centennial‐scale delays in retreat through interactions with topography. Our results highlight the need for long‐term ocean state observations and to include forcing variability in ice sheet model simulations of future change. Plain Language Summary: Antarctica's contribution to future sea‐level rise is dominated by uncertainties in how the ice sheet will respond to ocean warming. Totten Glacier, East Antarctica, currently contributes to sea‐level rise and is the major outlet glacier in a region with potential to significantly raise sea levels. We use an ice sheet model coupled to a combined box and plume ocean model to investigate how variability in ocean temperatures can impact ice mass loss in this region. For scenarios of ocean temperatures that are near observed, the grounding line position is close to its present‐day location. In these scenarios, interactions between the ice shelf and topography are important in controlling the timing between peak ice shelf melting and peak mass changes. For small increases in background ocean temperature, we see grounding line retreat. In these scenarios, variability in ocean temperatures can reduce ice flux across the grounding line compared with when the ocean temperature is constant. Certain amplitudes of variability can delay grounding line retreat by centuries through interactions with the topography. This highlights the need for long‐term ocean observations in this region and to include ocean variability in ice sheet model simulations of future change. Key Points: Totten's grounding line is steady and close to present‐day only for current ocean temperatures and retreats for small temperature increasesOcean variability of various amplitudes can reduce discharge and cause centennial scale delays in grounding line migrationForcing variability needs to be taken into account in ice sheet model simulations of future climate change [ABSTRACT FROM AUTHOR]

The safety band of Antarctica, consisting of floating glacier tongues and ice shelves, buttresses ice discharge of the Antarctic Ice Sheet. Recent disintegration events of ice shelves along with glacier retreat indicate a weakening of this important safety band. Predicting calving front retreat is a real challenge due to complex ice dynamics in a data-scarce environment that are unique for each ice shelf and glacier. We explore the extent to which easy-to-access remote sensing and modeling data can help to define environmental conditions leading to calving front retreat. For the first time, we present a circum-Antarctic record of glacier and ice shelf front change over the last two decades in combination with environmental variables such as air temperature, sea ice days, snowmelt, sea surface temperature, and wind direction. We find that the Antarctic Ice Sheet area decreased by - 29 618 ± 1193 km 2 in extent between 1997–2008 and gained an area of 7108 ± 1029 km 2 between 2009 and 2018. Retreat concentrated along the Antarctic Peninsula and West Antarctica including the biggest ice shelves (Ross and Ronne). In several cases, glacier and ice shelf retreat occurred in conjunction with one or several changes in environmental variables. Decreasing sea ice days, intense snowmelt, weakening easterlies, and relative changes in sea surface temperature were identified as enabling factors for retreat. In contrast, relative increases in mean air temperature did not correlate with calving front retreat. For future studies a more appropriate measure for atmospheric forcing should be considered, including above-zero-degree days and temperature extreme events. To better understand drivers of glacier and ice shelf retreat, it is critical to analyze the magnitude of basal melt through the intrusion of warm Circumpolar Deep Water that is driven by strengthening westerlies and to further assess surface hydrology processes such as meltwater ponding, runoff, and lake drainage. [ABSTRACT FROM AUTHOR]

The geometry of the sea floor immediately beyond Antarctica's marine-terminating glaciers is a fundamental control on warm-water routing, but it also describes former topographic pinning points that have been important for ice-shelf buttressing. Unfortunately, this information is often lacking due to the inaccessibility of these areas for survey, leading to modelled or interpolated bathymetries being used as boundary conditions in numerical modelling simulations. At Thwaites Glacier (TG) this critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project. We present more than 2000 km 2 of new multibeam echo-sounder (MBES) data acquired in exceptional sea-ice conditions immediately offshore TG, and we update existing bathymetric compilations. The cross-sectional areas of sea-floor troughs are under-predicted by up to 40 % or are not resolved at all where MBES data are missing, suggesting that calculations of trough capacity, and thus oceanic heat flux, may be significantly underestimated. Spatial variations in the morphology of topographic highs, known to be former pinning points for the floating ice shelf of TG, indicate differences in bed composition that are supported by landform evidence. We discuss links to ice dynamics for an overriding ice mass including a potential positive feedback mechanism where erosion of soft erodible highs may lead to ice-shelf ungrounding even with little or no ice thinning. Analyses of bed roughnesses and basal drag contributions show that the sea-floor bathymetry in front of TG is an analogue for extant bed areas. Ice flow over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting at the grounding zone today. We conclude that more can certainly be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. This work also addresses the requirements of recent numerical ice-sheet and ocean modelling studies that have recognised the need for accurate and high-resolution bathymetry to determine warm-water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour. [ABSTRACT FROM AUTHOR]

Ocean-induced basal melting is responsible for much of the Amundsen Sea Embayment ice loss in recent decades, but the total magnitude and spatiotemporal evolution of this melt is poorly constrained. To address this problem, we generated a record of high-resolution digital elevation models (DEMs) for Pine Island Glacier (PIG) using commercial sub-meter satellite stereo imagery and integrated additional 2002–2015 DEM and altimetry data. We implemented a Lagrangian elevation change (D h/ D t) framework to estimate ice shelf basal melt rates at 32–256 m resolution. We describe this methodology and consider basal melt rates and elevation change over the PIG ice shelf and lower catchment from 2008 to 2015. We document the evolution of Eulerian elevation change (d h/ d t) and upstream propagation of thinning signals following the end of rapid grounding line retreat around 2010. Mean full-shelf basal melt rates for the 2008–2015 period were ∼82 –93 Gt yr -1 , with ∼200 –250 m yr -1 basal melt rates within large channels near the grounding line, ∼10 –30 m yr -1 over the main shelf, and ∼0 –10 m yr -1 over the North shelf and South shelf, with the notable exception of a small area with rates of ∼50 –100 m yr -1 near the grounding line of a fast-flowing tributary on the South shelf. The observed basal melt rates show excellent agreement with, and provide context for, in situ basal melt-rate observations. We also document the relative melt rates for kilometer-scale basal channels and keels at different locations on the ice shelf and consider implications for ocean circulation and heat content. These methods and results offer new indirect observations of ice–ocean interaction and constraints on the processes driving sub-shelf melting beneath vulnerable ice shelves in West Antarctica. [ABSTRACT FROM AUTHOR]

Relatively little is known about the physical mechanisms that drive the dynamics of the East Antarctic outlet glaciers. Here we conduct a remote-sensing investigation of the Polar Record Glacier (PRG), East Antarctica to analyze its ice flow acceleration, ice front variations and ice surface melting. Ice flow speeds at PRG increased by up to 15% from 2005 to 2015, with substantial interannual fluctuations. The ice velocities also showed seasonal variations, accelerating by up to 9% between September and January. Multiple mechanisms contribute to the observed seasonal variations: the initial acceleration may result from the lost back-stress provided by the sea ice in the austral spring and the later speedup relate to the surface meltwater that leads to weakened ice shelf and shear margins. The sensitivity of the PRG to oceanic forcing is confirmed by comparing the secular ice velocity increases with ocean temperatures. These measurements suggest that the dynamics of East Antarctic ice shelves are sensitive to melt at both the surface and base, at a range of timescales. [ABSTRACT FROM AUTHOR]

The article cities a research study that describes a mechanism for preventing the thinning and weakening of Ross ice shelves in Antarctica. It has been found that this thinning and weakening is primarily driven by warm ocean water entering into the shelf cavities. Basal channels underneath the shelves cause ice thinning.

Prediction of future ice-sheet behavior in Antarctica and its contribution to sea-level rise depends on accurate understanding of ice-sheet response to a warm climate. Examination of how the ice sheet reacted to past warm episodes affords a means of assessing its tolerances to climate change. The West Antarctic Ice Sheet (WAIS), in particular, is thought to be highly susceptible to variations in ocean temperature at its grounding lines. Yet, detailed records of past ocean temperatures close to the continent are rare. Here, we present a record of past relative ocean-temperature and sea-ice change derived from the presence and then eventual abandonment of southern elephant seal (Mirounga leonina) occupation sites along the Victoria Land Coast (VLC) of the western Ross Embayment. Our results suggest greatly reduced landfast and likely pack ice, as well as potentially the incursion of relatively warm modified Circum-Polar Deep Water from ∼7100 to 500 yr BP, with the greatest reduction in ice/warmest water temperatures at ∼5200 and ∼2300-1800 yr BP. These changes in ocean conditions would have had a strong influence on VLC marine-terminating glaciers through variations in buttressing sea ice and melt rates on the underside of floating ice. Independent data suggest that these glaciers had restricted extent in the mid-Holocene, consistent with our inference of warm ocean temperatures and low sea ice. The glaciers subsequently expanded within the last millennium, coincident with the disappearance of southern elephant seals from the coast and the inferred return to icy conditions. Our relative sea-ice and ocean-temperature reconstruction also is consistent with the hypothesized retreat of the WAIS inland of its present position in the mid to late Holocene, although the distance between our sites and the current WAIS grounding lines is large (600–1000 km), and thus any linkage is speculative at present. Finally, limited pre-Holocene southern elephant seal data support the existence of warm ocean temperatures immediately prior to and perhaps even during build-up to the Last Glacial Maximum (LGM) ice position. If this could be confirmed, it would suggest that factors other than ocean temperatures, such as lowered sea level, might have been critical in causing ice-sheet advance in the Ross Embayment at the LGM. • Molted skin and remains show southern elephant seals once occupied the Ross Sea. • 305 radiocarbon dates show colonies on the Victoria Land Coast ∼7000-500 yr BP. • The colonies indicate reduced landfast sea ice at several times in the Holocene. • We attribute low sea ice to incursion of circumpolar deepwater into the Ross Sea. • Colony abandonment ∼1000 yr BP was accompanied by sea ice and glacier advance. [ABSTRACT FROM AUTHOR]

The Totten Glacier in East Antarctica is of major climate interest because of the large fluctuation of its grounding line and of its potential vulnerability to climate change. The ocean above the continental shelf in front of the Totten ice shelf exhibits large extents of landfast sea ice with low interannual variability. Landfast sea ice is either crudely or not at all represented in current climate models. These models are potentially omitting or misrepresenting important effects related to this type of sea ice, such as its influence on coastal polynya locations. Yet, the impact of the landfast sea ice on the ocean–ice shelf interactions is poorly understood. Using a series of high-resolution, regional NEMO-LIM-based experiments, including an explicit treatment of ocean–ice shelf interactions, over the years 2001–2010, we simulate a realistic landfast sea ice extent in the area of Totten Glacier through a combination of a sea ice tensile strength parameterisation and a grounded iceberg representation. We show that the presence of landfast sea ice impacts seriously both the location of coastal polynyas and the ocean mixed layer depth along the coast, in addition to favouring the intrusion of mixed Circumpolar Deep Water into the ice shelf cavities. Depending on the local bathymetry and the landfast sea ice distribution, landfast sea ice affects ice shelf cavities differently. The Totten ice shelf melt rate is increased by 16% on average and its variance decreased by 38%, while the Moscow University ice shelf melt rate is increased by + 54 % in winter. This highlights the importance of including an accurate landfast sea ice representation in regional and eventually global climate models. • Realistic landfast ice by combination of tensile strength and grounded icebergs. • Landfast ice impacts the polynya location and the sea ice production. • Landfast ice favours the inflow of Circumpolar Deep Water in both ice shelf. • Landfast ice increases the basal melt rate in Totten and Moscow University cavity. [ABSTRACT FROM AUTHOR]

The article explores the ice streams in the central regions of Antarctica. It discusses the emergence of a pressing problem regarding the causes that block the ice mass from free sliding and its avalanching into the ocean through the shelf ice. It highlights a concept concerning the slowing down influence of the lateral zones between which the ice stream moves. It notes that lateral zones are the limiting areas for the B stream of the slowly moving stagnant ice located in the Antarctic ice shield.

Studying the vulnerability of Antarctic Ice Shelves and Sheets to changes in the Southern Ocean has experienced increased attention in the last decade. Aiming to deepen the understanding of the interaction between sea ice production, ocean circulation and heat transport underneath Antarctic Ice Shelves we summarize results of different numerical experiments and present first glimpses from a circum-Antarctic ice sheet coupled ocean model. Overall results so far suggest that a dwindling sea ice production can temporarily reduce ocean driven deep ice shelf melting. Secondly stronger wind events may slow down the circulation over the Antarctic continental shelves and thus reduce heat transport where geostrophic flows appear driven by horizontal density gradients induced through sea ice formation. Furthermore changes in the physical shape of floating glacier tongues and small ice shelves can substantially alter regional circulation behaviour and influence the ocean connectivity of much larger neighbouring ice shelves. [ABSTRACT FROM AUTHOR]