Nunataks as barriers to ice flow: implications for palaeo ice-sheet reconstructions

Abstract. Numerical models predict that discharge from the polar ice sheets will become the largest contributor to sea level rise over the coming centuries. However, the predicted amount of ice discharge and associated thinning depends on how well ice sheet models reproduce glaciological processes, such as ice flow in regions of large topographic relief, where ice flows around bedrock summits (i.e. nunataks) and through outlet glaciers. The ability of ice sheet models to capture long-term ice loss is best tested by comparing model simulations against geological data. A benchmark for such models is ice surface elevation change, which has been constrained empirically at nunataks and along margins of outlet glaciers using cosmogenic exposure dating. However, the usefulness of this approach in quantifying ice sheet thinning relies on how well such records represent changes in regional ice surface elevation. Here we examine how ice surface elevations respond to the presence of obstacles that create large topographic relief by modeling ice flow around and between idealised nunataks during periods of imposed ice sheet thinning. We found that, for realistic Antarctic conditions, a single nunatak could exert an impact on ice thickness over 20 km away from its summit, with its most prominent effect being a local increase (decrease) of the ice surface elevation of hundreds of metres upstream (downstream) of the obstacle. A direct consequence of this differential surface response for cosmogenic exposure dating was a delay in the time of bedrock exposure upstream relative to downstream of a nunatak. A nunatak elongated transverse to ice flow, with a wide subglacial continuation, was able to increase ice retention and therefore impose steeper ice surface gradients, while efficient ice drainage through outlet glaciers alleviated the differential response. Such differences, however, are not typically captured by continent-wide ice sheet models due to their coarse grid resolutions. This appears to be a key reason why models overestimate ice-sheet surface elevations and underestimate the pace of ice sheet melt contributing to sea level rise compared to empirical reconstructions. We conclude that a model grid refinement over complex topography and information about sample position relative to ice flow near the nunatak are necessary to improve data-model comparisons of ice surface elevation, and therefore the ability of models to simulate ice discharge in regions of large topographic relief.

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History, mass loss, structure, and dynamic behavior of the Antarctic Ice Sheet

Science ◽

10.1126/science.aaz5489 ◽

2020 ◽

Vol 367 (6484) ◽

pp. 1321-1325 ◽

Cited By ~ 1

Author(s):

Robin E. Bell ◽

Helene Seroussi

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Satellite Measurements ◽

Ice Flow ◽

Antarctic Ice Sheet ◽

West Antarctic Ice Sheet ◽

Mountain Ranges ◽

West Antarctic ◽

The Antarctic

Antarctica contains most of Earth’s fresh water stored in two large ice sheets. The more stable East Antarctic Ice Sheet is larger and older, rests on higher topography, and hides entire mountain ranges and ancient lakes. The less stable West Antarctic Ice Sheet is smaller and younger and was formed on what was once a shallow sea. Recent observations made with several independent satellite measurements demonstrate that several regions of Antarctica are losing mass, flowing faster, and retreating where ice is exposed to warm ocean waters. The Antarctic contribution to sea level rise has reached ~8 millimeters since 1992. In the future, if warming ocean waters and increased surface meltwater trigger faster ice flow, sea level rise will accelerate.

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Greenland ice sheet contribution to sea-level rise from a new-generation ice-sheet model

The Cryosphere ◽

10.5194/tc-6-1561-2012 ◽

2012 ◽

Vol 6 (6) ◽

pp. 1561-1576 ◽

Cited By ~ 150

Author(s):

F. Gillet-Chaulet ◽

O. Gagliardini ◽

H. Seddik ◽

M. Nodet ◽

G. Durand ◽

...

Keyword(s):

Mass Balance ◽

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Mass Balance ◽

Ice Flow ◽

Surface Mass ◽

Continental Scale ◽

New Generation

Abstract. Over the last two decades, the Greenland ice sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise (SLR). The recent increases in ice loss appear to be due to changes in both the surface mass balance of the ice sheet and ice discharge (ice flux to the ocean). Rapid ice flow directly affects the discharge, but also alters ice-sheet geometry and so affects climate and surface mass balance. Present-day ice-sheet models only represent rapid ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic ice-sheet model which reproduces the current patterns of rapid ice flow. This requires three essential developments: the complete solution of the full system of equations governing ice deformation; a variable resolution unstructured mesh to resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled ice discharge is in good agreement with observations on the continental scale and for individual outlets. From this initial state, we investigate possible bounds for the next century ice-sheet mass loss. We run sensitivity experiments of the GrIS dynamical response to perturbations in climate and basal lubrication, assuming a fixed position of the marine termini. We find that increasing ablation tends to reduce outflow and thus decreases the ice-sheet imbalance. In our experiments, the GrIS initial mass (im)balance is preserved throughout the whole century in the absence of reinforced forcing, allowing us to estimate a lower bound of 75 mm for the GrIS contribution to SLR by 2100. In one experiment, we show that the current increase in the rate of ice loss can be reproduced and maintained throughout the whole century. However, this requires a very unlikely perturbation of basal lubrication. From this result we are able to estimate an upper bound of 140 mm from dynamics only for the GrIS contribution to SLR by 2100.

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Sensitivity of Greenland ice sheet projections to spatial resolution in higher-order simulations: the AWI contribution to ISMIP6-Greenland using ISSM

10.5194/tc-2019-329 ◽

2020 ◽

Cited By ~ 1

Author(s):

Martin Rückamp ◽

Heiko Goelzer ◽

Angelika Humbert

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Higher Order ◽

Initial State ◽

Ice Flow ◽

Outlet Glacier ◽

Flow Models ◽

High Emission

Abstract. Projections of the contribution of the Greenland ice sheet to future sea-level rise include uncertainties primarily due to the imposed climate forcing and the initial state of the ice sheet model. Several state-of-the-art ice flow models are currently being employed on various grid resolutions to estimate future mass changes in the framework of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). Here we investigate the sensitivity to grid resolution on centennial sea-level contributions from the Greenland ice sheet and study the mechanism at play. To this end, we employ the finite-element higher-order ice flow model ISSM and conduct experiments with four different horizontal resolutions, namely 4, 2, 1 and 0.75 km. We run the simulation based on the ISMIP6 core GCM MIROC5 under the high emission scenario RCP8.5 and consider both atmospheric and oceanic forcing in full and separate scenarios. Under the full scenarios, finer simulations unveil up to ~5 % more sea-level rise compared to the coarser resolution. The sensitivity depends on the magnitude of outlet glacier retreat, which is implemented as a series of retreat masks following the ISMIP6 protocol. Without imposed retreat under atmosphere-only forcing, the resolution dependency exhibits an opposite behaviour with about ~ 5 % more sea-level contribution in the coarser resolution. The sea-level contribution indicates a converging behaviour

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Sensitivity of Greenland ice sheet projections to spatial resolution in higher-order simulations: the AWI contribution to ISMIP6-Greenland using ISSM

10.5194/egusphere-egu2020-14565 ◽

2020 ◽

Author(s):

Martin Rückamp ◽

Heiko Goelzer ◽

Thomas Kleiner ◽

Angelika Humbert

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Higher Order ◽

Glacier Retreat ◽

Driving Mechanism ◽

Initial State ◽

Ice Flow ◽

Outlet Glacier

Projections of the contribution of the Greenland ice sheet to future sea-level rise include uncertainties primarily due to the imposed climate forcing and the initial state of the ice sheet model. Several state-of-the-art ice flow models are currently being employed on various grid resolutions to estimate future mass changes in the framework of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). Here we investigate the sensitivity to grid resolution on centennial sea-level contributions from the Greenland ice sheet and study the mechanism at play. To this end, we employ the finite-element higher-order ice flow model ISSM and conduct experiments with four different horizontal resolutions, namely 4, 2, 1 and 0.75 km. We run the simulation based on the ISMIP6 core GCM MIROC5 under the high emission scenario RCP8.5 and consider both atmospheric and oceanic forcing in full and separate scenarios. Under the full scenarios, finer simulations unveil up to 5% more sea-level rise compared to the coarser resolution. The sensitivity depends on the magnitude of outlet glacier retreat, which is implemented as a series of retreat masks following the ISMIP6 protocol. Without imposed retreat under atmosphere-only forcing, the resolution dependency exhibits an opposite behaviour with about 5% more sea-level contribution in the coarser resolution. The sea-level contribution indicates a converging behaviour &#8804; 1 km horizontal resolution. A driving mechanism for differences is the ability to resolve the bed topography, which highly controls ice discharge to the ocean. Additionally, thinning and acceleration emerge to propagate further inland in high resolution for many glaciers. A major response mechanism is sliding (despite no climate-induced hydrological feedback is invoked), with an enhanced feedback on the effective normal pressure N at higher resolution leading to a larger increase in sliding speeds under scenarios with outlet glacier retreat.

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Greenland Ice Sheet contribution to sea-level rise from a new-generation ice-sheet model

The Cryosphere Discussions ◽

10.5194/tcd-6-2789-2012 ◽

2012 ◽

Vol 6 (4) ◽

pp. 2789-2826 ◽

Cited By ~ 3

Author(s):

F. Gillet-Chaulet ◽

O. Gagliardini ◽

H. Seddik ◽

M. Nodet ◽

G. Durand ◽

...

Keyword(s):

Mass Balance ◽

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Mass Balance ◽

Ice Flow ◽

Surface Mass ◽

Continental Scale ◽

New Generation

Abstract. Over the last two decades, the Greenland Ice Sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise. The recent increases in ice loss appear to be due to changes in both the surface mass balance of the ice sheet and ice discharge (ice flux to the ocean). Rapid ice flow directly affects the discharge, but also alters ice-sheet geometry and so affects climate and surface mass balance. The most usual ice-sheet models only represent rapid ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic ice-sheet model which reproduces the current patterns of rapid ice flow. This requires three essential developments: the complete solution of the full system of equations governing ice deformation; an unstructured mesh to usefully resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled ice discharge is in good agreement with observations on the continental scale and for individual outlets. By conducting perturbation experiments, we investigate how current ice loss will endure over the next century. Although we find that increasing ablation tends to reduce outflow and on its own has a stabilising effect, if destabilisation processes maintain themselves over time, current increases in the rate of ice loss are likely to continue.

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Determining the contribution of Antarctica to sea-level rise using data assimilation methods

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2006.1801 ◽

2006 ◽

Vol 364 (1844) ◽

pp. 1841-1865 ◽

Cited By ~ 10

Author(s):

Robert J Arthern ◽

Richard C.A Hindmarsh

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Weather Prediction ◽

Ice Sheet ◽

Snow Accumulation ◽

Ice Flow ◽

Thickness Change ◽

Using Data ◽

Linearized Model ◽

The Antarctic

The problem of forecasting the future behaviour of the Antarctic ice sheet is considered. We describe a method for optimizing this forecast by combining a model of ice sheet flow with observations. Under certain assumptions, a linearized model of glacial flow can be combined with observations of the thickness change, snow accumulation, and ice-flow, to forecast the Antarctic contribution to sea-level rise. Numerical simulations show that this approach can potentially be used to test whether changes observed in Antarctica are consistent with the natural forcing of a stable ice sheet by snowfall fluctuations. To make predictions under less restrictive assumptions, improvements in models of ice flow are needed. Some of the challenges that this prediction problem poses are highlighted, and potentially useful approaches drawn from numerical weather prediction are discussed.

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Revised chronology of northwest Laurentide ice-sheet deglaciation from beryllium-10 exposure-dated erratics on the western Canadian Shield

10.5194/egusphere-egu2020-13198 ◽

2020 ◽

Author(s):

Alberto Reyes ◽

Anders Carlson ◽

Jesse Reimink

Keyword(s):

Climate Change ◽

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Canadian Shield ◽

Laurentide Ice Sheet ◽

Exposure Dating ◽

Poor Region ◽

A Site ◽

Exposure Ages

The timing of northwest Laurentide ice-sheet deglaciation is important for understanding how ice-sheet retreat, and associated meltwater discharge, may have been involved in abrupt climate change and rapid sea-level rise at the end of the last glaciation. However, the deglacial chronology across the western Canadian Shield is poorly understood, with only a handful of minimum-limiting 14C dates and sparse cosmogenic nuclide exposure dates constraining the timing and pattern of northwest Laurentide ice-sheet retreat across >1000 km of ice-sheet retreat to the marine limit west of Hudson Bay. We present cosmogenic 10Be surface exposure dating of glacial erratics at two sites, within a ~160,000 km2 region with no reliable temporal constraints on ice-margin retreat, to directly date the timing of northwest Laurentide ice-sheet deglaciation. Six erratics perched directly on bedrock at a site on the western edge of the Slave Craton have exposure ages between 12.8&#177;0.6 and 12.2&#177;0.6 thousand years ago (ka;&#160;&#177;1sigma). Five erratics on bedrock, sampled at a site 115 km up-ice to the east, yielded exposure ages between 10.8&#177;0.5 and 11.6&#177;0.5 ka. When corrected for decreased atmospheric depth due to isostatic uplift since deglaciation, the error-weighted mean ages for the two sites indicate that the Laurentide ice sheet retreated through this region of the western Canadian Shield between 13.3&#177;0.2 and 11.8&#177;0.2 ka, or at least 1 kyr earlier than inferred from the canonical compilation of minimum-limiting 14C dates for deglaciation and paleo-glaciological models. We tentatively infer a preliminary ice-margin retreat rate of ~0.1 m kyr-1&#160;over this interval spanning much of the Younger Dryas which, compared to earlier estimates, implies a substantially lower volume of meltwater flux from the retreating northwest Laurentide ice sheet at this time. &#160;Additional exposure ages on glacial erratics across this data-poor region are needed for validation of existing deglacial ice-sheet models, which can in turn contribute to comprehensive testing of hypotheses related to northwest Laurentide ice-sheet retreat rate, abrupt deglacial sea-level rise, and potential forcing of associated climate change events.

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Ice-dynamic projections of the Greenland ice sheet in response to atmospheric and oceanic warming

The Cryosphere Discussions ◽

10.5194/tcd-8-3851-2014 ◽

2014 ◽

Vol 8 (4) ◽

pp. 3851-3905 ◽

Cited By ~ 2

Author(s):

J. J. Fürst ◽

H. Goelzer ◽

P. Huybrechts

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Flow Model ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Melting ◽

Strong Impact ◽

Ice Dynamics ◽

Ice Flow ◽

Over Time

Abstract. Continuing global warming will have a strong impact on the Greenland ice sheet in the coming centuries. During the last decade, both increased surface melting and enhanced ice discharge from calving glaciers have contributed 0.6 ± 0.1 mm yr−1 to global sea-level rise, roughly in shares of respectively 60 and 40 per cent. Here we use a higher-order ice flow model, initialised to the present state, to simulate future ice volume changes driven by both atmospheric and oceanic temperature changes. For these projections, the ice flow model accounts for runoff-induced basal lubrication and ocean warming-induced discharge increase at the marine margins. For a suite of ten Atmosphere and Ocean General Circulation Models and four Representative Concentration Pathway scenarios, the projected sea-level rise lies in the range of +1.4 to +16.6 cm by the year 2100. For two low emission scenarios, the projections are conducted up to 2300. Ice loss rates are found to either abate when the warming already peaks in this century, allowing to preserve the ice sheet in a geometry close to the present-day state, or to remain at a constant level over three hundred years under moderate warming. The volume loss is predominantly caused by increased surface melting as the contribution from enhanced ice discharge decreases over time and is self-limited by thinning and retreat of the marine margin reducing the ice–ocean contact area. The effect of enhanced basal lubrication on the volume evolution is found to be negligible on centennial time scales. The presented projections show that the observed rates of volume change over the last decades cannot simply be extrapolated over the 21st century on account of a different balance of processes causing ice loss over time. The results also indicate that the largest source of uncertainty arises from the surface mass balance and the underlying climate change projections, and not from ice dynamics.

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Cordilleran Ice Sheet mass loss preceded climate reversals near the Pleistocene Termination

Science ◽

10.1126/science.aan3001 ◽

2017 ◽

Vol 358 (6364) ◽

pp. 781-784 ◽

Cited By ~ 26

Author(s):

B. Menounos ◽

B. M. Goehring ◽

G. Osborn ◽

M. Margold ◽

B. Ward ◽

...

Keyword(s):

Mass Loss ◽

Sea Level Rise ◽

Sea Level ◽

Numerical Models ◽

Ice Sheet ◽

Younger Dryas ◽

Climate Dynamics ◽

Southern Margin ◽

The Pacific ◽

Cordilleran Ice Sheet

The Cordilleran Ice Sheet (CIS) once covered an area comparable to that of Greenland. Previous geologic evidence and numerical models indicate that the ice sheet covered much of westernmost Canada as late as 12.5 thousand years ago (ka). New data indicate that substantial areas throughout westernmost Canada were ice free prior to 12.5 ka and some as early as 14.0 ka, with implications for climate dynamics and the timing of meltwater discharge to the Pacific and Arctic oceans. Early Bølling-Allerød warmth halved the mass of the CIS in as little as 500 years, causing 2.5 to 3.0 meters of sea-level rise. Dozens of cirque and valley glaciers, along with the southern margin of the CIS, advanced into recently deglaciated regions during the Bølling-Allerød and Younger Dryas.

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Eemian Greenland ice sheet simulated with a higher-order model shows strong sensitivity to surface mass balance forcing

The Cryosphere ◽

10.5194/tc-13-2133-2019 ◽

2019 ◽

Vol 13 (8) ◽

pp. 2133-2148

Author(s):

Andreas Plach ◽

Kerim H. Nisancioglu ◽

Petra M. Langebroek ◽

Andreas Born ◽

Sébastien Le clec'h

Keyword(s):

Mass Balance ◽

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Higher Order ◽

Interglacial Period ◽

Surface Mass Balance ◽

Ice Flow ◽

Surface Mass

Abstract. The Greenland ice sheet contributes increasingly to global sea level rise. Its history during past warm intervals is a valuable reference for future sea level projections. We present ice sheet simulations for the Eemian interglacial period (∼130 000 to 115 000 years ago), a period with warmer-than-present summer climate over Greenland. The evolution of the Eemian Greenland ice sheet is simulated with a 3-D higher-order ice sheet model, forced with a surface mass balance derived from regional climate simulations. Sensitivity experiments with various surface mass balances, basal friction, and ice flow approximations are discussed. The surface mass balance forcing is identified as the controlling factor setting the minimum in Eemian ice volume, emphasizing the importance of a reliable surface mass balance model. Furthermore, the results indicate that the surface mass balance forcing is more important than the representation of ice flow for simulating the large-scale ice sheet evolution. This implies that modeling of the future contribution of the Greenland ice sheet to sea level rise highly depends on an accurate surface mass balance.

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