275 research outputs found
Future Antarctic bed topography and its implications for ice sheet dynamics
The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45 mm yr−1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future
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Representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics: Application to Brunt/Stancomb-Wills Ice Shelf, Antarctica
Ice shelves play a major role in buttressing ice sheet flow into the ocean, hence the importance of accurate numerical modeling of their stress regime. Commonly used ice flow models assume a continuous medium and are therefore complicated by the presence of rupture features (crevasses, rifts, and faults) that significantly affect the overall flow patterns. Here we apply contact mechanics and penalty methods to develop a new ice shelf flow model that captures the impact of rifts and faults on the rheology and stress distribution of ice shelves. The model achieves a best fit solution to satellite observations of ice shelf velocities to infer the following: (1) a spatial distribution of contact and friction points along detected faults and rifts, (2) a more realistic spatial pattern of ice shelf rheology, and (3) a better representation of the stress balance in the immediate vicinity of faults and rifts. Thus, applying the model to the Brunt/Stancomb-Wills Ice Shelf, Antarctica, we quantify the state of friction inside faults and the opening rates of rifts and obtain an ice shelf rheology that remains relatively constant everywhere else on the ice shelf. We further demonstrate that better stress representation has widespread application in examining aspects affecting ice shelf structure and dynamics including the extent of ice mélange in rifts and the change in fracture configurations. All are major applications for better insight into the important question of ice shelf stability
Extensive winter subglacial water storage beneath the Greenland Ice Sheet
This is the final version of the article. Available from AGU via the DOI in this record.Surface meltwater that reaches the base of the Greenland Ice Sheet exerts a fundamental impact on ice flow, but observations of catchment-wide movement and distribution of subglacial water remain limited. Using radar-sounding data from two seasons, we identify the seasonal distribution of subglacial water in western Greenland. Our analysis provides evidence of widespread subglacial water storage beneath Greenland in the wintertime. The winter storage is located primarily on bedrock ridges with higher bed elevations in excess of 200 m. During the melt season water moves to the subglacial troughs. This inverse relationship with topography indicates that the material properties of the glacier bed strongly influence subglacial drainage development. Both the spatial variations in bed properties and the initial state of the subglacial hydrology system at the start of the melt season lead to differing glacier dynamical responses to surface melting across the Greenland Ice Sheet.W.C. is a recipient of the NASA Earth and
Space Science Fellowship. D.M.S. is
supported by a grant from the NASA
Cryospheric Sciences Program. H.S. is
supported by grants from the NASA
Cryospheric Sciences and Sea Level Rise
Programs. T.T.C and R.E.B are supported
by grants from National Science
Foundation (NSF) and NASA
Cryospheric Sciences. S.P. is supported
by the Natural Environment Research
Council’s Centre for Polar Observatio
High-resolution ice-thickness mapping in South Greenland
Airborne radar sounding is difficult in South Greenland because of the presence of englacial water, which prevents the signal from reaching the bed. Data coverage remains suboptimal for traditional methods of ice-thickness and bed mapping that rely on geostatistical techniques, such as kriging, because important features are missing. Here we apply two alternative approaches of highresolution (̃300 m) ice-thickness mapping, that are based on the conservation of mass, to two regions of South Greenland: (1) Qooqqup Sermia and Kiattuut Sermiat, and (2) Ikertivaq. These two algorithms solve optimization problems, for which the conservation of mass is either enforced as a hard constraint, or as a soft constraint. For the first region, very few measurements are available but there is no gap in ice motion data, whereas for Ikertivaq, more ice-thickness measurements are available, but there are gaps in ice motion data. We show that mass-conservation algorithms can be used as validation tools for radar sounding. We also show that it is preferable to apply mass conservation as a hard constraint, rather than a soft constraint, as it better preserves elongated features, such as glacial valleys and ridges
Retreat of Thwaites Glacier, West Antarctica, over the next 100 years using various ice flow models, ice shelf melt scenarios and basal friction laws
Thwaites Glacier (TG), West Antarctica, has experienced rapid,
potentially irreversible grounding line retreat and mass loss in response to
enhanced ice shelf melting. Results from recent numerical models suggest a
large spread in the evolution of the glacier in the coming decades to a
century. It is therefore important to investigate how different
approximations of the ice stress balance, parameterizations of basal
friction and ice shelf melt parameterizations may affect projections. Here,
we simulate the evolution of TG using ice sheet models of varying levels of
complexity, different basal friction laws and ice shelf melt to quantify
their effect on the projections. We find that the grounding line retreat and
its sensitivity to ice shelf melt are enhanced when a full-Stokes model is
used, a Budd friction is used and ice shelf melt is applied on partially
floating elements. Initial conditions also impact the model results. Yet, all
simulations suggest a rapid, sustained retreat of the glacier along the same
preferred pathway. The fastest retreat rate occurs on the eastern side of the
glacier, and the slowest retreat occurs across a subglacial ridge on the
western side. All the simulations indicate that TG will undergo an
accelerated retreat once the glacier retreats past the western subglacial
ridge. Combining all the simulations, we find that the uncertainty of the
projections is small in the first 30 years, with a cumulative contribution to
sea level rise of 5 mm, similar to the current rate. After 30 years, the
contribution to sea level depends on the model configurations, with
differences up to 300 % over the next 100 years, ranging from 14 to 42 mm.</p
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Ice Sheet Model Intercomparison Project (ISMIP6) contribution to CMIP6
Reducing the uncertainty in the past, present, and future contribution of ice sheets to sea-level change requires a coordinated effort between the climate and glaciology communities. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) is the primary activity within the Coupled Model Intercomparison Project – phase 6 (CMIP6) focusing on the Greenland and Antarctic ice sheets. In this paper, we describe the framework for ISMIP6 and its relationship with other activities within CMIP6. The ISMIP6 experimental design relies on CMIP6 climate models and includes, for the first time within CMIP, coupled ice-sheet–climate models as well as standalone ice-sheet models. To facilitate analysis of the multi-model ensemble and to generate a set of standard climate inputs for standalone ice-sheet models, ISMIP6 defines a protocol for all variables related to ice sheets. ISMIP6 will provide a basis for investigating the feedbacks, impacts, and sea-level changes associated with dynamic ice sheets and for quantifying the uncertainty in ice-sheet-sourced global sea-level change
Case-based decision support system for breast cancer management
Breast cancer is identified as the most common type of cancer in women worldwide with 1.6 million women around the world diagnosed every year. This prompts many active areas of research in identifying better ways to prevent, detect, and treat breast cancer. DESIREE is a European Union funded project, which aims at developing a web-based software ecosystem for the multidisciplinary management of primary breast cancer. The development of an intelligent clinical decision support system offering various modalities of decision support is one of the key objectives of the project. This paper explores case-based reasoning as a problem solving paradigm and discusses the use of an explicit domain knowledge ontology in the development of a knowledge-intensive case-based decision support system for breast cancer management
Optimal numerical solvers for transient simulations of ice flow using the Ice Sheet System Model (ISSM versions 4.2.5 and 4.11)
Identifying fast and robust numerical solvers is a critical issue that needs to
be addressed in order to improve projections of polar ice sheets evolving in a
changing climate. This work evaluates the impact of using advanced numerical
solvers for transient ice-flow simulations conducted with the JPL–UCI Ice Sheet
System Model (ISSM). We identify optimal numerical solvers by testing a broad
suite of readily available solvers, ranging from direct sparse solvers to
preconditioned iterative methods, on the commonly used Ice Sheet Model
Intercomparison Project for Higher-Order ice sheet Models benchmark tests.
Three types of analyses are considered: mass transport, horizontal stress
balance, and incompressibility. The results of the fastest solvers for
each analysis type are ranked based on their scalability across mesh size and
basal boundary conditions. We find that the fastest iterative solvers are
∼ 1.5–100 times faster than the default direct solver used in ISSM, with
speed-ups improving rapidly with increased mesh resolution. We provide a set of
recommendations for users in search of efficient solvers to use for transient
ice-flow simulations, enabling higher-resolution meshes and faster turnaround
time. The end result will be improved transient simulations for short-term,
highly resolved forward projections (10–100 year time scale) and also improved
long-term paleo-reconstructions using higher-order representations of stresses
in the ice. This analysis will also enable a new generation of comprehensive
uncertainty quantification assessments of forward sea-level rise projections,
which rely heavily on ensemble or sampling approaches that are inherently
expensive
Modeling the response of northwest Greenland to enhanced ocean thermal forcing and subglacial discharge
Calving-front dynamics is an important
control on Greenland's ice mass balance. Ice front retreat of
marine-terminating glaciers may, for example, lead to a loss in resistive
stress, which ultimately results in glacier acceleration and thinning. Over
the past decade, it has been suggested that such retreats may be triggered by
warm and salty Atlantic Water, which is typically found at a depth below
200–300 m. An increase in subglacial water discharge at glacier ice fronts
due to enhanced surface runoff may also be responsible for an intensification
of undercutting and calving. An increase in ocean thermal forcing or
subglacial discharge therefore has the potential to destabilize
marine-terminating glaciers along the coast of Greenland. It remains unclear
which glaciers are currently stable but may retreat in the future and how far
inland and how fast they will retreat. Here, we quantify the sensitivity and
vulnerability of marine-terminating glaciers along the northwest coast of
Greenland (from 72.5 to 76∘ N) to ocean forcing and subglacial
discharge using the Ice Sheet System Model (ISSM). We rely on a
parameterization of undercutting based on ocean thermal forcing and
subglacial discharge and use ocean temperature and salinity from
high-resolution ECCO2 (Estimating the Circulation and Climate of the Ocean,
Phase II) simulations at the fjord mouth to constrain the ocean thermal
forcing. The ice flow model includes a calving law based on a tensile von
Mises criterion. We find that some glaciers, such as Dietrichson Gletscher or
Alison Glacier, are sensitive to small increases in ocean thermal forcing,
while others, such as Illullip Sermia or Cornell
Gletscher, are remarkably stable,
even in a +3 ∘C ocean warming scenario. Under the most intense
experiment, we find that Hayes Gletscher retreats by more than 50 km inland by 2100 into a deep trough,
and its velocity increases by a factor of 3 over only 23 years. The model
confirms that ice–ocean interactions can trigger extensive and rapid glacier
retreat, but the bed controls the rate and magnitude of the retreat. Under
current oceanic and atmospheric conditions, we find that this sector of the
Greenland ice sheet alone will contribute more than 1 cm to sea level rise
and up to 3 cm by 2100 under the most extreme scenario.</p
Sensitivity of the dynamics of Pine Island Glacier, West Antarctica, to climate forcing for the next 50 years
Pine Island Glacier, a major contributor to sea level rise in West
Antarctica, has been undergoing significant changes over the last few
decades. Here, we employ a three-dimensional, higher-order model to simulate
its evolution over the next 50 yr in response to changes in its surface mass
balance, the position of its calving front and ocean-induced ice shelf
melting. Simulations show that the largest climatic impact on ice dynamics is
the rate of ice shelf melting, which rapidly affects the glacier speed over
several hundreds of kilometers upstream of the grounding line. Our
simulations show that the speedup observed in the 1990s and 2000s is
consistent with an increase in sub-ice-shelf melting. According to our
modeling results, even if the grounding line stabilizes for a few decades, we
find that the glacier reaction can continue for several decades longer.
Furthermore, Pine Island Glacier will continue to change rapidly over the
coming decades and remain a major contributor to sea level rise, even if
ocean-induced melting is reduced
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