Abstracts of presentations and poster sessions for the 2018 WAIS Workshop can be found at the first following linked website. From the eight pages of linked abstracts, I have selected three that highlight the risks of MICI types of ice mass loss from the WAIS and I have underlined some sentences that are particularly concerning from these three abstracts (but feel free to post and comment on other abstracts that I did not select):
Title: "WAIS 2018 Agenda"
Title: "Poster Session 1, Monday afternoon: The Vulcan Mind Meld"
Title: "Poster Session 2, Tuesday Afternoon: The Picard Maneuver"
https://docs.google.com/document/d/170BqLjDDVX2cAe0Mr_qwtwTIdPoPtYC6fQYcWB0l7As/edit&
Title: "Across the Great Divide: The Flow-to-Fracture Transition and the Future of the West Antarctic Ice Sheet", by Richard B. Alley, Byron R. Parizek, Knut Christianson, Robert M. DeConto, David Pollard and Sridhar Anandakrishna
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/Alley_R.pdfAbstract: "Physical understanding, modeling, and available data indicate that sufficient warming and retreat of Thwaites Glacier, West Antarctica will remove its ice shelf and generate a calving cliff taller than any extant calving fronts, and that beyond some threshold this will generate faster retreat than any now observed. Persistent ice shelves are restricted to cold environments. Ice-shelf removal has been observed in response to atmospheric warming, with an important role for meltwater wedging open crevasses, and in response to oceanic warming, by mechanisms that are not fully characterized. Some marine-terminating glaciers lacking ice shelves “calve” from cliffs that are grounded at sea level or in relatively shallow water, but more-vigorous flows advance until the ice is close to flotation before calving. For these vigorous flows, a calving event shifts the ice front to a position that is slightly too thick to float, and generates a stress imbalance that causes the ice front to flow faster and thin to flotation, followed by another calving event; the rate of retreat thus is controlled by ice flow even though the retreat is achieved by fracture. Taller cliffs generate higher stresses, however, favoring fracture over flow.
Deformational processes are often written as power-law functions of stress, with ice deformation increasing as approximately the third power of stress, but subcritical crack growth as roughly the thirtieth power, accelerating to elastic-wave speeds with full failure. Physical understanding, models based on this understanding, and the limited available data agree that, above some threshold height, brittle processes will become rate-limiting, generating faster calving at a rate that is not well known but could be very fast. Subaerial slumping followed by basal-crevasse growth of the unloaded ice is the most-likely path to this rapid calving. This threshold height is probably not too much greater than the tallest modern cliffs, which are roughly 100 m."
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Title: "Towards Process-based Models of Marine Ice-cliff instability", by Doug Benn, Jeremy Bassis, Jan Åström, Joe Todd & Thomas Zwinger
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/Benn_D.pdfAbstract: "The finite strength of ice places a limit on ice cliff height above sea level (freeboard) (Hanson and Hooke, 2003; Bassis and Walker, 2012). When this height is exceeded, ice cliff failure can occur. This calving mechanism is currently hypothetical, but could become widespread if deep calving cliffs are exposed on marine ice sheets following the disintegration of fringing ice shelves.
Runaway ice-cliff failure or the Marine Ice-cliff Instability (MICI) could lead to much more rapid ice loss of the West Antarctic Ice Sheet than the well-established Marine Ice-Sheet Instability (MISI) processes (Pollard et al., 2015; DeConto and Pollard, 2016). It is relatively straightforward to define height thresholds for ice-cliff failure based on field or laboratory measurements of the yield strength of ice, but methods for rates of ice loss remain rudimentary. The current generation of models employs simple rate functions tuned to match observed or prescribed calving rates. Because of the potentially great importance of MICI for future sea-level rise, there is an urgent need for well-founded models of ice cliff instability to enable reliable predictions of ice loss under different forcings.
In this talk, we present preliminary results of investigations into marine ice cliff instability using the Helsinki Discrete Element Model (HiDEM) and Elmer/Ice. We find that the large longitudinal stress gradients at tall ice fronts trigger complex mixed-mode dynamic behaviors, including brittle failure, viscous deformation, and enhanced viscous flow along shear zones. Crucially, brittle and viscous processes are complexly linked: the rate and pattern of fracture development depends on the rheology and stress history of the glacier. Furthermore, fractures influence the larger scale flow of ice tens of ice thicknesses away from the calving front. This creates considerable challenges for modelling, because approaches that rely on alternating between elastic and viscous models (e.g. Vallot et al., 2018) yield results that depend on time-step size. We shall address this problem using a fully visco-elastic version of HiDEM (in development) to explore how interactions between brittle and viscous processes control rates of ice flow and ice-front retreat where the ice-cliff stability threshold is exceeded. The ultimate aim is to use insights from process-based models to develop parameterizations of MICI for regional scale predictive models."
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Title: "Climatic Thresholds for WAIS Retreat: Onset of Widespread Ice Shelf Hydrofracturing and Ice Cliff Calving in a Warming World", by Rob DeConto, David Pollard, Knut Christianson, Richard B. Alley & Byron R. Parizek
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/WAIS2018.pdfAbstract: "The loss or thinning of buttressing ice shelves and accompanying changes in grounding zone stress balance are commonly implicated as the primary trigger for grounding line retreat, such as that observed in Amundsen Sea outlet glaciers today. Ice-shelf thinning is mostly attributed to the presence of warm ocean waters beneath the shelves. However, climate model projections indicate that summer air temperatures could soon exceed the threshold for widespread meltwater production on ice-shelf surfaces. This has serious implications for the future stability of ice shelves, because they are vulnerable to the propagation of water-induced flexural stresses and water-aided crevasse penetration, often referred to as ‘hydrofracturing’. Once initiated, the rate of shelf loss through hydrofracturing can far exceed that caused by sub-surface oceanic melting, and could result in the complete loss of some buttressing ice shelves, with marine-terminating grounding lines suddenly becoming calving ice fronts. In places where those exposed (unbuttressed) ice fronts are thick enough (>900m), deviatoric stresses can exceed the strength of the ice, and the cliff face will fail through brittle processes leading to rapid calving like that seen in analogous settings on Greenland such as Jakobshavn and Helheim.
Here we explore the implications of hydrofacturing and subsequent ice-cliff collapse in a warming climate, by parameterizing these processes in a hybrid ice sheet-shelf model. Model physical parameters controlling sensitivity of surface crevasse penetration to meltwater and ice-cliff calving rate (a function of cliff height above the stress-balance threshold for brittle failure) are based on observations of calving in analogous settings, and model performance relative to observed mass loss and paleo sea-level estimates. Including these processes and exploring a range of atmospheric and ocean climate forcing scenarios,
we find the potential for major future WAIS retreat if global mean temperature rises more than ~2ºC above preindustrial. We also find that strict mitigation, with net negative carbon emissions initiated ~2060 substantially reduces the magnitude and rate of long-term WAIS retreat.
In simulations following a ‘worst case’ RCP8.5 scenario, the model produces rates of equivalent sea level rise that would be measured in cm per year by the end of this century. Importantly, parameterized Antarctic calving rates at thick ice fronts are not allowed to exceed those observed in Greenland today. This may be an overly conservative assumption, considering the very different spatial scales and physical settings of Antarctic outlet glaciers like Thwaites. Clearly the potential for mechanical/brittle processes to deliver ice to the ocean, in addition to viscous and basal processes, needs to be better constrained through more complete, physically based model representations of calving."
Edit, for those who do not understand the implications of Alley et al. (2018)'s comment that ice deformation is a power-function of stress, I attach an image that translates this underlying ice-cliff behavior into terms of calving rate (deformation) per year for various values of marine glacier freeboard (ice face height minus water depth) and relative water depth (which combine determine the primary stresses near the ice cliff face).