scholarly journals Present-day high-resolution ice velocity map of the Antarctic ice sheet

2018 ◽  
Author(s):  
Qiang Shen ◽  
Hansheng Wang ◽  
C. K. Shum ◽  
Liming Jiang ◽  
Hou Tse Hsu ◽  
...  
Keyword(s):  
Sea Level ◽  
Displacement Vector ◽  
Ice Sheet ◽  
Landsat 8 ◽  
Ice Streams ◽  
Coupled Models ◽  
Ice Dynamics ◽  
Antarctic Ice Sheet ◽  
Ice Velocity ◽  
The Antarctic

Abstract. Ice velocity constitutes a key parameter for estimating ice-sheet discharge rates and is crucial for improving coupled models of the Antarctic ice sheet to accurately predict its future fate and contribution to sea-level change. Here, we present a new Antarctic ice velocity map at a 100-m grid spacing inferred from Landsat 8 imagery data collected from December 2013 through March 2016 and robustly processed using the feature tracking method. These maps were assembled from over 73,000 displacement vector scenes inferred from over 32,800 optical images. Our maps cover nearly all the ice shelves, landfast ice, ice streams, and most of the ice sheet. The maps have an estimated uncertainty of less than 10 m yr-1 based on robust internal and external validations. These datasets will allow for a comprehensive continent-wide investigation of ice dynamics and mass balance combined with the existing and future ice velocity measurements and provide researchers access to better information for monitoring local changes in ice glaciers. Other uses of these datasets include control and calibration of ice-sheet modelling, developments in our understanding of Antarctic ice-sheet evolution, and improvements in the fidelity of projects investigating sea-level rise (https://doi.pangaea.de/10.1594/PANGAEA.895738).

scholarly journals Changes in ice dynamics and mass balance of the Antarctic ice sheet

2006 ◽  
Vol 364 (1844) ◽  
pp. 1637-1655 ◽  
Author(s):  
Eric Rignot
Keyword(s):  
Mass Balance ◽  
Sea Level ◽  
Ice Sheet ◽  
East Antarctica ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Dynamics ◽  
Antarctic Ice Sheet ◽  
Ice Shelves ◽  
The Antarctic

The concept that the Antarctic ice sheet changes with eternal slowness has been challenged by recent observations from satellites. Pronounced regional warming in the Antarctic Peninsula triggered ice shelf collapse, which led to a 10-fold increase in glacier flow and rapid ice sheet retreat. This chain of events illustrated the vulnerability of ice shelves to climate warming and their buffering role on the mass balance of Antarctica. In West Antarctica, the Pine Island Bay sector is draining far more ice into the ocean than is stored upstream from snow accumulation. This sector could raise sea level by 1 m and trigger widespread retreat of ice in West Antarctica. Pine Island Glacier accelerated 38% since 1975, and most of the speed up took place over the last decade. Its neighbour Thwaites Glacier is widening up and may double its width when its weakened eastern ice shelf breaks up. Widespread acceleration in this sector may be caused by glacier ungrounding from ice shelf melting by an ocean that has recently warmed by 0.3 °C. In contrast, glaciers buffered from oceanic change by large ice shelves have only small contributions to sea level. In East Antarctica, many glaciers are close to a state of mass balance, but sectors grounded well below sea level, such as Cook Ice Shelf, Ninnis/Mertz, Frost and Totten glaciers, are thinning and losing mass. Hence, East Antarctica is not immune to changes.

scholarly journals The role of history and strength of the oceanic forcing in sea-level projections from Antarctica with the Parallel Ice Sheet Model

2020 ◽  
Author(s):  
Ronja Reese ◽  
Anders Levermann ◽  
Torsten Albrecht ◽  
Hélène Seroussi ◽  
Ricarda Winkelmann
Keyword(s):  
Mass Loss ◽  
Sea Level ◽  
Ice Sheet ◽  
Ocean Warming ◽  
Ice Streams ◽  
Antarctic Ice Sheet ◽  
Median Response ◽  
Ice Shelves ◽  
Order Of Magnitude ◽  
The Antarctic

<p>Mass loss from the Antarctic Ice Sheet constitutes the largest uncertainty in projections of future sea-level rise. Ocean-driven melting underneath the floating ice shelves and subsequent acceleration of the inland ice streams is the major reason for currently observed mass loss from Antarctica and is expected to become more important in the future. Here we show that for projections of future mass loss from the Antarctic Ice Sheet, it is essential (1) to better constrain the sensitivity of sub-shelf melt rates to ocean warming, and (2) to include the historic trajectory of the ice sheet. In particular, we find that while the ice-sheet response in simulations using the Parallel Ice Sheet Model is comparable to the median response of models in three Antarctic Ice Sheet Intercomparison projects – initMIP, LARMIP-2 and ISMIP6 – conducted with a range of ice-sheet models, the projected 21st century sea-level contribution differs significantly depending on these two factors. For the highest emission scenario RCP8.5, this leads to projected ice loss ranging from 1.4 to 4.3 cm of sea-level equivalent in the ISMIP6 simulations where the sub-shelf melt sensitivity is comparably low, opposed to a likely range of 9.2 to 35.9 cm using the exact same initial setup, but emulated from the LARMIP-2 experiments with a higher melt sensitivity based on oceanographic studies. Furthermore, using two initial states, one with and one without a previous historic simulation from 1850 to 2014, we show that while differences between the ice-sheet configurations in 2015 are marginal, the historic simulation increases the susceptibility of the ice sheet to ocean warming, thereby increasing mass loss from 2015 to 2100 by about 50%. Our results emphasize that the uncertainty that arises from the forcing is of the same order of magnitude as the ice-dynamic response for future sea-level projections.</p>

scholarly journals The role of history and strength of the oceanic forcing in sea-level projections from Antarctica with the Parallel Ice Sheet Model

2020 ◽  
Author(s):  
Ronja Reese ◽  
Anders Levermann ◽  
Torsten Albrecht ◽  
Hélène Seroussi ◽  
Ricarda Winkelmann
Keyword(s):  
Mass Loss ◽  
Sea Level ◽  
Ice Sheet ◽  
Ocean Warming ◽  
Ice Streams ◽  
Antarctic Ice Sheet ◽  
Median Response ◽  
Ice Shelves ◽  
Order Of Magnitude ◽  
The Antarctic

Abstract. Mass loss from the Antarctic Ice Sheet constitutes the largest uncertainty in projections of future sea-level rise. Ocean-driven melting underneath the floating ice shelves and subsequent acceleration of the inland ice streams is the major reason for currently observed mass loss from Antarctica and is expected to become more important in the future. Here we show that for projections of future mass loss from the Antarctic Ice Sheet, it is essential (1) to better constrain the sensitivity of sub-shelf melt rates to ocean warming, and (2) to include the historic trajectory of the ice sheet. In particular, we find that while the ice-sheet response in simulations using the Parallel Ice Sheet Model is comparable to the median response of models in three Antarctic Ice Sheet Intercomparison projects – initMIP, LARMIP-2 and ISMIP6 – conducted with a range of ice-sheet models, the projected 21st century sea-level contribution differs significantly depending on these two factors. For the highest emission scenario RCP8.5, this leads to projected ice loss ranging from 1.4 to 4.0 cm of sea-level equivalent in the ISMIP6 simulations where the sub-shelf melt sensitivity is comparably low, opposed to a likely range of 9.2 to 35.9 cm using the exact same initial setup, but emulated from the LARMIP-2 experiments with a higher melt sensitivity based on oceanographic studies. Furthermore, using two initial states, one with and one without a previous historic simulation from 1850 to 2014, we show that while differences between the ice-sheet configurations in 2015 are marginal, the historic simulation increases the susceptibility of the ice sheet to ocean warming, thereby increasing mass loss from 2015 to 2100 by about 50 %. Our results emphasize that the uncertainty that arises from the forcing is of the same order of magnitude as the ice-dynamic response for future sea-level projections.

Sliding conditions beneath the Antarctic Ice Sheet

2020 ◽  
Author(s):  
Rupert Gladstone ◽  
John Moore ◽  
Michael Wolovick ◽  
Thomas Zwinger
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Ice Streams ◽  
Basal Resistance ◽  
Sliding Speed ◽  
Antarctic Ice Sheet ◽  
Friction Heat ◽  
Hydrologic System ◽  
The Antarctic

<p>Computer models for ice sheet dynamics are the primary tools for making future predictions of ice sheet behaviour, the marine ice sheet instability, and ice sheet contributions to sea level rise. However, the dominant mode of flow for ice streams is sliding at the bed, and the physical processes that control sliding are not well understood. Ice sheet models often use hard-bed (often Weertman-type) sliding rules for computational efficiency.  However, soft beds with deformable sediments, which are known from laboratory experiments and direct glacier observations to exhibit Coulomb plastic behaviour, are ubiquitous beneath fast flowing ice streams. Using hard-bed sliding rules leads to actively misleading rates of inland surface diffusion and grounding line migration as compared to plastic beds, leading to incorrect forecasts of future sea level rise. Here, we use a 3D Stokes-flow ice sheet model along with observations of the Antarctic Ice Sheet to infer, through inversions and steady temperature simulations, key basal properties, most important of which are sliding speed, basal resistance, friction heat and grounded ice basal melt rate.  In addition to simulations of the whole Antarctic Ice Sheet we implement fine resolution simulations of the Pine Island Glacier and its catchment.  Contrary to the predictions of most hard-bed sliding relations, we find no correlation between basal resistance and sliding speed for fast moving ice streams. These results emphasize the importance of Coulomb plastic sliding, and strongly suggest that ice sheet modelers should devote greater efforts to developing models that can incorporate Coulomb plastic sliding relations without generating numerical instabilities.  We use our model results, along with some assumptions, to infer properties of the sub-glacial hydrologic system.  Assumptions about connectivity of the sub-glacial hydrologic system to the ocean limit our capacity to assess sliding relations that incorporate a dependence on effective pressure, and likely cause underestimates of ice sheet mass loss in model-based predictions utilising such sliding relations.  Hydrology modelling is likely essential both to further assess sliding relations and to use sliding relations in future predictions.  We estimate that the dominant source of basal meltwater for Pine Island Glacier is due to friction heat caused by basal sliding, despite recent estimates of high heating due to volcanic activity.</p><p> </p>

scholarly journals Glacial cycles simulation of the Antarctic Ice Sheet with PISM – Part 2: Parameter ensemble analysis

2019 ◽  
Author(s):  
Torsten Albrecht ◽  
Ricarda Winkelmann ◽  
Anders Levermann
Keyword(s):  
Sea Level ◽  
Ice Sheet ◽  
Model Parameters ◽  
Large Ensemble ◽  
Glacial Cycles ◽  
Relevant Model ◽  
Ice Dynamics ◽  
Antarctic Ice Sheet ◽  
The Antarctic ◽  
Ensemble Analysis

Abstract. The Parallel Ice Sheet Model (PISM) is applied to the Antarctic Ice Sheet over the last two glacial cycles (≈ 210,000 years) with a resolution of 16 km. A Large Ensemble of 256 model runs is analyzed in which four relevant model parameters have been systematically varied using full-factorial parameter sampling. Parameters and plausible parameter ranges have been identified in a companion paper (Albrecht et al., 2019) and are associated with ice dynamics, climatic forcing, basal sliding and bed deformation and represent distinct classes of model uncertainties. The model is calibrated against both modern and geologic data, including reconstructed grounding line locations, elevation-age data, ice thickness and surface velocities as well as uplift rates. An aggregated score is computed for each ensemble member that measures the overall model-data misfit, including measurement uncertainty in terms of a Gaussian error model (Briggs and Tarasov, 2013). The statistical method used to analyze the ensemble simulation results follows closely the simple averaging method described in Pollard et al. (2016). This analysis further constrains relevant model and boundary parameters by revealing clusters of best fit parameter combinations. The ensemble of reconstructed histories of Antarctic Ice Sheet volumes provides a score-weighted likely range of sea-level contributions since the Last Glacial Maximum of 9.4 ± 4.1 m (or 6.5 ± 2.0 × 106 km3), which is at the upper range of previous studies. The last deglaciation occurs in all ensemble simulations after around 12,000 years before present, and hence after the Meltwater Pulse-1A. Our Large Ensemble analysis also provides well-defined parametric uncertainty bounds and a probabilistic range of present-day states that can be used for PISM projections of future sea-level contributions from the Antarctic Ice Sheet.

scholarly journals Comparison of hybrid schemes for the combination of shallow approximations in numerical simulations of the Antarctic Ice Sheet

2017 ◽  
Vol 11 (1) ◽  
pp. 247-265 ◽  
Author(s):  
Jorge Bernales ◽  
Irina Rogozhina ◽  
Ralf Greve ◽  
Maik Thomas
Keyword(s):  
Ice Sheet ◽  
Force Balance ◽  
Iterative Technique ◽  
Ice Streams ◽  
Ice Dynamics ◽  
Antarctic Ice Sheet ◽  
Hybrid Schemes ◽  
Basal Sliding ◽  
Calibration Techniques ◽  
The Antarctic

Abstract. The shallow ice approximation (SIA) is commonly used in ice-sheet models to simplify the force balance equations within the ice. However, the SIA cannot adequately reproduce the dynamics of the fast flowing ice streams usually found at the margins of ice sheets. To overcome this limitation, recent studies have introduced heuristic hybrid combinations of the SIA and the shelfy stream approximation. Here, we implement four different hybrid schemes into a model of the Antarctic Ice Sheet in order to compare their performance under present-day conditions. For each scheme, the model is calibrated using an iterative technique to infer the spatial variability in basal sliding parameters. Model results are validated against topographic and velocity data. Our analysis shows that the iterative technique compensates for the differences between the schemes, producing similar ice-sheet configurations through quantitatively different results of the sliding coefficient calibration. Despite this we observe a robust agreement in the reconstructed patterns of basal sliding parameters. We exchange the calibrated sliding parameter distributions between the schemes to demonstrate that the results of the model calibration cannot be straightforwardly transferred to models based on different approximations of ice dynamics. However, easily adaptable calibration techniques for the potential distribution of basal sliding coefficients can be implemented into ice models to overcome such incompatibility, as shown in this study.

scholarly journals Acceleration of Dynamic Ice Loss in Antarctica From Satellite Gravimetry

2021 ◽  
Vol 9 ◽  
Author(s):  
Theresa Diener ◽  
Ingo Sasgen ◽  
Cécile Agosta ◽  
Johannes J. Fürst ◽  
Matthias H. Braun ◽  
...  
Keyword(s):  
Mass Loss ◽  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Snow Accumulation ◽  
Ice Dynamics ◽  
Antarctic Ice Sheet ◽  
Surface Ice ◽  
The Antarctic ◽  
Dynamic Discharge

The dynamic stability of the Antarctic Ice Sheet is one of the largest uncertainties in projections of future global sea-level rise. Essential for improving projections of the ice sheet evolution is the understanding of the ongoing trends and accelerations of mass loss in the context of ice dynamics. Here, we examine accelerations of mass change of the Antarctic Ice Sheet from 2002 to 2020 using data from the GRACE (Gravity Recovery and Climate Experiment; 2002–2017) and its follow-on GRACE-FO (2018-present) satellite missions. By subtracting estimates of net snow accumulation provided by re-analysis data and regional climate models from GRACE/GRACE-FO mass changes, we isolate variations in ice-dynamic discharge and compare them to direct measurements based on the remote sensing of the surface-ice velocity (2002–2017). We show that variations in the GRACE/GRACE-FO time series are modulated by variations in regional snow accumulation caused by large-scale atmospheric circulation. We show for the first time that, after removal of these surface effects, accelerations of ice-dynamic discharge from GRACE/GRACE-FO agree well with those independently derived from surface-ice velocities. For 2002–2020, we recover a discharge acceleration of -5.3 ± 2.2 Gt yr−2 for the entire ice sheet; these increasing losses originate mainly in the Amundsen and Bellingshausen Sea Embayment regions (68%), with additional significant contributions from Dronning Maud Land (18%) and the Filchner-Ronne Ice Shelf region (13%). Under the assumption that the recovered rates and accelerations of mass loss persisted independent of any external forcing, Antarctica would contribute 7.6 ± 2.9 cm to global mean sea-level rise by the year 2100, more than two times the amount of 2.9 ± 0.6 cm obtained by linear extrapolation of current GRACE/GRACE-FO mass loss trends.

IODP Exp. 374 provides clues into the Antarctic Ice Sheet contribution to sea level changes

2021 ◽  
Author(s):  
Laura De Santis ◽  
Denise Kulhanek ◽  
Robert McKay
Keyword(s):  
Continental Shelf ◽  
Sea Level ◽  
Ocean Circulation ◽  
Ross Sea ◽  
Ice Sheet ◽  
Outer Shelf ◽  
Ice Streams ◽  
Antarctic Ice Sheet ◽  
Global Sea Level ◽  
The Antarctic

<p>The five sites drilled during International Ocean Discovery Program (IODP) Expedition 374 recovered the distal geological component of a Neogene latitudinal and depth transect across the Ross Sea continental shelf, slope and rise, that can be combined with previous records of ANDRILL and the Deep Sea Drilling Project Leg 28. This transect provides clues into the ocean and atmospheric forcings on marine ice sheet instabilities and provides new direct constraints for reconstructing the Antarctic Ice Sheet contribution to global sea level change. Site U1521 recovered a middle Miocene record that allows identification of the different processes that lead to the expansion and retreat of ice streams emanating from the East and West Antarctic Ice Sheets across the Ross Sea continental shelf. This site also recovered a semi-continuous, expanded, high-resolution record of the Miocene Climatic Optimum in an ice-proximal location. Site U1522 recovered a Pleistocene to upper Miocene sequence from the outer shelf, dating the step-wise continental shelf–wide expansion and coalescing of marine-based ice streams from West Antarctica. Thin diatom-rich mudstone and diatomite beds were recovered in some intervals that provide snapshot records of a deglaciated outer shelf environment in the late Miocene. Site U1523 targeted a Miocene to Pleistocene sediment drift on the outermost continental shelf and informs about the changing vigor of the eastward flowing Antarctic Slope Current (ASC) through time. Changes in ASC vigor is a key control on regulating heat flux onto the continental shelf, making the ASC a key control on ice sheet mass balance. Sites U1524 and U1525 cored a continental rise levee system near the flank of the Hillary Canyon. The upper ~50 m at Site U1525 belong to a large trough-mouth fan deposited to the west of the site. The lower 100 m at Site U1525 and the entire 400 m succession of sediment at Site U1524 recovered near-continuous records of the downslope flow of Ross Sea Bottom Water and turbidity currents, but also of ASC vigor and iceberg discharge. Analyses of Exp. 374 sediments is ongoing, but following initial shipboard characterization, the intial results of sample analysis, the correlation between downhole synthetic logs and the associated seismic sections provide insight into the ages and the processes of erosion and deposition of glacial and marine strata. Exp. 374 sediments are providing key chronological constraints on the major Ross Sea seismic unconformities, enabling reconstruction of paleo-bathymetry and assessment of the geomorphological changes associated with Neogene ice sheet and ocean circulation changes. Exp. 374 results are fundamental for improving the boundary conditions of numerical ice sheet, ocean, and coupled climate models, which are critically required for understanding past ice sheet and global sea level response during warm climate intervals. Such data will enable more accurate predictions of ice sheet behavior and sea level rise anticipated with future warming. </p>

scholarly journals Glacial-cycle simulations of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM) – Part 2: Parameter ensemble analysis

2020 ◽  
Vol 14 (2) ◽  
pp. 633-656 ◽  
Author(s):  
Torsten Albrecht ◽  
Ricarda Winkelmann ◽  
Anders Levermann
Keyword(s):  
Sea Level ◽  
Ice Sheet ◽  
Model Parameters ◽  
Last Deglaciation ◽  
Relevant Model ◽  
Ice Dynamics ◽  
Antarctic Ice Sheet ◽  
The Antarctic ◽  
Best Fit ◽  
Ensemble Analysis

Abstract. The Parallel Ice Sheet Model (PISM) is applied to the Antarctic Ice Sheet over the last two glacial cycles (≈210 000 years) with a resolution of 16 km. An ensemble of 256 model runs is analyzed in which four relevant model parameters have been systematically varied using full-factorial parameter sampling. Parameters and plausible parameter ranges have been identified in a companion paper (Albrecht et al., 2020) and are associated with ice dynamics, climatic forcing, basal sliding and bed deformation and represent distinct classes of model uncertainties. The model is scored against both modern and geologic data, including reconstructed grounding-line locations, elevation–age data, ice thickness, surface velocities and uplift rates. An aggregated score is computed for each ensemble member that measures the overall model–data misfit, including measurement uncertainty in terms of a Gaussian error model (Briggs and Tarasov, 2013). The statistical method used to analyze the ensemble simulation results follows closely the simple averaging method described in Pollard et al. (2016). This analysis reveals clusters of best-fit parameter combinations, and hence a likely range of relevant model and boundary parameters, rather than individual best-fit parameters. The ensemble of reconstructed histories of Antarctic Ice Sheet volumes provides a score-weighted likely range of sea-level contributions since the Last Glacial Maximum (LGM) of 9.4±4.1 m (or 6.5±2.0×106km3), which is at the upper range of most previous studies. The last deglaciation occurs in all ensemble simulations after around 12 000 years before present and hence after the meltwater pulse 1A (MWP1a). Our ensemble analysis also provides an estimate of parametric uncertainty bounds for the present-day state that can be used for PISM projections of future sea-level contributions from the Antarctic Ice Sheet.

Antarctic ice sheet response to upper-bound scenarios

2021 ◽  
Author(s):  
Sainan Sun ◽  
Frank Pattyn
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Upper Bound ◽  
Ice Sheet ◽  
Climate Forcing ◽  
Ice Shelf ◽  
Antarctic Ice Sheet ◽  
Ice Shelves ◽  
Basal Sliding ◽  
The Antarctic

<p>Mass loss of the Antarctic ice sheet contributes the largest uncertainty of future sea-level rise projections. Ice-sheet model predictions are limited by uncertainties in climate forcing and poor understanding of processes such as ice viscosity. The Antarctic BUttressing Model Intercomparison Project (ABUMIP) has investigated the 'end-member' scenario, i.e., a total and sustained removal of buttressing from all Antarctic ice shelves, which can be regarded as the upper-bound physical possible, but implausible contribution of sea-level rise due to ice-shelf loss. In this study, we add successive layers of ‘realism’ to the ABUMIP scenario by considering sustained regional ice-shelf collapse and by introducing ice-shelf regrowth after collapse with the inclusion of ice-sheet and ice-shelf damage (Sun et al., 2017). Ice shelf regrowth has the ability to stabilize grounding lines, while ice shelf damage may reinforce ice loss. In combination with uncertainties from basal sliding and ice rheology, a more realistic physical upperbound to ice loss is sought. Results are compared in the light of other proposed mechanisms, such as MICI due to ice cliff collapse.</p>

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