scholarly journals Sliding dominates slow-flowing margin regions, Greenland Ice Sheet

2019 ◽  
Vol 5 (7) ◽  
pp. eaaw5406 ◽  
Author(s):  
Nathan Maier ◽  
Neil Humphrey ◽  
Joel Harper ◽  
Toby Meierbachtol
Keyword(s):  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ablation Zone ◽  
Ice Flow ◽  
Dimensional Array ◽  
Theoretical Maximum ◽  
Resistance To Sliding ◽  
Lower Elevation ◽  
Tilt Sensors

On the Greenland Ice Sheet (GrIS), ice flow due to deformation and sliding across the bed delivers ice to lower-elevation marginal regions where it can melt. We measured the two mechanisms of motion using a three-dimensional array of 212 tilt sensors installed within a network of boreholes drilled to the bed in the ablation zone of GrIS. Unexpectedly, sliding completely dominates ice motion all winter, despite a hard bedrock substrate and no concurrent surface meltwater forcing. Modeling constrained by detailed tilt observations made along the basal interface suggests that the high sliding is due to a slippery bed, where sparsely spaced bedrock bumps provide the limited resistance to sliding. The conditions at the site are characterized as typical of ice sheet margins; thus, most ice flow near the margins of GrIS is mainly from sliding, and marginal ice fluxes are near their theoretical maximum for observed surface speeds.

scholarly journals Englacial latent-heat transfer has limited influence on seaward ice flux in western Greenland

2016 ◽  
Vol 63 (237) ◽  
pp. 1-16 ◽  
Author(s):  
KRISTIN POINAR ◽  
IAN JOUGHIN ◽  
JAN T. M. LENAERTS ◽  
MICHIEL R. VAN DEN BROEKE
Keyword(s):  
Heat Transfer ◽  
Latent Heat ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Dynamic Evolution ◽  
Ablation Zone ◽  
Slow Movement ◽  
Limited Effect ◽  
Ice Flow ◽  
Numeric Model

ABSTRACTSurface meltwater can refreeze within firn layers and crevasses to warm ice through latent-heat transfer on decadal to millennial timescales. Earlier work posited that the consequent softening of the ice might accelerate ice flow, potentially increasing ice-sheet mass loss. Here, we calculate the effect of meltwater refreezing on ice temperature and softness in the Pâkitsoq (near Swiss Camp) and Jakobshavn Isbræ regions of western Greenland using a numeric model and existing borehole measurements. We show that in the Jakobshavn catchment, meltwater percolation within the firn warms the ice at depth by 3–5°C. By contrast, meltwater refreezing in crevasses (cryo-hydrologic warming) at depths of ~300 m warms the ice in Pâkitsoq by up to 10°C, but this causes minimal increase in ice motion (<10 m a−1). Pâkitsoq is representative of western Greenland's land-terminating ice, where the slow movement of ice through a wide ablation zone provides ideal conditions for cryo-hydrologic warming to occur. We find that only ~37% of the western Greenland ice flux, however, travels through such areas. Overall, our findings suggest that cryo-hydrologic warming will likely have only a limited effect on the dynamic evolution of the Greenland ice sheet.

scholarly journals Sensitivity of ice loss to uncertainty in flow law parameters in an idealized one-dimensional geometry

2020 ◽  
Vol 14 (10) ◽  
pp. 3537-3550
Author(s):  
Maria Zeitz ◽  
Anders Levermann ◽  
Ricarda Winkelmann
Keyword(s):  
Mass Loss ◽  
Flow Line ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Surface Mass ◽  
Scientific Debate ◽  
Flow Parameters ◽  
Flow Law

Abstract. Acceleration of the flow of ice drives mass losses in both the Antarctic and the Greenland Ice Sheet. The projections of possible future sea-level rise rely on numerical ice-sheet models, which solve the physics of ice flow, melt, and calving. While major advancements have been made by the ice-sheet modeling community in addressing several of the related uncertainties, the flow law, which is at the center of most process-based ice-sheet models, is not in the focus of the current scientific debate. However, recent studies show that the flow law parameters are highly uncertain and might be different from the widely accepted standard values. Here, we use an idealized flow-line setup to investigate how these uncertainties in the flow law translate into uncertainties in flow-driven mass loss. In order to disentangle the effect of future warming on the ice flow from other effects, we perform a suite of experiments with the Parallel Ice Sheet Model (PISM), deliberately excluding changes in the surface mass balance. We find that changes in the flow parameters within the observed range can lead up to a doubling of the flow-driven mass loss within the first centuries of warming, compared to standard parameters. The spread of ice loss due to the uncertainty in flow parameters is on the same order of magnitude as the increase in mass loss due to surface warming. While this study focuses on an idealized flow-line geometry, it is likely that this uncertainty carries over to realistic three-dimensional simulations of Greenland and Antarctica.

scholarly journals Generation and fate of basal meltwater during winter, western Greenland Ice Sheet

2021 ◽  
Vol 15 (12) ◽  
pp. 5409-5421
Author(s):  
Joel Harper ◽  
Toby Meierbachtol ◽  
Neil Humphrey ◽  
Jun Saito ◽  
Aidan Stansberry
Keyword(s):  
Sliding Friction ◽  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Winter Period ◽  
Ablation Zone ◽  
Ice Flow ◽  
Flow Acceleration ◽  
Basal Cavity ◽  
Surface Melt

Abstract. Basal sliding in the ablation zone of the Greenland Ice Sheet is closely associated with water from surface melt introduced to the bed in summer, yet melting of basal ice also generates subglacial water year-round. Assessments of basal melt rely on modeling with results strongly dependent upon assumptions with poor observational constraints. Here we use surface and borehole measurements to investigate the generation and fate of basal meltwater in the ablation zone of Isunnguata Sermia basin, western Greenland. The observational data are used to constrain estimates of the heat and water balances, providing insights into subglacial hydrology during the winter months when surface melt is minimal or nonexistent. Despite relatively slow ice flow speeds during winter, the basal meltwater generation from sliding friction remains manyfold greater than that due to geothermal heat flux. A steady acceleration of ice flow over the winter period at our borehole sites can cause the rate of basal water generation to increase by up to 20 %. Borehole measurements show high but steady basal water pressure rather than monotonically increasing pressure. Ice and groundwater sinks for water do not likely have sufficient capacity to accommodate the meltwater generated in winter. Analysis of basal cavity dynamics suggests that cavity opening associated with flow acceleration likely accommodates only a portion of the basal meltwater, implying that a residual is routed to the terminus through a poorly connected drainage system. A forcing from cavity expansion at high pressure may explain observations of winter acceleration in western Greenland.

scholarly journals Dynamic inland propagation of thinning due to ice loss at the margins of the Greenland ice sheet

2012 ◽  
Vol 58 (210) ◽  
pp. 734-740 ◽  
Author(s):  
Weili Wang ◽  
Jun Li ◽  
H. Jay Zwally
Keyword(s):  
Mass Loss ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Large Mass ◽  
Dynamic Impact ◽  
Ice Flow ◽  
Ice Cloud ◽  
Elevation Changes ◽  
Balance Analysis

AbstractMass-balance analysis of the Greenland ice sheet based on surface elevation changes observed by the European Remote-sensing Satellite (ERS) (1992-2002) and Ice, Cloud and land Elevation Satellite (ICESat) (2003-07) indicates that the strongly increased mass loss at lower elevations (<2000 m) of the ice sheet, as observed during 2003-07, appears to induce interior ice thinning at higher elevations. In this paper, we perform a perturbation experiment with a three-dimensional anisotropic ice-flow model (AIF model) to investigate this upstream propagation. Observed thinning rates in the regions below 2000 m elevation are used as perturbation inputs. The model runs with perturbation for 10 years show that the extensive mass loss at the ice-sheet margins does in fact cause interior thinning on short timescales (i.e. decadal). The modeled pattern of thinning over the ice sheet agrees with the observations, which implies that the strong mass loss since the early 2000s at low elevations has had a dynamic impact on the entire ice sheet. The modeling results also suggest that even if the large mass loss at the margins stopped, the interior ice sheet would continue thinning for 300 years and would take thousands of years for full dynamic recovery.

scholarly journals Sensitivity of ice flow to uncertainty in flow law parameters in an idealized one-dimensional geometry

2020 ◽  
Author(s):  
Maria Zeitz ◽  
Anders Levermann ◽  
Ricarda Winkelmann
Keyword(s):  
Mass Loss ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Flow Parameters ◽  
Surface Temperatures ◽  
Ice Sheet Modeling ◽  
Standard Values ◽  
Flow Law

Abstract. The flow of ice drives mass losses in both, the Antarctic and the Greenland Ice Sheet. The projections of possible future sea-level rise rely on numerical ice-sheet models, which solve the physics of ice flow and melt. While a number of important uncertainties have been addressed by the ice-sheet modeling community, the flow law, which is at the center of most process-based ice-sheet models, has so far been assumed certain. Unfortunately, recent studies show that the parameters in the flow law might be uncertain and different from the widely accepted standard values. Here, we use an idealized flowline setup to investigate how uncertainties in the flow law translate into uncertainties in flow-driven mass loss given a step-wise increase of surface temperatures. We find that the measured range of flow parameters can double the flow-driven mass loss within the first centuries of warming, compared to a setting with standard parameters. The spread of ice loss due to an uncertainty in flow parameters is of the same order as the increase in mass loss due to increasing surface temperatures. While this study focuses on an idealized setting in order to disentangle the effect of the flow law from other effects, it is likely that this uncertainty carries over to realistic three-dimensional simulations of Greenland and Antarctica.

scholarly journals Variability of Basal Meltwater Generation During Winter, Western Greenland Ice Sheet

2021 ◽  
Author(s):  
Joel Harper ◽  
Toby Meierbachtol ◽  
Neil Humphrey ◽  
Jun Saito ◽  
Aidan Stansberry
Keyword(s):  
Sliding Friction ◽  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Winter Period ◽  
Ablation Zone ◽  
Ice Flow ◽  
Flow Acceleration ◽  
Basal Cavity ◽  
Surface Melt

Abstract. Basal sliding in the ablation zone of the Greenland Ice Sheet is closely associated with water from surface melt introduced to the bed in summer, yet melting of basal ice also generates subglacial water year-round. Assessments of basal melt rely on modelling with results strongly dependent upon assumptions with poor observational constraint. Here we use surface and borehole measurements to investigate the generation and fate of basal meltwater in the ablation zone of Isunnguata Sermia basin, Western Greenland. The observational data are used to constrain estimates of the heat and water balances, providing insights into subglacial hydrology during the winter months when surface melt is minimal or non-existent. Despite relatively slow ice flow speeds during winter, the basal meltwater generation from sliding friction remains many fold greater than that due to geothermal heat flux. A steady acceleration of ice flow over the winter period at our borehole sites can cause the rate of basal water generation to increase by up to 20 %. Borehole measurements show high but steady basal water pressure, rather than monotonically increasing pressure. Ice and groundwater sinks for water do not likely have sufficient capacity to accommodate the meltwater generated in winter. Analysis of basal cavity dynamics suggests that cavity opening associated with flow acceleration likely accommodates only a portion of the basal meltwater, implying a residual is routed to the terminus through a poorly connected drainage system. A forcing from cavity expansion at high pressure may explain observations of winter acceleration in Western Greenland.

scholarly journals Thermomechanical modelling of the Scandinavian ice sheet: implications for ice-stream formation

1999 ◽  
Vol 28 ◽  
pp. 83-89 ◽  
Author(s):  
A. J. Payne ◽  
D.J. Baldwin
Keyword(s):  
Three Dimensional ◽  
Ice Sheet ◽  
Ice Streams ◽  
Ice Flow ◽  
Low Viscosity ◽  
Ice Stream ◽  
Scandinavian Ice Sheet ◽  
The Baltic ◽  
Stream Formation ◽  
Initial Results

AbstractThis work attempts to explain the fan-like landform assemblages observed in satellite images of the area covered by the former Scandinavian ice sheet (SIS). These assemblages have been interpreted as evidence of large ice streams within the SIS. If this interpretation is correct, then it calls into doubt current theories on the formation of ice streams. These theories regard soft sediment and topographic troughs as being the key determinants of ice-stream location. Neither can be used to explain the existence of ice streams on the flat, hard-rock area of the Baltic Shield. Initial results from a three-dimensional, thermomechanical ice-sheet model indicate that interactions between ice flow, form and temperature can create patterns similar to those mentioned above. The model uses a realistic, 20 km resolution gridded topography and a simple parameterization of accumulation and ablation. It produces patterns of maximum ice-sheet extent, which are similar to those reconstructed from the area’s glacial geomorphology. Flow in the maximum, equilibrium ice sheet is dominated by wedges of warm, low-viscosity, fast-flowing ice. These are separated by areas of cold, slow-flowing ice. This patterning appears to develop spontaneously as the modelled ice sheet grows.

scholarly journals Sensitivity of Greenland Ice Sheet Projections to Model Formulations

2013 ◽  
Vol 59 (216) ◽  
pp. 733-749 ◽  
Author(s):  
H. Goelzer ◽  
P. Huybrechts ◽  
J.J. Fürst ◽  
F.M. Nick ◽  
M.L. Andersen ◽  
...  
Keyword(s):  
Regional Climate Model ◽  
Sea Level ◽  
Flow Model ◽  
Climate Model ◽  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Outlet Glacier ◽  
Glacier Dynamics

AbstractPhysically based projections of the Greenland ice sheet contribution to future sea-level change are subject to uncertainties of the atmospheric and oceanic climatic forcing and to the formulations within the ice flow model itself. Here a higher-order, three-dimensional thermomechanical ice flow model is used, initialized to the present-day geometry. The forcing comes from a high-resolution regional climate model and from a flowline model applied to four individual marine-terminated glaciers, and results are subsequently extended to the entire ice sheet. The experiments span the next 200 years and consider climate scenario SRES A1B. The surface mass-balance (SMB) scheme is taken either from a regional climate model or from a positive-degree-day (PDD) model using temperature and precipitation anomalies from the underlying climate models. Our model results show that outlet glacier dynamics only account for 6–18% of the sea-level contribution after 200 years, confirming earlier findings that stress the dominant effect of SMB changes. Furthermore, interaction between SMB and ice discharge limits the importance of outlet glacier dynamics with increasing atmospheric forcing. Forcing from the regional climate model produces a 14–31 % higher sea-level contribution compared to a PDD model run with the same parameters as for IPCC AR4.

scholarly journals A Three-Dimensional Polar Ice-Sheet Model

1977 ◽  
Vol 18 (80) ◽  
pp. 373-389 ◽  
Author(s):  
D. Jenssen
Keyword(s):  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Phase Changes ◽  
Three Dimensions ◽  
Dimensional Model ◽  
Polar Ice ◽  
Three Dimensional Model ◽  
Temperature Dependent ◽  
Mass Depletion

AbstractA three-dimensional model of the temperature and velocity distribution within any arbitrary-shaped ice mass is described. There is a mutual interaction in the model between the flow of the ice and its thermodynamics, since the flow law used in the model is temperature-dependent.Ice growth in three dimensions is governed by mass accumulation through precipitation, by mass depletion through loss of ice over the ocean, and by continuity requirements. Phase changes at the base of the ice are accounted for. The model has been applied in art exploratory manner to the Greenland ice sheet. Changes in the ice shape and temperature are presented and discussed. The basic shortcoming of the model as here presented appears primarily due to the coarse finite-difference mesh used, and to an unsophisticated approach to modelling the boundary ice.

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