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At any one time 130 000 icebergs are afloat in the Southern Ocean; 97% of these are too small to be registered in current satellite-based databases, yet the melting of these small icebergs provides a major input to the Southern Ocean. We use a unique set of visual size observations of 53 000 icebergs in the South Atlantic Ocean, the SCAR International Iceberg Database, to derive average iceberg dissolution rates. Fracture into two parts is the dominant dissolution process for tabular icebergs, with an average half-life of 30 days for icebergs <4 km length and 60 days for larger icebergs. Complete shatter producing many icebergs <1 km length is rare. A side attrition rate of 0.23 m d−1 combined with drift speed of 6 km d−1, or any proportional change in both numbers fits the observed changes in iceberg distribution. The largest injection into the Southern Ocean of fresh water and any iceberg-transported material takes place in a ~2.3 × 10⁶ km2 zone extending east-northeast from the Antarctic Peninsula to the Greenwich meridian. The iceberg contribution to salinities and temperatures, with maximum contribution north of the Weddell Sea, differs in some regions, from those indicated by tracking large icebergs. [ABSTRACT FROM AUTHOR]

We review successes and challenges from five recent subglacial bedrock drilling campaigns intended to find evidence for Antarctic Ice Sheet retreat during warm periods in the geologic past. Insights into times when the polar ice sheets were smaller than present serve as guiding information for modeling efforts that aim to predict the rate and magnitude of future sea level rise that would accompany major retreat of the Antarctic Ice Sheet. One method to provide direct evidence for the timing of deglaciations and minimum extent of prior ice sheets is to extract subglacial bedrock cores for cosmogenic nuclide analysis from beneath the modern ice sheet surface. Here we summarize the lessons learned from five field seasons tasked with obtaining bedrock cores from shallow depths (

We investigate two ice rises, Kupol Moskovskij and Kupol Ciolkovskogo, in the Fimbul Ice Shelf, East Antarctica, situated ~60 km from each other but differing in their glaciological settings. We apply a thermo-mechanically coupled Elmer/Ice model to profiles going across these ice rises and use it to investigate their past evolution covering present to several millennia ago. We constrain the model results using field measurements, including surface-velocity measurements, and surface mass balance estimated by isochronous radar stratigraphy dated with firn cores. We find that the ice rises are thickening at present (2012–2014), which started only in recent decades. Investigation of deeper radar reflectors suggests a stronger upwind-downwind contrast in surface mass balance in the past for both ice rises, with varying details. This result matches what was previously found on Blåskimen Island ice rise, which is also in the Fimbul Ice Shelf. Moreover, Kupol Moskovskij, situated at a shear margin, shows signs of recent changes in its ice-divide position, while Kupol Ciolkovskogo shows a more stable divide position. This study highlights the long-term influence of surface mass balance on ice rises, as well as the strong influence of local glaciological settings on their evolution. [ABSTRACT FROM AUTHOR]

Subglacial lake water-volume changes produce ice-elevation anomalies that provide clues about water flow beneath glaciers and ice sheets. Significant challenges remain in the quantitative interpretation of these elevation-change anomalies because the surface expression of subglacial lake activity depends on basal conditions, rate of water-volume change, and ice rheology. To address these challenges, we introduce an inverse method that reconstructs subglacial lake activity from altimetry data while accounting for the effects of viscous ice flow. We use a linearized approximation of a Stokes ice-flow model under the assumption that subglacial lake activity only induces small perturbations relative to a reference ice-flow state. We validate this assumption by accurately reconstructing lake activity from synthetic data that are produced with a fully nonlinear model. We then apply the method to estimate the water-volume changes of several active subglacial lakes in Antarctica by inverting data from NASA's Ice, Cloud, and land Elevation Satellite 2 (ICESat-2) laser altimetry mission. The results show that there can be substantial discrepancies (20% or more) between the inversion and traditional estimation methods due to the effects of viscous ice flow. The inverse method will help refine estimates of subglacial water transport and further constrain the role of subglacial hydrology in ice-sheet evolution.

Stable water isotope records of six firn cores retrieved from two adjacent plateaus on the northern Antarctic Peninsula between 2014 and 2016 are presented and investigated for their connections with firn-core glacio-chemical data, meteorological records and modelling results. Average annual accumulation rates of 2500 kg m−2 a−1 largely reduce the modification of isotopic signals in the snowpack by post-depositional processes, allowing excellent signal preservation in space and time. Comparison of firn-core and ECHAM6-wiso modelled δ18O and d-excess records reveals a large agreement on annual and sub-annual scales, suggesting firn-core stable water isotopes to be representative of specific synoptic situations. The six firn cores exhibit highly similar isotopic patterns in the overlapping period (2013), which seem to be related to temporal changes in moisture sources rather than local near-surface air temperatures. Backward trajectories calculated with the HYSPLIT model suggest that prominent δ18O minima in 2013 associated with elevated sea salt concentrations are related to long-range moisture transport dominated by westerly winds during positive SAM phases. In contrast, a broad δ18O maximum in the same year accompanied by increased concentrations of black carbon and mineral dust corresponds to the advection of more locally derived moisture with northerly flow components (South America) when the SAM is negative.

Converting measurements of ice-sheet surface elevation change to mass change requires measurements of accumulation and knowledge of the evolution of the density profile in the firn. Most firn-densification models are tuned using measured depth–density profiles, a method which is based on an assumption that the density profile in the firn is invariant through time. Here we present continuous measurements of firn-compaction rates in 12 boreholes near the South Pole over a 2 year period. To our knowledge, these are the first continuous measurements of firn compaction on the Antarctic plateau. We use the data to derive a new firn-densification algorithm framed as a constitutive relationship. We also compare our measurements to compaction rates predicted by several existing firn-densification models. Results indicate that an activation energy of 60 kJ mol−1, a value within the range used by current models, best predicts the seasonal cycle in compaction rates on the Antarctic plateau. Our results suggest models can predict firn-compaction rates with at best 7% uncertainty and cumulative firn compaction on a 2 year timescale with at best 8% uncertainty.

A major consideration for maritime activity in the Southern Hemisphere is the northern limit of icebergs, or the Southern Ocean Limit Of Known Ice (SOLOKI). This analysis of historical reports of icebergs during Southern Hemisphere voyages from 1687 to 1933 provides a basis for examination of their geographical and chronological occurrence during ~250 years. The analyses use tabulated data from 742 voyages and other reports from many sources, some including first-person descriptions. While these data are dependent on icebergs being reported by mariners, as well as the variable frequency of voyages, they demonstrate distinct periods of exceptional frequency of icebergs occurring in certain localities, particularly the far South Atlantic. Based upon historical records the evidence suggests unprecedented numbers of icebergs were present in southern shipping channels in the 1890s. When these historical observations are combined with modern iceberg drift trajectories, their possible origin can be elucidated. Owing to the numbers of icebergs seen and their geographical spread, our results suggest that this was possibly the largest near-synchronous calvings in the last 300 years, and the northernmost extent of the SOLOKI.

From 2017 to 2020, three significant calving events took place on Pine Island Glacier, West Antarctica. Ice-shelf velocities changed over this period and the calving events have been suggested as possible drivers. However, satellite observations also show significant changes in the areal extent of fracture zones, especially in the marginal areas responsible for providing lateral support to the ice shelf. Here, we conduct a model study to identify and quantify drivers of recent ice-flow changes of the Pine Island Ice Shelf. In agreement with recent studies, we find that the calving events caused significant velocity changes over the ice shelf. However, calving alone cannot explain observed velocity changes. Changes in the structural rigidity, i.e. ice damage, further significantly impacted ice flow. We suggest that ice damage evolution of the ice-shelf margins may have influenced recent calving events, and these two processes are linked.

Hydrologic pathways beneath ice sheets and glaciers play an important role in regulating ice flow. Antarctica has experienced, and will continue to experience, changes in ice dynamics and geometry, but the associated changes in subglacial hydrology have received less attention. Here, we use the GlaDS subglacial hydrology model to examine drainage evolution beneath an idealised Antarctic glacier in response to steepening ice surface slopes, accelerating ice velocities and subglacial lake drainages. Ice surface slope changes exerted a dominant influence, redirecting basal water to different outlet locations and substantially increasing channelised discharge crossing the grounding line. Faster ice velocities had comparatively negligible effects. Subglacial lake drainage results indicated that lake refilling times play a key role in drainage system evolution, with lake flux more readily accommodated following shorter refilling times. Our findings are significant for vulnerable Antarctic regions currently experiencing dynamic thinning since subglacial water re-routing could destabilise ice shelves through enhanced sub-shelf melting, potentially hastening irreversible retreat. These changes could also affect subglacial lake activity. We, therefore, emphasise that including a nuanced and complex representation of subglacial hydrology in ice-sheet models could provide critical information on the timing and magnitude of sea-level change contributions from Antarctica.

Rifts are full-thickness fractures that propagate laterally across an ice shelf. They cause ice-shelf weakening and calving of tabular icebergs, and control the initial size of calved icebergs. Here, we present a joint inverse and forward computational modeling framework to capture rifting by combining the vertically integrated momentum balance and anisotropic continuum damage mechanics formulations. We incorporate rift–flank boundary processes to investigate how the rift path is influenced by the pressure on rift–flank walls from seawater, contact between flanks, and ice mélange that may also transmit stress between flanks. To illustrate the viability of the framework, we simulate the final 2 years of rift propagation associated with the calving of tabular iceberg A68 in 2017. We find that the rift path can change with varying ice mélange conditions and the extent of contact between rift flanks. Combinations of parameters associated with slower rift widening rates yield simulated rift paths that best match observations. Our modeling framework lays the foundation for robust simulation of rifting and tabular calving processes, which can enable future studies on ice-sheet–climate interactions, and the effects of ice-shelf buttressing on land ice flow.

We developed a novel laser melting sampler (LMS) for ice cores to measure the stable water isotope ratios (δ18O and δD) as temperature proxies at sub-centimeter depth resolutions. In this LMS system, a 2 mm diameter movable evacuation nozzle holds an optical fiber through which a laser beam irradiates the ice core. The movable nozzle intrudes into the ice core, the laser radiation meanwhile melts the ice cylindrically, and the meltwater is pumped away simultaneously through the same nozzle and transferred to a vial for analysis. To avoid isotopic fractionation of the ice through vaporization, the laser power is adjusted to ensure that the temperature of the meltwater is always kept well below its boiling point. A segment of a Dome Fuji shallow ice core (Antarctica), using the LMS, was then demonstrated to have been discretely sampled with a depth resolution as small as 3 mm: subsequent analysis of δ18O, δD, and deuterium excess (d) was consistent with results obtained by hand segmentation within measurement uncertainties. With system software to control sampling resolution, the LMS will enable us to identify temperature variations that may be detectable only at sub-centimeter resolutions in ice cores.

To investigate the mechanisms driving recent changes in outlet glaciers in Antarctica, we measured the glacier front position, flow velocity and surface elevation of five outlet glaciers flowing into Lützow-Holm Bay in East Antarctica. After a steady advance from 2008 to 2015, all the glaciers synchronously retreated by 0.4–6.0 km between 2016 and 2018. The initiation of the retreat coincided with the breakup of land-fast sea ice in Lützow-Holm Bay in 2016, which resulted in the largest sea-ice loss in the region since 1998. Similar flow variations and surface elevation changes were observed near the grounding line of Shirase, Skallen and Telen glaciers. The slowdown in 2011–15 (by 13%) and the speedup in 2016–18 (by 7%) coincided with the respective increase and decrease in surface elevation. Simultaneous retreat and acceleration after the land-fast sea-ice breakup implies that sea ice has a significant influence on glacier dynamics. Thickening/thinning observed near the grounding line was attributed to a reduced/enhanced stretching flow regime during the deceleration/acceleration period. Our results demonstrate that land-fast sea ice affects not only terminus positions, but also the flow speed and ice thickness of the Antarctic glaciers.

Changes in iceberg calving fluxes and oceanographic conditions around Antarctica have likely influenced the spatial and temporal distribution of iceberg fresh water fluxes to the surrounding ocean basins. However, Antarctic iceberg melt rate estimates have been limited to very large icebergs in the open ocean. Here we use a remote-sensing approach to estimate iceberg melt rates from 2011 to 2022 for 15 study sites around Antarctica. Melt rates generally increase with iceberg draft and follow large-scale variations in ocean temperature: maximum melt rates for the western peninsula, western ice sheet, eastern ice sheet and eastern peninsula are ~50, ~40, ~5 and ~5 m a−1, respectively. Iceberg melt sensitivity to thermal forcing varies widely, with a best-estimate increase in melting of ~24 m a−1°C−1 and range from near-zero to ~100 m a−1°C−1. Variations in water shear likely contribute to the apparent spread in thermal forcing sensitivity across sites. Although the sensitivity of iceberg melt rates to water shear prevents the use of melt rates as a proxy to infer coastal water mass temperature variability, additional coastal iceberg melt observations will likely improve models of Southern Ocean fresh water fluxes and have potential for subglacial discharge plume mapping.

Mass loss from the Amundsen Sea Embayment of the West Antarctic Ice Sheet is a major contributor to global sea-level rise (SLR) and has been increasing over recent decades. Predictions of future SLR are increasingly modelled using ensembles of simulations within which model parameters and external forcings are varied within credible ranges. Accurately reporting the uncertainty associated with these predictions is crucial in enabling effective planning for, and construction of defences against, rising sea levels. Calibrating model simulations against current observations of ice-sheet behaviour enables the uncertainty to be reduced. Here we calibrate an ensemble of BISICLES ice-sheet model simulations of ice loss from the Amundsen Sea Embayment using remotely sensed observations of surface elevation and ice speed. Each calibration type is shown to be capable of reducing the 90% credibility bounds of predicted contributions to SLR by 34 and 43% respectively.

One of the largest contributors to uncertainty in predictions of sea-level rise from ice-sheet models is a lack of knowledge about the bed topography beneath ice sheets. Bed topography maps are normally made by interpolating between linear radar surveys using methods that include kriging, mass conservation and flowline diffusion, all of which may miss influential mesoscale (2–30 km) bedforms. Previous works have explored an Ice-Flow Perturbation Analysis (IFPA) approach for estimating bed topography using the surface expression of these mesoscale bedforms. Using regions of Pine Island Glacier that have been intensively surveyed by ice-penetrating radar as test sites, and a refined IFPA methodology, we find that IFPA detects bedforms capable of influencing ice flow which are not represented in Bedmachine Antarctica and other interpolated bed products. We further explore the ability of IFPA to estimate relative bed slipperiness, finding higher slipperiness in the main trunk and tributaries. Alongside other methods which estimate ice thickness, bed topography maps from IFPA have the potential to constrain projections of future sea-level rise, especially where radar data are sparse.

Estimates of ice shelf mass loss are typically based on surface height measurements, assuming hydrostatic equilibrium and estimated firn thickness. Recent investigations, however, challenge the assumption that ice shelves are freely floating, particularly in proximity to narrow structures such as basal channels and shear margins. We compare contemporaneous measurements of Antarctic ice shelf thickness, from ice-penetrating radar, to freeboard height, from laser altimetry, acquired during multiple airborne surveys. On average, the hydrostatic thickness differs from observed thickness by at least ~17 ± 98 m, but this difference varies well beyond the propagated error within and among ice shelves, and depends on the corrections applied. We find that uncertainty in firn thickness can account for most, but not all, of the imbalance. Overall, errors in hydrostatic thickness do not significantly impact estimated basal melt rates. Our results indicate that localized approaches to estimating ice shelf thickness and rates of change are not applicable at large scales, and vice versa, and point to the need for more abundant and accurate firn and ice thickness measurements to improve estimates and predictions of ice shelf mass loss.

As part of the Coupled Model Intercomparison Project Phase 6 (CMIP6), the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) was devised to assess the likely sea-level-rise contribution from the Earth's ice sheets. Here, we construct an ensemble of climate forcings for Antarctica until the year 2300 based on original ISMIP6 forcings until 2100, combined with climate indices from simulations with the MIROC4m climate model until 2300. We then use these forcings to run simulations for the Antarctic ice sheet with the SICOPOLIS model. For the unabated warming pathway RCP8.5/SSP5-8.5, the ice sheet suffers a severe mass loss, amounting to ~ 1.5 m SLE (sea-level equivalent) for the fourteen-experiment mean, and ~ 3.3 m SLE for the most sensitive experiment. Most of this loss originates from West Antarctica. For the reduced emissions pathway RCP2.6/SSP1-2.6, the loss is limited to a three-experiment mean of ~ 0.16 m SLE. The means are approximately two times larger than what was found in a previous study (Chambers and others, 2022, doi:10.1017/jog.2021.124) that assumed a sustained late-21st-century climate beyond 2100, demonstrating the importance of post-2100 climate trends on Antarctic mass changes in the 22nd and 23rd centuries.

Although over 600 Antarctic subglacial lakes have been identified using radar and satellite observations, the bathymetry and bed properties, which are key to understanding conditions within the lake, have been determined in very few localities. We present measurements of water column thickness and lakebed properties from Lago Subglacial CECs (SLC), located beneath 2653 m of ice at the Rutford-Institute-Minnesota divide in Antarctica. Seismic profiles indicate a maximum water column thickness of 301.3 ± 1.5 m, at the widest part of the lake, with an estimated lake volume of 2.5 ± 0.3 km3. Seismic imaging and measurements of the reflection strength at the ice base and lakebed indicate >15 m of high-porosity fine-grained sediment in the central section of the lakebed, consistent with a depositional sequence with an age of up to 0.5 Ma. These observations, along with previous radar measurements and modelling, indicate a low-energy sedimentary environment with a long water-residence time. As such, SLC is a suitable target for exploration via direct access to recover sediment records of ice sheet and climate history and investigate microbial life with long periods of isolation.

The Antarctic Peninsula's widespread glacier retreat and ice shelf collapse have been attributed to atmospheric and oceanic warming. Following the initial post-collapse period of retreat, several former tributary glaciers of the Larsen A and B ice shelves have been slowly re-advancing for more than a decade. Here, we use a flowline model of Crane Glacier to gauge the sensitivity of former tributary glaciers to future climate change following this period of long-term dynamic adjustment. The glacier's long-term geometry and speed changes are similar to those of other former Larsen A and B tributaries, suggesting that Crane Glacier is a reasonable representation of regional dynamics. For the unperturbed climate simulations, discharge remains nearly unchanged in 2018–2100, indicating that dynamic readjustment to shelf collapse took ~15 years. Despite large uncertainties in Crane Glacier's past and future climate forcing, a wide range of future climate scenarios leads to a relatively modest range in grounding line discharge (0.8–1.5 Gt a−1) by 2100. Based on the model results for Crane, we infer that although former ice shelf tributaries may readvance following collapse, similar to the tidewater glacier cycle, their dynamic response to future climate perturbations should be less than their response to ice shelf collapse.

Antarctica's ice shelves stabilize the ice sheet and, therefore, understanding processes affecting the mass budgets of ice shelves is important for estimating grounded ice loss. To study the ice shelf dynamics, we analyzed seismological and GNSS data from the Ekström Ice Shelf in Dronning Maud Land. We extracted probabilistic power spectral densities (PPSD) in the frequency band 3.4–6.8 Hz, typical of icequakes, from seismological data and observed pronounced signals in the PPSD with near 3 and 4 cycles per day (cpd) corresponding to tidal overharmonics, in addition to the main tidal constituents near 1 and 2 cpd. GNSS data reveal the same components in ice flow speed but not in vertical displacements. Generally, tide-induced grounding line migration modulates the flow velocity of an entire ice shelf. We find that this velocity modulation causes the increased icequake activity in the tidal overharmonics with 3 and 4 cpd in an ice shear zone where the flow velocity drops to nearly zero.

Over the last two decades, anomalous warming events have been observed in coastal Antarctic regions. While these events have been documented in the Ross Sea sector, the Antarctic interior is believed to have been buffered from warming. In this work, we present data from lakes located near Mt. Heekin and Thanksgiving Valley (~85° S) along the Shackleton Glacier, which are believed to be the southern-most Antarctic dry valley lakes. In 2018, the lakes were characterized, repeat satellite images were examined, and lake water chemistry was measured. Our analysis shows that lake areas recently increased, and the water-soluble ion chemistry indicates a flushing of salts from periglacial soils, likely from increased glacial melt as illustrated by water isotope data. Our results show that high southern latitude ice-free areas have likely been affected by warm pulses over the past 60 years and these pulses may be quasi-synchronous throughout the Transantarctic Mountains.

At any one time 130 000 icebergs are afloat in the Southern Ocean; 97% of these are too small to be registered in current satellite-based databases, yet the melting of these small icebergs provides a major input to the Southern Ocean. We use a unique set of visual size observations of 53 000 icebergs in the South Atlantic Ocean, the SCAR International Iceberg Database, to derive average iceberg dissolution rates. Fracture into two parts is the dominant dissolution process for tabular icebergs, with an average half-life of 30 days for icebergs

During Cook's 1772–75 Antarctic circumnavigation on the HMS Resolution, he recorded the positions of hundreds of icebergs. This paper compares Cook's observations and those of Halley in 1700, Bouvet in 1739 and Riou in 1789, with the Brigham Young University/National Ice Center (BYU/NIC) and the Alfred Wegener Institute datasets. Cook's description of the iceberg plume east of the Amery Ice Shelf and the iceberg distributions in the Weddell, Ross and Amundsen Seas agree with modern data. In January 1774, Cook reached his farthest south on the shelf of the Amundsen Sea Embayment, the site of the current International Thwaites Glacier Collaboration field study. Cook's largest iceberg had a 2.5 km diameter, where power-law models show that icebergs of this size or smaller comprise 92% of their total number. In the eastern Weddell, Cook's observation of a sea-ice tongue with a much greater extent than in satellite imagery remains unexplained. Although Riou's icebergs lie 1000 km east of the BYU/NIC trajectories, application of the England and others (2020) fracture and drift model to the trajectories removes the discrepancy and means that all the ship observations are consistent with modern observations and theory.

The analysis of Martian meteorites is a key research to understand the mineralogical composition of Mars. However, they suffer different types of alteration due to the environment where they fall on Earth. These differences should be identified in order to characterize correctly the original Martian compounds. Most of the meteorites found on Earth are collected in Antarctica where the environmental conditions are such that, in general and based on many previous investigations, produce fewer alterations compared with other terrestrial environments such as hot deserts. In this study, the weathering alterations of minerals from the MIL 090030 Martian Nakhlite found in Antarctica were analyzed to determine which minerals are formed by Antarctic environmental conditions. It was confirmed that the south-polar region environment can contaminate Martian meteorites and, in this study, several minerals like halite, nitratine and niter, were detected as weathering alteration products that have not been referenced in the literature before.

The Antarctic Peninsula Ice Sheet (APIS) has become a significant contributor to sea-level rise over recent decades. Accurately estimating the ice discharge from the outlet glaciers of the APIS is crucial to quantify the mass balance of the Antarctic Peninsula. We here compute the ice discharge from the outlet glaciers of the APIS north of 70${^\circ }$S for the five most widely used ice-thickness reconstructions, using a common surface velocity field and a common set of flux gates, so the differences in ice discharge can be solely attributed to the differences in ice thickness at the flux gates. The total volumetric ice discharge for 2015–2017 ranges within 45–141 km3 a−1, depending on the ice-thickness model, with a mean of 87 ± 44 km3 a−1. The substantial differences between the ice-discharge results, and a multi-model normalized root-mean-squared deviation of 0.91 for the whole data set, reveal large differences and inconsistencies between the ice-thickness models, giving an indication of the large uncertainty in the current ice-discharge estimates for the APIS. This manifests a fundamental problem of the region: the scarcity of appropriate ice-thickness measurements and the difficulty of the current models to reconstruct the ice-thickness distribution in this complex region.

Subglacial lake water-volume changes produce ice-elevation anomalies that provide clues about water flow beneath glaciers and ice sheets. Significant challenges remain in the quantitative interpretation of these elevation-change anomalies because the surface expression of subglacial lake activity depends on basal conditions, rate of water-volume change, and ice rheology. To address these challenges, we introduce an inverse method that reconstructs subglacial lake activity from altimetry data while accounting for the effects of viscous ice flow. We use a linearized approximation of a Stokes ice-flow model under the assumption that subglacial lake activity only induces small perturbations relative to a reference ice-flow state. We validate this assumption by accurately reconstructing lake activity from synthetic data that are produced with a fully nonlinear model. We then apply the method to estimate the water-volume changes of several active subglacial lakes in Antarctica by inverting data from NASA's Ice, Cloud, and land Elevation Satellite 2 (ICESat-2) laser altimetry mission. The results show that there can be substantial discrepancies (20% or more) between the inversion and traditional estimation methods due to the effects of viscous ice flow. The inverse method will help refine estimates of subglacial water transport and further constrain the role of subglacial hydrology in ice-sheet evolution. [ABSTRACT FROM AUTHOR]

Converting measurements of ice-sheet surface elevation change to mass change requires measurements of accumulation and knowledge of the evolution of the density profile in the firn. Most firn-densification models are tuned using measured depth–density profiles, a method which is based on an assumption that the density profile in the firn is invariant through time. Here we present continuous measurements of firn-compaction rates in 12 boreholes near the South Pole over a 2 year period. To our knowledge, these are the first continuous measurements of firn compaction on the Antarctic plateau. We use the data to derive a new firn-densification algorithm framed as a constitutive relationship. We also compare our measurements to compaction rates predicted by several existing firn-densification models. Results indicate that an activation energy of 60 kJ mol−1, a value within the range used by current models, best predicts the seasonal cycle in compaction rates on the Antarctic plateau. Our results suggest models can predict firn-compaction rates with at best 7% uncertainty and cumulative firn compaction on a 2 year timescale with at best 8% uncertainty. [ABSTRACT FROM AUTHOR]

A major consideration for maritime activity in the Southern Hemisphere is the northern limit of icebergs, or the Southern Ocean Limit Of Known Ice (SOLOKI). This analysis of historical reports of icebergs during Southern Hemisphere voyages from 1687 to 1933 provides a basis for examination of their geographical and chronological occurrence during ~250 years. The analyses use tabulated data from 742 voyages and other reports from many sources, some including first-person descriptions. While these data are dependent on icebergs being reported by mariners, as well as the variable frequency of voyages, they demonstrate distinct periods of exceptional frequency of icebergs occurring in certain localities, particularly the far South Atlantic. Based upon historical records the evidence suggests unprecedented numbers of icebergs were present in southern shipping channels in the 1890s. When these historical observations are combined with modern iceberg drift trajectories, their possible origin can be elucidated. Owing to the numbers of icebergs seen and their geographical spread, our results suggest that this was possibly the largest near-synchronous calvings in the last 300 years, and the northernmost extent of the SOLOKI. [ABSTRACT FROM AUTHOR]

Stable water isotope records of six firn cores retrieved from two adjacent plateaus on the northern Antarctic Peninsula between 2014 and 2016 are presented and investigated for their connections with firn-core glacio-chemical data, meteorological records and modelling results. Average annual accumulation rates of 2500 kg m−2 a−1 largely reduce the modification of isotopic signals in the snowpack by post-depositional processes, allowing excellent signal preservation in space and time. Comparison of firn-core and ECHAM6-wiso modelled δ18O and d-excess records reveals a large agreement on annual and sub-annual scales, suggesting firn-core stable water isotopes to be representative of specific synoptic situations. The six firn cores exhibit highly similar isotopic patterns in the overlapping period (2013), which seem to be related to temporal changes in moisture sources rather than local near-surface air temperatures. Backward trajectories calculated with the HYSPLIT model suggest that prominent δ18O minima in 2013 associated with elevated sea salt concentrations are related to long-range moisture transport dominated by westerly winds during positive SAM phases. In contrast, a broad δ18O maximum in the same year accompanied by increased concentrations of black carbon and mineral dust corresponds to the advection of more locally derived moisture with northerly flow components (South America) when the SAM is negative. [ABSTRACT FROM AUTHOR]

From 2017 to 2020, three significant calving events took place on Pine Island Glacier, West Antarctica. Ice-shelf velocities changed over this period and the calving events have been suggested as possible drivers. However, satellite observations also show significant changes in the areal extent of fracture zones, especially in the marginal areas responsible for providing lateral support to the ice shelf. Here, we conduct a model study to identify and quantify drivers of recent ice-flow changes of the Pine Island Ice Shelf. In agreement with recent studies, we find that the calving events caused significant velocity changes over the ice shelf. However, calving alone cannot explain observed velocity changes. Changes in the structural rigidity, i.e. ice damage, further significantly impacted ice flow. We suggest that ice damage evolution of the ice-shelf margins may have influenced recent calving events, and these two processes are linked. [ABSTRACT FROM AUTHOR]

Rifts are full-thickness fractures that propagate laterally across an ice shelf. They cause ice-shelf weakening and calving of tabular icebergs, and control the initial size of calved icebergs. Here, we present a joint inverse and forward computational modeling framework to capture rifting by combining the vertically integrated momentum balance and anisotropic continuum damage mechanics formulations. We incorporate rift–flank boundary processes to investigate how the rift path is influenced by the pressure on rift–flank walls from seawater, contact between flanks, and ice mélange that may also transmit stress between flanks. To illustrate the viability of the framework, we simulate the final 2 years of rift propagation associated with the calving of tabular iceberg A68 in 2017. We find that the rift path can change with varying ice mélange conditions and the extent of contact between rift flanks. Combinations of parameters associated with slower rift widening rates yield simulated rift paths that best match observations. Our modeling framework lays the foundation for robust simulation of rifting and tabular calving processes, which can enable future studies on ice-sheet–climate interactions, and the effects of ice-shelf buttressing on land ice flow. [ABSTRACT FROM AUTHOR]

Hydrologic pathways beneath ice sheets and glaciers play an important role in regulating ice flow. Antarctica has experienced, and will continue to experience, changes in ice dynamics and geometry, but the associated changes in subglacial hydrology have received less attention. Here, we use the GlaDS subglacial hydrology model to examine drainage evolution beneath an idealised Antarctic glacier in response to steepening ice surface slopes, accelerating ice velocities and subglacial lake drainages. Ice surface slope changes exerted a dominant influence, redirecting basal water to different outlet locations and substantially increasing channelised discharge crossing the grounding line. Faster ice velocities had comparatively negligible effects. Subglacial lake drainage results indicated that lake refilling times play a key role in drainage system evolution, with lake flux more readily accommodated following shorter refilling times. Our findings are significant for vulnerable Antarctic regions currently experiencing dynamic thinning since subglacial water re-routing could destabilise ice shelves through enhanced sub-shelf melting, potentially hastening irreversible retreat. These changes could also affect subglacial lake activity. We, therefore, emphasise that including a nuanced and complex representation of subglacial hydrology in ice-sheet models could provide critical information on the timing and magnitude of sea-level change contributions from Antarctica. [ABSTRACT FROM AUTHOR]

To investigate the mechanisms driving recent changes in outlet glaciers in Antarctica, we measured the glacier front position, flow velocity and surface elevation of five outlet glaciers flowing into Lützow-Holm Bay in East Antarctica. After a steady advance from 2008 to 2015, all the glaciers synchronously retreated by 0.4–6.0 km between 2016 and 2018. The initiation of the retreat coincided with the breakup of land-fast sea ice in Lützow-Holm Bay in 2016, which resulted in the largest sea-ice loss in the region since 1998. Similar flow variations and surface elevation changes were observed near the grounding line of Shirase, Skallen and Telen glaciers. The slowdown in 2011–15 (by 13%) and the speedup in 2016–18 (by 7%) coincided with the respective increase and decrease in surface elevation. Simultaneous retreat and acceleration after the land-fast sea-ice breakup implies that sea ice has a significant influence on glacier dynamics. Thickening/thinning observed near the grounding line was attributed to a reduced/enhanced stretching flow regime during the deceleration/acceleration period. Our results demonstrate that land-fast sea ice affects not only terminus positions, but also the flow speed and ice thickness of the Antarctic glaciers. [ABSTRACT FROM AUTHOR]

Mass loss from the Amundsen Sea Embayment of the West Antarctic Ice Sheet is a major contributor to global sea-level rise (SLR) and has been increasing over recent decades. Predictions of future SLR are increasingly modelled using ensembles of simulations within which model parameters and external forcings are varied within credible ranges. Accurately reporting the uncertainty associated with these predictions is crucial in enabling effective planning for, and construction of defences against, rising sea levels. Calibrating model simulations against current observations of ice-sheet behaviour enables the uncertainty to be reduced. Here we calibrate an ensemble of BISICLES ice-sheet model simulations of ice loss from the Amundsen Sea Embayment using remotely sensed observations of surface elevation and ice speed. Each calibration type is shown to be capable of reducing the 90% credibility bounds of predicted contributions to SLR by 34 and 43% respectively. [ABSTRACT FROM AUTHOR]

Changes in iceberg calving fluxes and oceanographic conditions around Antarctica have likely influenced the spatial and temporal distribution of iceberg fresh water fluxes to the surrounding ocean basins. However, Antarctic iceberg melt rate estimates have been limited to very large icebergs in the open ocean. Here we use a remote-sensing approach to estimate iceberg melt rates from 2011 to 2022 for 15 study sites around Antarctica. Melt rates generally increase with iceberg draft and follow large-scale variations in ocean temperature: maximum melt rates for the western peninsula, western ice sheet, eastern ice sheet and eastern peninsula are ~50, ~40, ~5 and ~5 m a−1, respectively. Iceberg melt sensitivity to thermal forcing varies widely, with a best-estimate increase in melting of ~24 m a−1°C−1 and range from near-zero to ~100 m a−1°C−1. Variations in water shear likely contribute to the apparent spread in thermal forcing sensitivity across sites. Although the sensitivity of iceberg melt rates to water shear prevents the use of melt rates as a proxy to infer coastal water mass temperature variability, additional coastal iceberg melt observations will likely improve models of Southern Ocean fresh water fluxes and have potential for subglacial discharge plume mapping. [ABSTRACT FROM AUTHOR]

We developed a novel laser melting sampler (LMS) for ice cores to measure the stable water isotope ratios (δ18O and δD) as temperature proxies at sub-centimeter depth resolutions. In this LMS system, a 2 mm diameter movable evacuation nozzle holds an optical fiber through which a laser beam irradiates the ice core. The movable nozzle intrudes into the ice core, the laser radiation meanwhile melts the ice cylindrically, and the meltwater is pumped away simultaneously through the same nozzle and transferred to a vial for analysis. To avoid isotopic fractionation of the ice through vaporization, the laser power is adjusted to ensure that the temperature of the meltwater is always kept well below its boiling point. A segment of a Dome Fuji shallow ice core (Antarctica), using the LMS, was then demonstrated to have been discretely sampled with a depth resolution as small as 3 mm: subsequent analysis of δ18O, δD, and deuterium excess (d) was consistent with results obtained by hand segmentation within measurement uncertainties. With system software to control sampling resolution, the LMS will enable us to identify temperature variations that may be detectable only at sub-centimeter resolutions in ice cores. [ABSTRACT FROM AUTHOR]

One of the largest contributors to uncertainty in predictions of sea-level rise from ice-sheet models is a lack of knowledge about the bed topography beneath ice sheets. Bed topography maps are normally made by interpolating between linear radar surveys using methods that include kriging, mass conservation and flowline diffusion, all of which may miss influential mesoscale (2–30 km) bedforms. Previous works have explored an Ice-Flow Perturbation Analysis (IFPA) approach for estimating bed topography using the surface expression of these mesoscale bedforms. Using regions of Pine Island Glacier that have been intensively surveyed by ice-penetrating radar as test sites, and a refined IFPA methodology, we find that IFPA detects bedforms capable of influencing ice flow which are not represented in Bedmachine Antarctica and other interpolated bed products. We further explore the ability of IFPA to estimate relative bed slipperiness, finding higher slipperiness in the main trunk and tributaries. Alongside other methods which estimate ice thickness, bed topography maps from IFPA have the potential to constrain projections of future sea-level rise, especially where radar data are sparse. [ABSTRACT FROM AUTHOR]

Estimates of ice shelf mass loss are typically based on surface height measurements, assuming hydrostatic equilibrium and estimated firn thickness. Recent investigations, however, challenge the assumption that ice shelves are freely floating, particularly in proximity to narrow structures such as basal channels and shear margins. We compare contemporaneous measurements of Antarctic ice shelf thickness, from ice-penetrating radar, to freeboard height, from laser altimetry, acquired during multiple airborne surveys. On average, the hydrostatic thickness differs from observed thickness by at least ~17 ± 98 m, but this difference varies well beyond the propagated error within and among ice shelves, and depends on the corrections applied. We find that uncertainty in firn thickness can account for most, but not all, of the imbalance. Overall, errors in hydrostatic thickness do not significantly impact estimated basal melt rates. Our results indicate that localized approaches to estimating ice shelf thickness and rates of change are not applicable at large scales, and vice versa, and point to the need for more abundant and accurate firn and ice thickness measurements to improve estimates and predictions of ice shelf mass loss. [ABSTRACT FROM AUTHOR]

As part of the Coupled Model Intercomparison Project Phase 6 (CMIP6), the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) was devised to assess the likely sea-level-rise contribution from the Earth's ice sheets. Here, we construct an ensemble of climate forcings for Antarctica until the year 2300 based on original ISMIP6 forcings until 2100, combined with climate indices from simulations with the MIROC4m climate model until 2300. We then use these forcings to run simulations for the Antarctic ice sheet with the SICOPOLIS model. For the unabated warming pathway RCP8.5/SSP5-8.5, the ice sheet suffers a severe mass loss, amounting to ~ 1.5 m SLE (sea-level equivalent) for the fourteen-experiment mean, and ~ 3.3 m SLE for the most sensitive experiment. Most of this loss originates from West Antarctica. For the reduced emissions pathway RCP2.6/SSP1-2.6, the loss is limited to a three-experiment mean of ~ 0.16 m SLE. The means are approximately two times larger than what was found in a previous study (Chambers and others, 2022, doi:10.1017/jog.2021.124) that assumed a sustained late-21st-century climate beyond 2100, demonstrating the importance of post-2100 climate trends on Antarctic mass changes in the 22nd and 23rd centuries. [ABSTRACT FROM AUTHOR]

Although over 600 Antarctic subglacial lakes have been identified using radar and satellite observations, the bathymetry and bed properties, which are key to understanding conditions within the lake, have been determined in very few localities. We present measurements of water column thickness and lakebed properties from Lago Subglacial CECs (SLC), located beneath 2653 m of ice at the Rutford-Institute-Minnesota divide in Antarctica. Seismic profiles indicate a maximum water column thickness of 301.3 ± 1.5 m, at the widest part of the lake, with an estimated lake volume of 2.5 ± 0.3 km3. Seismic imaging and measurements of the reflection strength at the ice base and lakebed indicate >15 m of high-porosity fine-grained sediment in the central section of the lakebed, consistent with a depositional sequence with an age of up to 0.5 Ma. These observations, along with previous radar measurements and modelling, indicate a low-energy sedimentary environment with a long water-residence time. As such, SLC is a suitable target for exploration via direct access to recover sediment records of ice sheet and climate history and investigate microbial life with long periods of isolation. [ABSTRACT FROM AUTHOR]

Antarctica's ice shelves stabilize the ice sheet and, therefore, understanding processes affecting the mass budgets of ice shelves is important for estimating grounded ice loss. To study the ice shelf dynamics, we analyzed seismological and GNSS data from the Ekström Ice Shelf in Dronning Maud Land. We extracted probabilistic power spectral densities (PPSD) in the frequency band 3.4–6.8 Hz, typical of icequakes, from seismological data and observed pronounced signals in the PPSD with near 3 and 4 cycles per day (cpd) corresponding to tidal overharmonics, in addition to the main tidal constituents near 1 and 2 cpd. GNSS data reveal the same components in ice flow speed but not in vertical displacements. Generally, tide-induced grounding line migration modulates the flow velocity of an entire ice shelf. We find that this velocity modulation causes the increased icequake activity in the tidal overharmonics with 3 and 4 cpd in an ice shear zone where the flow velocity drops to nearly zero. [ABSTRACT FROM AUTHOR]

The Antarctic Peninsula's widespread glacier retreat and ice shelf collapse have been attributed to atmospheric and oceanic warming. Following the initial post-collapse period of retreat, several former tributary glaciers of the Larsen A and B ice shelves have been slowly re-advancing for more than a decade. Here, we use a flowline model of Crane Glacier to gauge the sensitivity of former tributary glaciers to future climate change following this period of long-term dynamic adjustment. The glacier's long-term geometry and speed changes are similar to those of other former Larsen A and B tributaries, suggesting that Crane Glacier is a reasonable representation of regional dynamics. For the unperturbed climate simulations, discharge remains nearly unchanged in 2018–2100, indicating that dynamic readjustment to shelf collapse took ~15 years. Despite large uncertainties in Crane Glacier's past and future climate forcing, a wide range of future climate scenarios leads to a relatively modest range in grounding line discharge (0.8–1.5 Gt a−1) by 2100. Based on the model results for Crane, we infer that although former ice shelf tributaries may readvance following collapse, similar to the tidewater glacier cycle, their dynamic response to future climate perturbations should be less than their response to ice shelf collapse. [ABSTRACT FROM AUTHOR]

AbstractRapid Access Ice Drill is a new drilling technology capable of quickly accessing the glacial bed of Antarctic ice sheets, retrieving ice core and rock core samples, and providing boreholes for downhole logging of physical properties. Scientific goals include searching for old ice near the glacial bed and sampling subglacial bedrock. During field trials near McMurdo Station on a piedmont glacier at Minna Bluff in the 2019–20 austral summer, we successfully completed a ‘top-to-bottom’ operational sequence in three boreholes by (1) augering through firn, (2) creating a borehole packer seal in non-porous ice, (3) establishing fluid circulation, (4) quickly drilling a borehole in ice at penetration rates up to 1.2 m min−1, (5) acquiring a short ice core at depth, (6) penetrating the glacial bed at a depth of ~677 m, (7) recovering a 3.2 m core of ice, basal till and subglacial bedrock, (8) optically logging the borehole on wireline, (9) testing hydrofracture potential by overpressuring the borehole fluid and (10) operating in an environmentally benign yet rapid field mode. Minna Bluff testing, therefore, demonstrates the effectiveness of this integrated system to drill rapidly through thick ice and penetrate across the glacial bed to take cores of bedrock.

Hercules Dome, Antarctica, has long been identified as a prospective deep ice core site due to the undisturbed internal layering, climatic setting and potential to obtain proxy records from the Last Interglacial (LIG) period when the West Antarctic ice sheet may have collapsed. We performed a geophysical survey using multiple ice-penetrating radar systems to identify potential locations for a deep ice core at Hercules Dome. The surface topography, as revealed with recent satellite observations, is more complex than previously recognized. The most prominent dome, which we term ‘West Dome’, is the most promising region for a deep ice core for the following reasons: (1) bed-conformal radar reflections indicate minimal layer disturbance and extend to within tens of meters of the ice bottom; (2) the bed is likely frozen, as evidenced by both the shape of the measured vertical ice velocity profiles beneath the divide and modeled ice temperature using three remotely sensed estimates of geothermal flux and (3) models of layer thinning have 132 ka old ice at 45–90 m above the bed with an annual layer thickness of ~1 mm, satisfying the resolution and preservation needed for detailed analysis of the LIG period.

The British Antarctic Survey's (BAS) Rapid Access Isotope Drill (RAID), designed for rapid drilling to survey prospective ice core sites, has been deployed at multiple Antarctic locations over 6 years. This drilling method creates ice chippings that can be discretely sampled and analysed for their chemical and water isotopic composition. Ice sampling methods have evolved since the first uses of the BAS RAID, enabling a more quantifiable sample resolution. Here, we show that water isotope records obtained from RAID ice are comparable to those of equivalent depth resolution from proximal ice cores. Records of chemical impurities also show good agreement with nearby cores. Our findings suggest that the RAID is suitable for both chemical and isotopic reconnaissance of drilling sites. Residual contamination of certain ions is discussed, with proposed design changes to avoid this issue with future use.

Accurate ice flow velocity data are essential for studying the mass balance of the Antarctic ice sheet. However, there is a lack of ice velocity maps of 1960s–80s in basin-wide regions or the entire ice sheet. In this study, an enhanced hierarchical network densification approach is developed for basin-wide Antarctic velocity mapping using historical ARGON and Landsat images. The produced multiple historical velocity maps from 1963 to 1989 in the region of the Fimbul and Jelbart ice shelves, East Antarctica, achieved an accuracy better than 29 m a−1. They revealed that the ice flow velocity had no significant changes over the period. Combining the surface mass balance estimate with the ice discharge estimated from our historical velocity maps and recently published velocity maps, we estimated a positive mass balance of 8.6 ± 3.9 Gt a−1 in the study area from 1963 and 2015. Our results indicate that the region's positive mass balance, as estimated in recently published studies, has been maintained since the 1960s. It is also in concordance with the low level of mass balance from 1992 to 2017 in East Antarctica. This suggests that the study area has been stable since the 1960s.

At any one time 130 000 icebergs are afloat in the Southern Ocean; 97% of these are too small to be registered in current satellite-based databases, yet the melting of these small icebergs provides a major input to the Southern Ocean. We use a unique set of visual size observations of 53 000 icebergs in the South Atlantic Ocean, the SCAR International Iceberg Database, to derive average iceberg dissolution rates. Fracture into two parts is the dominant dissolution process for tabular icebergs, with an average half-life of 30 days for icebergs <4 km length and 60 days for larger icebergs. Complete shatter producing many icebergs <1 km length is rare. A side attrition rate of 0.23 m d−1 combined with drift speed of 6 km d−1, or any proportional change in both numbers fits the observed changes in iceberg distribution. The largest injection into the Southern Ocean of fresh water and any iceberg-transported material takes place in a ~2.3 × 10⁶ km2 zone extending east-northeast from the Antarctic Peninsula to the Greenwich meridian. The iceberg contribution to salinities and temperatures, with maximum contribution north of the Weddell Sea, differs in some regions, from those indicated by tracking large icebergs. [ABSTRACT FROM AUTHOR]

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

Ice cliffs are features along ice sheet margins, along tropical mountain glaciers, at termini of mountain glaciers and on debris-covered glacier tongues, that have received scattered attention in literature. They cover small relative areas of glacier or margin surface respectively, but have been involved in two apparent anomalies. On the one hand, they have been identified as potential hotspots of extreme melt rates on debris-covered tongues contributing to their relatively rapid ablation, compared to the surrounding glacier surface. On the other hand, they appear where the ice margin is stable (or temporarily advancing) even under conditions of negative mass balance. In this manuscript, we recapitulate why ice cliffs remain interesting features to investigate and what we know about them so far. We conclude by suggesting to further investigate their genesis and variable morphology and their potential as windows into past climates and processes. [ABSTRACT FROM AUTHOR]

To increase inclusivity, diversity, equity and accessibility in Antarctic science, we must build more positive and inclusive Antarctic field work environments. The International Thwaites Glacier Collaboration (ITGC) has engaged in efforts to contribute to that goal through a variety of activities since 2018, including creating an open-access 'Field and Ship Best Practices' guide, engaging in pre-field season team dynamics meetings, and surveying post-field season reflections and experiences. We report specific actions taken by ITGC and their outcomes. We found that strong and supported early career researchers brought new and important perspectives regarding strategies for transforming culture. We discovered that engaged and involved senior leadership was also critical for expanding participation and securing funding to support efforts. Pre-field discussions involving all field team members were particularly helpful for setting expectations, improving sense of belonging, describing field work best practices, and co-creating a positive work culture. [ABSTRACT FROM AUTHOR]