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rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #100 on: March 27, 2015, 05:39:37 PM »
Thank you for the help.
.
.
Once tried to understand the Wikipedia things, but....couldn't get those. Anyways, will try again and let you know !!

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #101 on: March 27, 2015, 05:42:10 PM »
Can you please clearly define : "What is Newtonian fluid?" "Their characteristics"
.
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This time please no links of definitions..... I want some simple sentence definition.

Thanks in advance.

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #102 on: March 27, 2015, 05:43:36 PM »
also can you provide
                                 ----- "the List of Notations that are used for glacier modelling studies ?"

Thanks in advance

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #103 on: March 27, 2015, 05:55:41 PM »
Can you please clearly define : "What is Newtonian fluid?" "Their characteristics"
.
.
This time please no links of definitions..... I want some simple sentence definition.

Thanks in advance.

A Newtonian fluid is a fluid in which the viscous stresses arising from its flow, at every point, are linearly proportional to the local strain rate—the rate of change of its deformation over time.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #104 on: March 27, 2015, 06:04:06 PM »
also can you provide
                                 ----- "the List of Notations that are used for glacier modelling studies ?"

Thanks in advance

Unfortunately, as I am a civil engineer and not a glaciologist I cannot provide a comprehensive list of notations.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #105 on: March 27, 2015, 06:04:32 PM »
Why is it called Newtonian fluid ?

Laurent

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #106 on: March 27, 2015, 06:14:52 PM »
May be you could have looked for it by yourself ?
https://en.wikipedia.org/wiki/Newtonian_fluid
More precisely, a fluid is Newtonian only if the tensors that describe the viscous stress and the strain rate are related by a constant viscosity tensor that does not depend on the stress state and velocity of the flow.

Oh yes why Mister Newton ?
Newtonian fluids are named after Isaac Newton, who first derived the relation between the rate of shear strain rate and shear stress for such fluids in differential form.

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #107 on: March 27, 2015, 06:17:03 PM »
Why is it called Newtonian fluid ?

Newton's laws of mechanics are linear and Newtonian fluids respond linearly to local strain rates.  Non-Newtonian fluids response depends (non-linearly) on the shear rate and/or the shear history (like corn starch).
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rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #108 on: March 27, 2015, 06:45:19 PM »
Anyways, thanks for the help. Along with that, sorry for asking silly questions.

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #109 on: March 27, 2015, 06:46:54 PM »
It will be helpful if anyone can provide me any list of glacial flow model notations.

Thanks

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #110 on: July 07, 2015, 02:17:23 AM »
The linked reference discusses the finding of a relatively new altimetric tool for measuring land-ice elevation changes in Antarctica:

Young, Duncan A.; Lindzey, Laura E.; Blankenship, Donald D.; Greenbaum, Jamin S.; Garcia De Gorordo, Alvaro; Kempf, Scott D.; Roberts, Jason L.; Warner, Roland C.; Van Ommen, Tas; Siegert, Martin J.; Le Meur, Emmanuel (February 2015), "Land-ice elevation changes from photon-counting swath altimetry: first applications over the Antarctic ice sheet", Journal of Glaciology, Volume 61, Number 225, pp. 17-28(12), DOI: http://dx.doi.org/10.3189/2015JoG14J048

http://www.ingentaconnect.com/content/igsoc/jog/2015/00000061/00000225/art00003

Abstract: "Satellite altimetric time series allow high-precision monitoring of ice-sheet mass balance. Understanding elevation changes in these regions is important because outlet glaciers along ice-sheet margins are critical in controlling flow of inland ice. Here we discuss a new airborne altimetry dataset collected as part of the ICECAP (International Collaborative Exploration of the Cryosphere by Airborne Profiling) project over East Antarctica. Using the ALAMO (Airborne Laser Altimeter with Mapping Optics) system of a scanning photon-counting lidar combined with a laser altimeter, we extend the 2003–09 surface elevation record of NASA's ICESat satellite, by determining cross-track slope and thus independently correcting for ICESat's cross-track pointing errors. In areas of high slope, cross-track errors result in measured elevation change that combines surface slope and the actual Δz/Δt signal. Slope corrections are particularly important in coastal ice streams, which often exhibit both rapidly changing elevations and high surface slopes. As a test case (assuming that surface slopes do not change significantly) we observe a lack of ice dynamic change at Cook Ice Shelf, while significant thinning occurred at Totten and Denman Glaciers during 2003–09."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #111 on: July 11, 2015, 12:49:43 PM »
The linked reference (& associated image) provides a more coherent interpretation of the past 25,000 years of Antarctic ice sheet history; which, will hopefully improve our understanding of ice-ocean-geothermal history:
http://planetearth.nerc.ac.uk/features/story.aspx?id=1774&cookieConsent=A
Extract: "The Scientific Committee for Antarctic Research (SCAR) commissioned an international team of 78 scientists from 14 countries to compile the most complete review of the history of the ice sheet to date. The team includes scientists from many backgrounds, including marine geologists, those studying changes recorded in lake muds, ice-sheet modellers, terrestrial glacial geologists and glaciologists.
The review looked at the ice sheet's extent and thickness at the Last Glacial Maximum – the peak of the last ice age, 25,000 years ago – and then every 5,000 years until the present. The challenge we faced was to take information from many sources and many different international teams, and turn it into a unified history of the Antarctic ice cap over the last few tens of thousands of years."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

wili

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #112 on: July 13, 2015, 04:54:26 PM »
Study finds surprisingly high geothermal heating beneath West Antarctic Ice Sheet

UC Santa Cruz team reports first direct measurement of heat flow from deep within the Earth to the bottom of the West Antarctic ice sheet

Lead author Andrew Fisher, professor of Earth and planetary sciences at UC Santa Cruz, emphasized that the geothermal heating reported in this study does not explain the alarming loss of ice from West Antarctica that has been documented by other researchers.

“The ice sheet developed and evolved with the geothermal heat flux coming up from below–it’s part of the system. But this could help explain why the ice sheet is so unstable. When you add the effects of global warming, things can start to change quickly,” he said.

http://news.ucsc.edu/2015/07/antarctic-heating.html

(Apologies if this was already posted. Thanks to COBob at robertscribbler's blog for this.)
"A force de chercher de bonnes raisons, on en trouve; on les dit; et après on y tient, non pas tant parce qu'elles sont bonnes que pour ne pas se démentir." Choderlos de Laclos "You struggle to come up with some valid reasons, then cling to them, not because they're good, but just to not back down."

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #113 on: September 27, 2015, 04:09:54 PM »
The linked article discusses another basic tool for assessment pre-cliff failure/hydrofracturing ice flow across the AIS.  The skillful use of such a tool could assist in the determination of model parameters that will help to determine when main phase cliff failure and hydrofracturing will begin for key Antarctic marine glaciers (like those in the ASE):

Felix S. L. Ng (2015), "Spatial complexity of ice flow across the Antarctic Ice Sheet", Nature Geoscience, doi:10.1038/ngeo2532


http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2532.html

Abstract: "Fast-flowing ice streams carry ice from the interior of the Antarctic Ice Sheet towards the coast. Understanding how ice-stream tributaries operate and how networks of them evolve is essential for developing reliable models of the ice sheet’s response to climate change.  A particular challenge is to unravel the spatial complexity of flow within and across tributary networks. Here I define a measure of planimetric flow convergence, which can be calculated from satellite measurements of the ice sheet’s surface velocity, to explore this complexity. The convergence map of Antarctica clarifies how tributaries draw ice from its interior. The map also reveals curvilinear zones of convergence along lateral shear margins of streaming, and abundant ripples associated with nonlinear ice rheology and changes in bed topography and friction. Convergence on ice-stream tributaries and their feeding zones is uneven and interspersed with divergence. For individual drainage basins, as well as the ice sheet as a whole, fast flow cannot converge or diverge as much as slow flow. I therefore deduce that flow in the ice-stream networks is subject to mechanical regulation that limits flow-orthonormal strain rates. These findings provide targets for ice-sheet simulations and motivate more research into the origin and dynamics of tributarization."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #114 on: October 04, 2015, 02:55:52 AM »
The linked reference presents both analysis and field observations of a major calving event at Greenland’s Helheim Glacier.  The research identified the physical mechanisms operating during calving events, which result in "… glacial earthquakes, globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in Greenland and Antarctica."  This include remote monitoring of calving for key outlet glaciers like Jakobshavn, PIG and Thwaites.

T. Murray, M. Nettles, N. Selmes, L. M. Cathles, J. C. Burton, T. D. James, S. Edwards, I. Martin, T. O’Farrell, R. Aspey, I. Rutt and T. Baugé (17 July 2015, Published Online June 25 2015), " Reverse glacier motion during iceberg calving and the cause of glacial earthquakes", Science; Vol. 349, no. 6245, pp. 305-308, DOI: 10.1126/science.aab0460


http://www.sciencemag.org/content/349/6245/305.abstract?sid=76ef658e-614c-409d-848f-b8e53f184c97


Abstract: "Nearly half of Greenland’s mass loss occurs through iceberg calving, but the physical mechanisms operating during calving are poorly known and in situ observations are sparse. We show that calving at Greenland’s Helheim Glacier causes a minutes-long reversal of the glacier’s horizontal flow and a downward deflection of its terminus. The reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front. The downward motion results from a hydrodynamic pressure drop behind the capsizing berg, which also causes an upward force on the solid Earth. These forces are the source of glacial earthquakes, globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in Greenland and Antarctica."

See also:
http://www.latimes.com/science/sciencenow/la-sci-sn-glacier-earthquake-iceberg-greenland-ice-20150625-story.html
http://www.npr.org/sections/thetwo-way/2015/06/25/417457888/study-reveals-what-happens-during-a-glacial-earthquake
http://phys.org/news/2015-08-glacial-earthquakes-sea-level.html

Extract: "It is only recently that scientists learned of the existence of glacial earthquakes–measurable seismic rumblings produced as massive chunks fall off the fronts of advancing glaciers into the ocean. In Greenland, these quakes have grown sevenfold over the last two decades and they are advancing northward, suggesting that ice loss is increasing as climate warms. But exactly what drives the quakes has been poorly understood. Now, a new study elucidating the quakes' mechanics may give scientists a way to measure ice loss remotely, and thus refine predictions of future sea-level rise. The study appears this week in the early online edition of the leading journal Science.
It shows that as the glacier front falls off into the water, or calves, there is a kickback. The rest of the glacier moves rapidly downward and backward–something like a skateboard that slips out from under a rider's feet and goes backward as the rider falls forward. This is what produces the quake, say the researchers. The force of that kickback can be so great, it can reverse the glacier's flow for a few minutes, from the equivalent of about 95 feet per day forward to about 130 feet per day backward. Earlier studies have shown that glaciers often speed up after calving, but did not show the more immediate backward motion that apparently produces the quakes.

"This gives us a far better explanation for the source of the earthquakes than we had before," said Meredith Nettles, a seismologist at Columbia University's Lamont-Doherty Earth Observatory and a coauthor of the study. "It will move us a long way towards being able to use remotely detectable seismic signals to estimate mass loss from a major class of events in both Greenland and Antarctica.""
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #115 on: March 12, 2016, 03:44:05 AM »
The linked article discusses field work about the influence of tides on Antarctic ice shelves in order to develop better ice shelf/ice sheet models.  I note that the Nansen Ice Shelf is about to break free:

http://phys.org/news/2016-03-nasa-tracking-tides-ice-shelves.html

Extract: "he NASA scientists worked with personnel from the Korea Polar Research Institute to install instruments on the Nansen Ice Shelf, a roughly 30-mile-long ice shelf sticking out from the coast of Antarctica's Victoria Land. The ice shelf is near the new South Korean Jang Bogo Station, where Walker and Dow stayed during their field campaign.
"Nansen is a smaller ice shelf but we're hoping that it's representative of many of the smaller ice shelves that ring Antarctica," Walker said. "We also hope that the techniques that we're testing out in this campaign can be used in the future on larger ice shelves."
"Ice shelves are very important for holding back ice flow behind them because what they're essentially doing is acting as a plug; as soon as you remove them, there's nothing there preventing the ice mass from moving quickly down," Dow said. "It's a particular worry at the moment that the ice shelves around Antarctica are going to break up, and we're going to see an unprecedented speed-up in the ice coming from the center of the ice sheet."he NASA scientists worked with personnel from the Korea Polar Research Institute to install instruments on the Nansen Ice Shelf, a roughly 30-mile-long ice shelf sticking out from the coast of Antarctica's Victoria Land. The ice shelf is near the new South Korean Jang Bogo Station, where Walker and Dow stayed during their field campaign.
"Nansen is a smaller ice shelf but we're hoping that it's representative of many of the smaller ice shelves that ring Antarctica," Walker said. "We also hope that the techniques that we're testing out in this campaign can be used in the future on larger ice shelves."
"Ice shelves are very important for holding back ice flow behind them because what they're essentially doing is acting as a plug; as soon as you remove them, there's nothing there preventing the ice mass from moving quickly down," Dow said. "It's a particular worry at the moment that the ice shelves around Antarctica are going to break up, and we're going to see an unprecedented speed-up in the ice coming from the center of the ice sheet.""


“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #116 on: April 09, 2016, 05:04:45 PM »
Here are some more basics w.r.t. glacial dynamics:

Krabbendam, M.: Basal sliding of temperate basal ice on a rough, hard bed: pressure melting, creep mechanisms and implications for ice streaming, The Cryosphere Discuss., doi:10.5194/tc-2016-52, in review, 2016.


http://www.the-cryosphere-discuss.net/tc-2016-52/

Abstract. Basal ice motion is crucial to ice dynamics of ice sheets. The Weertman sliding model for basal sliding over bedrock obstacles proposes that sliding velocity is controlled by pressure melting and/or ductile flow, whichever is the fastest; it further assumes that stoss-side melting is limited by heat flow through the obstacle and ductile flow is controlled by Power Law Creep. These last two assumptions, it is argued here, are invalid if a substantial basal layer of temperate (T ~ Tmelt) ice is present. In that case, frictional melting results in excess basal meltwater and efficient water flow, leading to near-thermal equilibrium. Stoss-side melting is controlled by melt water production, heat advection by flowing meltwater to the next obstacle, and heat conduction through ice/rock over half the obstacle height. No heat flow through the obstacle is required. High temperature ice creep experiments have shown a sharp weakening of a factor 5–10 close to Tmelt, implying breakdown of Power Law Creep and probably caused by a deformation-mechanism switch to grain boundary pressure melting. Ice streaming over a rough, hard bed, as likely in the Northeast Greenland Ice Stream, may be explained by enhanced basal motion in a thick temperate ice layer.

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solartim27

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #117 on: April 21, 2016, 05:00:19 AM »
Nansen broke apart on Apr 8th.  Very considerate to do so before we lost the view to winter.
http://earthobservatory.nasa.gov/IOTD/view.php?id=87859

http://earthobservatory.nasa.gov/IOTD/view.php?id=87657
« Last Edit: April 21, 2016, 05:09:49 AM by solartim27 »
FNORD

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #118 on: June 29, 2016, 05:56:41 PM »
The linked reference on low-porosity crusts at the WAIS Divide site in West Antarctica could help future interpretations of paleoclimate data from this area:

Fegyveresi, J. M., Alley, R. B., Muto, A., Orsi, A. J., and Spencer, M. K.: Surface formation, preservation, and history of low-porosity crusts at the WAIS Divide site, West Antarctica, The Cryosphere Discuss., doi:10.5194/tc-2016-155, in review, 2016.

http://www.the-cryosphere-discuss.net/tc-2016-155/

Abstract. Observations at the WAIS Divide site show that near-surface snow is strongly altered by weather-related processes such as strong winds and temperature fluctuations, producing features that are recognizable in the deep ice core. Prominent "glazed" surface crusts develop frequently at the site during summer seasons. Surface, snow pit, and ice core observations made in this study during summer field seasons from 2008–09 to 2012–13, supplemented by Automated Weather Station (AWS) data with insolation sensors, revealed that such crusts formed during relatively low-wind, low-humidity, clear-sky periods with intense daytime sunshine. After formation, such glazed surfaces typically developed cracks in a polygonal pattern with few-meter spacing, likely from thermal contraction at night. Cracking was commonest when several clear days occurred in succession, and was generally followed by surface hoar growth; vapor escaping through the cracks during sunny days may have contributed to the high humidity that favored nighttime formation of surface hoar. Temperature and radiation observations showed that daytime solar heating often warmed the near-surface snow above the air temperature, contributing to mass transfer favoring crust formation and then surface hoar formation. Subsequent investigation of the WDC06A deep ice core revealed that crusts are preserved through the bubbly ice, and some occur in snow accumulated during winters, although not as commonly as in summertime deposits. Although no one has been on site to observe crust formation during winter, it may be favored by greater wintertime wind-packing from stronger peak winds, high temperatures and steep temperature gradients from rapid midwinter warmings reaching as high as −15 °C, and perhaps longer intervals of surface stability. Time-variations in crust occurrence in the core may provide paleoclimatic information, although additional studies are required. Discontinuity and cracking of crusts likely explain why crusts do not produce significant anomalies in other paleoclimatic records.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #119 on: July 06, 2016, 06:30:45 PM »
The linked (open access) reference provides guidance for improved modelling of marine ice sheets:

Gladstone, R. M., Warner, R. C., Galton-Fenzi, B. K., Gagliardini, O., Zwinger, T., and Greve, R.: Marine ice sheet model performance depends on basal sliding physics and sub-shelf melting, The Cryosphere Discuss., doi:10.5194/tc-2016-149, in review, 2016

http://www.the-cryosphere-discuss.net/tc-2016-149/

Abstract. Computer models are necessary for understanding and predicting marine ice sheet behaviour. However, there is uncertainty over implementation of physical processes at the ice base, both for grounded and floating glacial ice. Here we implement several sliding relations in a marine ice sheet flowline model accounting for all stress components, and demonstrate that model resolution requirements are strongly dependent on both the choice of basal sliding relation and the spatial distribution of ice shelf basal melting.

Sliding relations that reduce the magnitude of the step change in basal drag from grounded ice to floating ice (where basal drag is set to zero) show reduced dependence on resolution compared to a commonly used relation, in which basal drag is purely a power law function of basal ice velocity. Sliding relations in which basal drag goes smoothly to zero as the grounding line is approached from inland (due to a physically motivated incorporation of effective pressure at the bed) provide further reduction to resolution dependence.

A similar issue is found with the imposition of basal melt under the floating part of the ice shelf: melt parameterisations that reduce the abruptness of change in basal melting from grounded ice (where basal melt is set to zero) to floating ice provide improved convergence with resolution compared to parameterisations in which high melt occurs adjacent to the grounding line. Thus physical processes, such as sub-glacial outflow (which could cause high melt near the grounding line), would impact on capability to simulate marine ice sheets.

For any given marine ice sheet the basal physics, both grounded and floating, governs the feasibility of simulating the system. The combination of a physical dependency of basal drag on effective pressure and low ice shelf basal melt rates near the grounding line mean that some marine ice sheet systems can be reliably simulated at a coarser resolution than currently thought necessary.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #120 on: September 12, 2016, 07:23:13 PM »
The linked reference addresses the balance of driving stress vs basal drag on the Siple Coast Ice Streams; however, if future global warming leads to hydrofracturing of the RIS, these indicated balance of stresses will be lost:

Wohland, J., Albrecht, T., and Levermann, A.: Balance between driving stress and basal drag results in linearity between driving stress and basal yield stress in Antarctica's Siple Coast Ice Streams, The Cryosphere Discuss., doi:10.5194/tc-2016-191, in review, 2016.

http://www.the-cryosphere-discuss.net/tc-2016-191/

Abstract. Ice streams are distinct, fast-flowing regimes within ice sheets that exhibit fundamentally different characteristics as compared to the slow-moving inner parts of the ice sheets. While along-flow surface profiles of ice sheets are typically convex, some ice streams show linearly sloping or even concave surface profiles. We use observational data of the Siple Coast in Antarctica to inversely calculate membrane stresses, driving stresses and basal yield stresses based on the Shallow Shelf Approximation. Herein we assume that these marine-based ice streams are isothermal and in neglecting vertical shear we assume that their flow is dominated by sliding. We find that in the Siple Coast ice streams the membrane stresses are negligible and the driving stress balances the basal drag. It follows directly that in the Coulomb limit (i.e. basal drag independent of velocity) the driving stress is linear in the basal yield stress. In addition, we find that the ice topography and the basal conditions developed such that the driving stress is linear in the basal yield stress regardless of the choice of the pseudo plastic exponent in the basal drag parameterization.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #121 on: October 06, 2016, 12:09:36 PM »
The linked research is being used to improve the ice sheet model used by ACME (Accelerated Climate Modeling for Energy):


Zhu H, Petra N, Stadler G, Isaac T, Hughes TJR, Ghattas O. "Inversion of Geothermal Heat Flux in a Thermo-Mechanically Coupled Nonlinear Stokes Ice Sheet Model.". 2016.

http://www.the-cryosphere.net/10/1477/2016/


Abstract: “We address the inverse problem of inferring the basal geothermal heat flux from surface velocity observations using a steady-state thermomechanically coupled nonlinear Stokes ice flow model. This is a challenging inverse problem since the map from basal heat flux to surface velocity observables is indirect: the heat flux is a boundary condition for the thermal advection–diffusion equation, which couples to the nonlinear Stokes ice flow equations; together they determine the surface ice flow velocity. This multiphysics inverse problem is formulated as a nonlinear least-squares optimization problem with a cost functional that includes the data misfit between surface velocity observations and model predictions. A Tikhonov regularization term is added to render the problem well posed. We derive adjoint-based gradient and Hessian expressions for the resulting partial differential equation (PDE)-constrained optimization problem and propose an inexact Newton method for its solution. As a consequence of the Petrov–Galerkin discretization of the energy equation, we show that discretization and differentiation do not commute; that is, the order in which we discretize the cost functional and differentiate it affects the correctness of the gradient. Using two- and three-dimensional model problems, we study the prospects for and limitations of the inference of the geothermal heat flux field from surface velocity observations. The results show that the reconstruction improves as the noise level in the observations decreases and that short-wavelength variations in the geothermal heat flux are difficult to recover. We analyze the ill-posedness of the inverse problem as a function of the number of observations by examining the spectrum of the Hessian of the cost functional. Motivated by the popularity of operator-split or staggered solvers for forward multiphysics problems – i.e., those that drop two-way coupling terms to yield a one-way coupled forward Jacobian – we study the effect on the inversion of a one-way coupling of the adjoint energy and Stokes equations. We show that taking such a one-way coupled approach for the adjoint equations can lead to an incorrect gradient and premature termination of optimization iterations. This is due to loss of a descent direction stemming from inconsistency of the gradient with the contours of the cost functional. Nevertheless, one may still obtain a reasonable approximate inverse solution particularly if important features of the reconstructed solution emerge early in optimization iterations, before the premature termination.”
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

A-Team

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #122 on: October 08, 2016, 03:12:45 PM »
linked research is being used to improve the ice sheet model
Not according to the abstract. The authors explored an ambitious approach but found the mathematical obstacles insuperable. The details of why things didn't work out will prove very interesting to others in the field but ultimately nothing has been learned about geothermal heat flux under Greenland that will improve understanding of the past, present or future of this ice sheet.

Meanwhile, there has been quite a bit of experimental progress reported lately on geothermal heat flux and the equally important melt state of the bottom ice. Ultimately the lack of data on matters such as temperature, hydration state and topography of basal till are essential boundary conditions. PDEs propagate from there, they can't go anywhere without them.

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #123 on: October 08, 2016, 06:03:39 PM »
linked research is being used to improve the ice sheet model
Not according to the abstract. The authors explored an ambitious approach but found the mathematical obstacles insuperable. The details of why things didn't work out will prove very interesting to others in the field but ultimately nothing has been learned about geothermal heat flux under Greenland that will improve understanding of the past, present or future of this ice sheet.

Meanwhile, there has been quite a bit of experimental progress reported lately on geothermal heat flux and the equally important melt state of the bottom ice. Ultimately the lack of data on matters such as temperature, hydration state and topography of basal till are essential boundary conditions. PDEs propagate from there, they can't go anywhere without them.

Not that I have any desire to over-sell the value of "Inversion of Geothermal Heat Flux in a Thermo-Mechanically Coupled Nonlinear Stokes Ice Sheet Model"; however, the abstract does state: "Nevertheless, one may still obtain a reasonable approximate inverse solution particularly if important features of the reconstructed solution emerge early in optimization iterations, before the premature termination.”  Therefore, the research did provide some guidance in certain "optimized" circumstances.  Thus even if these finds are of little practical value for either the GIS or the EAIS; they may well be of some value for the WAIS (which is currently more critical, w.r.t. collapse risk); as the following references indicate that the geothermal heat flux beneath portions of the WAIS (particularly near the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD).  Also, for what is worth, ACME continues to provide funding for this research; thus what is impracticable today, may (with more findings) be practicable in the not too distant future:

C. Ramirez, A. Nyblade, S.E. Hansen, D.A. Wiens, S. Anandakrishnan, R.C. Aster, A.D. Huerta, P. Shore and T. Wilson (March, 2016) "Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow", Geophys. J. Int., 204 (3): 1636-1648. doi: 10.1093/gji/ggv542

https://gji.oxfordjournals.org/content/204/3/1636.refs?related-urls=yes&legid=gji;204/3/1636

Abstract: "S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ∼6–8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ∼5–6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ∼3–4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ∼41 to 58 mW m−2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland–Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland–Iberia margins (40–65 mW m−2) and North Sea rift (60–85 mW m−2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out."

See also:
Andrew T. Fisher et. al. (July 2015), "High geothermal heat flux measured below the West Antarctic Ice Sheet", Science Advances 1(6):e1500093-e1500093, DOI: 10.1126/sciadv.1500093

http://advances.sciencemag.org/content/1/6/e1500093
http://advances.sciencemag.org/content/advances/1/6/e1500093.full.pdf

Abstract: "The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m(2), significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m(2). The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region."


See also:

Theresa M. Damiani, Tom A. Jordan, Fausto Ferraccioli, Duncan A. Young, and Donald D. Blankenship, (2014), "Variable crustal thickness beneath Thwaites Glacier revealed from airborne gravimetry, possible implications for geothermal heat flux in West Antarctica", Earth and Planetary Science Letters Volume 407, 1, Pages 109–122, DOI: 10.1016/j.epsl.2014.09.023

http://www.sciencedirect.com/science/article/pii/S0012821X14005780

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #124 on: January 30, 2017, 06:58:25 PM »
The linked reference presents a sensitivity study of SLR to the loss of Antarctic ice shelves.

Pattyn, F.: Sea-level response to melting of Antarctic ice shelves on multi-centennial time scales with the fast Elementary Thermomechanical Ice Sheet model (f.ETISh v1.0), The Cryosphere Discuss., doi:10.5194/tc-2017-8, in review, 2017.

http://www.the-cryosphere-discuss.net/tc-2017-8/
&
http://www.the-cryosphere-discuss.net/tc-2017-8/tc-2017-8.pdf

Abstract: "The magnitude of the Antarctic ice sheet's contribution to global sea-level rise is dominated by the potential of its marine sectors to become unstable and collapse as a response to ocean (and atmospheric) forcing. This paper presents Antarctic sea-level response to sudden atmospheric and oceanic forcings on multi-centennial time scales with the newly developed fast Elementary Thermomechanical Ice Sheet (f.ETISh) model. The f.ETISh model is a vertically integrated hybrid ice sheet/ice shelf model with an approximate implementation of ice sheet thermomechanics, making the model two-dimensional. Its marine boundary is represented by two different flux conditions, coherent with power-law basal sliding and Coulomb basal friction. The model has been compared to a series of existing benchmarks.

Modelled Antarctic ice sheet response to forcing is dominated by sub-ice shelf melt and the sensitivity is highly dependent on basal conditions at the grounding line. Coulomb friction in the grounding-line transition zone leads to significantly higher mass loss in both West and East Antarctica on centennial time scales, leading to 2 m sea level rise after 500 years for a moderate melt scenario of 20 m a−1 under freely-floating ice shelves, up to 6 m for a 50 m a−1 scenario. The higher sensitivity is attributed to higher driving stresses upstream from the grounding line.

Removing the ice shelves altogether results in a disintegration of the West Antarctic ice sheet and (partially) marine basins in East Antarctica. After 500 years, this leads to a 4.5 m and a 12.2 m sea level rise for the power-law basal sliding and Coulomb friction conditions at the grounding line, respectively. The latter value agrees with simulations by DeConto and Pollard (2016) over a similar period (but with different forcing and including processes of hydro-fracturing and cliff failure)."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #125 on: May 05, 2017, 06:37:43 PM »
The linked reference discusses progress being made in our understanding of large ice sheet rheology:

Graham, F. S., Morlighem, M., Warner, R. C., and Treverrow, A.: Implementing an empirical scalar tertiary anisotropic rheology (ESTAR) into large-scale ice sheet models, The Cryosphere Discuss., doi:10.5194/tc-2017-54, in review, 2017.

http://www.the-cryosphere-discuss.net/tc-2017-54/

Abstract. The microstructural evolution that occurs in polycrystalline ice during deformation leads to the development of anisotropic rheological properties that are not adequately described by the most common, isotropic, ice flow relation used in large-scale ice sheet models – the Glen flow relation. We present a preliminary assessment of the implementation in the Ice Sheet System Model (ISSM) of a computationally-efficient, empirical, scalar, tertiary, anisotropic rheology (ESTAR). The effect of this anisotropic rheology on ice flow dynamics is investigated by comparing idealised simulations using ESTAR with those using the isotropic Glen flow relation, where the latter includes a flow enhancement factor. For an idealised embayed ice shelf, the Glen flow relation overestimates velocities by up to 17 % when using an enhancement factor equivalent to the maximum value prescribed by ESTAR. Importantly, no single Glen enhancement factor can accurately capture the spatial variations in flow over the ice shelf. For flow-line studies of idealised grounded flow over a bumpy topography or a sticky base – both scenarios dominated at depth by bed-parallel shear – the differences between simulated velocities using ESTAR and the Glen flow relation vary according to the value of the enhancement factor used to calibrate the Glen flow relation. These results demonstrate the importance of describing the anisotropic rheology of ice in a physically realistic manner, and have implications for simulations of ice sheet evolution used to reconstruct paleo-ice sheet extent and predict future ice sheet contributions to sea level.
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #126 on: May 18, 2017, 05:58:27 PM »
The linked article is entitled: "Massive Landforms Have Just Been Discovered Under The Antarctic Ice Sheet", addresses how eskers can cause deep under-base channels in associated ice shelves, thereby increasing the susceptibility of Antarctic ice shelves to break-up.

https://www.sciencealert.com/massive-landforms-have-just-been-discovered-under-the-antarctic-ice-sheet

Also see the associated reference" Drews et. al. (2017), "Actively evolving subglacial conduits and eskers initiate ice shelf channels at an Antarctic grounding line", Nature Communications, doi:10.1038/ncomms15228.

https://www.nature.com/articles/ncomms15228
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #127 on: May 21, 2017, 10:57:02 PM »
The following selected abstracts are from the linked: "Proceedings of the Wellington Symposium", held 12–17 February 2017
Wellington, New Zealand

https://www.igsoc.org/symposia/2017/newzealand/proceedings/proceedings.html


75A2218
The RICE ice core: timing and drivers of the deglaciation in the Ross Sea region
Nancy Bertler, Howard Conway, Dorthe Dahl-Jensen
Corresponding author: Nancy Bertler
Corresponding author e-mail: nancy.bertler@vuw.ac.nz
Geological evidence and modelling experiments suggest that the removal of ice shelves from marine-based ice sheets can lead to catastrophic collapse. Roosevelt and Ross Islands are thought to be stabilization anchors for the Ross Ice Shelf and thus the West Antarctic Ice Sheet. As part of the Roosevelt Island climate evolution (RICE) project, a 763 m deep ice core was recovered during 2011–13 from Roosevelt Island, at the northern edge of the Ross Ice Shelf. The ice at Roosevelt Island is grounded 210 m below sea level and accumulates in situ, with the Ross Ice Shelf flowing around the rise. High-resolution radar surveys show a well developed Raymond Bump at the divide of the ice dome. The RICE age model is developed using high-resolution methane data tied to the WAIS Divide ice core record, supported with annual layer count, tephra ages and a glacial flow model. Here we show data spanning the past 30 ka and discuss reconstructions of sea surface and air temperature, sea-ice extent, atmospheric circulation patterns and ice-shelf grounding-line retreat. An ensemble of sensitivity modelling experiments is used to determine thresholds for the removal of ice on Roosevelt Island and correlated grounding-line and ice-volume changes of the Ross Ice Shelf and the West Antarctic Ice Sheet. Our data suggest that the delayed onset of the Ross Ice Shelf grounding-line retreat during the deglaciation was driven at least in part by the early onset of deglaciation in West Antarctica as recorded in the WAIS ice core. The Ross Ice Shelf grounding line started to retreat rapidly with the initiation of an ice shelf cavity. RICE TEAM: Bertler N, Conway H, Dahl-Jensen D, Baccolo G, Baisden T, Blunier T, Brightley H, Brook E, Buizert C, Carter L, Ciobanu G, Dadic R, Delmonte B, Dongqi Z, Edwards R, Eling L, Ellis A, Emanuelsson D, Fudge T, Golledge N , Hindmarsh R, Hawley R, Jiao Y, Johnson K, Keller L, Kingslake J, Kipfstuhl S, Kjær H, Korokikth E , Kurbatov A, Lee J, Lowry D, Mayewski P, Naish T, Neff P, Scherer R, Schoeneman S, Severinghaus J, Simonsen M, Steig E, Ulaylottil Venugopal A, Vallelonga P, Waddington E, Winton H

75A2226
Sea ice drives the large-scale Southern Ocean overturning circulation
Violaine Pellichero, Jean-Baptiste Sallée, Sunke Schmidtko, Fabien Roquet, Jean-Benoît Charrassin
Corresponding author: Violaine Pellichero
Corresponding author e-mail: violaine.pellichero@locean-ipsl.upmc.fr
In the Southern Ocean, deep waters well up towards the ocean surface under sea ice, where water masses are transformed in the mixed layer and re-injected back in deeper or shallower layers. The role of the Southern Ocean in ventilating global deep waters and redistributing heat and fresh water within the upper ocean is particularly important for the climate as a whole. However, significant uncertainty exists about the processes responsible for the Southern Ocean water mass overturning circulation south of 30° S. Working on elephant-seal-derived data as well as ship-based observations and Argo float data, we have investigated the processes that lead to the under-ice transformation of the upper circumpolar deep water (UCDW) and Antarctic intermediate water (AAIW). Air–sea flux data from several sources and in situ observations are used to describe the diapycnal flux at the ocean surface from one density class to the next, including UCDW and AAIW density ranges. In the sea-ice sector, our results show that surface buoyancy fluxes drive an upwelling of about 6 Sv in the UCDW ranges and a subduction of about 4.5 Sv in the AAIW ranges. The freshwater flux dominates over most of the density ranges, highlighting the role of the sea ice in driving this Southern Ocean branch of the meridional overturning circulation and fresh-water transport. Moreover, the regional distribution of the cross-isopycnal flux is computed in order to identify the regions where the UCDW upwells and AAIW sinks around the Antarctic continent. Our conclusions suggest that changes in regional sea-ice distribution or sea-ice seasonal cycle duration, as currently observed, would widely affect the buoyancy budget of the underlying mixed layer, and impacts large-scale water-mass formation and transformation

75A2272
Paleoclimate earth system modelling of cryosphere–ocean interactions in the Southern Hemisphere
Elizabeth Keller, Nicholas Golledge, Richard Levy
Corresponding author: Elizabeth Keller
Corresponding author e-mail: l.keller@gns.cri.nz
We present paleoclimate model experiments designed to explore ocean–ice interactions in the Southern Hemisphere under warmer-than-present conditions. We examine the changes in ocean circulation and biochemistry associated with the retreat of the West Antarctic Ice Sheet (WAIS) with steady-state simulations of Pliocene and Miocene interglacials, and the role of ocean dynamics in the expansion and retreat of WAIS during glacial/interglacial transitions. We use intermediate-complexity earth system models LOVECLIM and the UVic ESCM for initial exploration, with the goal of moving to a full GCM and a high-resolution ocean model to examine more detailed ice–ocean dynamics and processes.

75A2296
Role of tropical teleconnections in changes in the Southern Ocean dynamics and Antarctic sea-ice extent in the ACME Earth System Model
Rahul Sivankutty, Diana Francis, Eayrs Clare, David Holland, Stephen Price
Corresponding author: Rahul Sivankutty
Corresponding author e-mail: rs5521@nyu.edu
Recent studies suggest that changes in the Southern Ocean, particularly the Antarctic Circumpolar Current, can influence the thermal structure of the upper ocean and thus affect sea-ice concentration in the Antarctic region. The poleward shifting of subtropical westerlies can result in changes in ocean circulation pattern. The changes in the Southern Annular Mode, and its linkage to tropical SST variability, prove that tropical teleconnections can play an important role in Antarctic climate variability. Using a state-of-the-art Earth system model – the US Department of Energy’s Accelerated Climate Model for Energy (ACME) – which includes coupled representations of all of the components of the physical climate system (atmosphere, land, ocean, sea ice and land ice), we study the tropical linkages to the variability in the Southern Ocean and Antarctic sea ice. The study validates the model’s ability to capture the observed teleconnection patterns. The mechanisms by which the tropical climate influences the dynamics of the Southern Ocean and thereby Antarctic sea ice variability are highlighted.

75A2308
Glacial Antarctic warm events as captured by RICE ice core
Abhijith UV, Nancy Bertler, Giuseppe Cortese
Corresponding author: Abhijith UV
Corresponding author e-mail: Abhijith.Uv@vuw.ac.nz
The last glacial period in Antarctica has been punctuated by several episodes of warm events, where air temperature rose between 1 and 3°C, which are referred to as Antarctic isotope maxima (AIM). On correlating high-resolution Antarctic and Greenland ice-core records for AIM events, an out-of-phase relationship has been observed between both the hemispheres, with Antarctica warming when Greenland is under a cold phase and Antarctica cooling when Greenland stays in a warm state. This out-of-phase relationship is called the ‘bipolar seesaw’. Possible explanations include oceanic teleconnections via a shift in strength of the Atlantic Meridional Overturning Circulation (AMOC) and Antarctic bottom water (AABW) formation. A recent comparison between the WAIS Divide and NGRIP records identified a Northern Hemisphere lead of about 218 ± 92 a and 208 ± 96 a for the onset and termination of Dangaard/Oeschger and AIM events, further evidence for an important oceanic role in the interhemispheric energy distribution. Roosevelt Island is a local ice rise at the northern edge of the Ross Ice Shelf. A 764 m deep ice core, the Roosevelt Island Climate Evolution (RICE) core, was obtained over two field seasons in 2011/12 and 2012/13. Due to its proximity to the Ross Sea, one of the major contributors to AABW, the RICE records have the potential to provide new insights into the drivers and consequences during the evolution of AIM events. Here, we will present preliminary data of the major ion record from the RICE ice core covering an age range of 18–60 ka with the main focus of understanding core aspects of AABW during AIM events, including its strength and mode of formation and further to test the bipolar seesaw hypothesis.


75A2377
Subsurface geomorphology and post-Last-Glacial-Maximum deglaciation of Pine Island Glacier, Antarctica
Gerhard Kuhn, Johann Philipp Klages, Claus-Dieter Hillenbrand, James A. Smith, Frank O. Nitsche, Karsten Gohl, Sabine Kasten
Corresponding author: Gerhard Kuhn
Corresponding author e-mail: gerhard.kuhn@awi.de
Subglacial meltwater largely facilitates rapid but nonlinear ice flow beneath concurrent ice streams, and there is widespread evidence for a dynamic subglacial water system beneath the Antarctic Ice Sheet. It steers and affects the pattern of ice flow and is a direct result of boundary processes acting at the ice sheet bed, i.e. pressure-induced basal melting. Consequently, the occurrence of subglacial meltwater plays an important role in bedrock erosion, subsequent re-deposition, and shaping of the topography of ice-sheet beds. Here we present new geological and geochemical data from sediment cores recovered from the West Antarctic continental shelf in Pine Island Bay. We have interpreted the data as a reliable indicator for deposition in palaeo-subglacial lakes beneath the formerly expanded West Antarctic Ice Sheet, presumably following the Last Glacial Maximum (LGM). Characteristic changes of sedimentary facies and geochemical profiles within these cores taken on RV Polarstern expeditions ANT-XXIII/4 (2006) and ANT-XXVI/3 (2010) support the presence of an active and expanded subglacial lake system in at least five basins that were carved into bedrock during the last glaciations and filled with some meters of post-LGM sediments. These findings have important implications for palaeo ice-sheet dynamics, suggesting the presence of considerable amounts of water lubricating the ice–bed interface, eventually leading to the subglacial deposition of water-saturated subglacial lake sediments and soft tills. Based on our recent findings, we suggest the transition from a subglacial lake to an ocean-influenced environment took place during deglaciation at the transition from glacial marine isotope stage (MIS) 2 to the early Holocene. We suggest that the ice sheet thinned and the sub-ice lakes successively transformed to sub-ice cavities flushed by tidal currents at this time. Based on bathymetric maps, a glacial isostatic adjustment model, a global sea level curve and age information, we estimate ice thickness for buoyancy at the grounding line, as this grounding line retreated further inland across the rim of the subglacial lake. Our findings may have implications for ice-sheet models, which have to consider the predominantly non-linear effects related to subglacial hydrology


75A2401
IODP Expedition 374: ocean–ice-sheet interactions and West Antarctic Ice Sheet vulnerability
Rob McKay, Laura De Santis, Denise Kulhanek
Corresponding author: Rob McKay
Corresponding author e-mail: robert.mckay@vuw.ac.nz
Observations from the past several decades indicate that the Southern Ocean is warming significantly, while Southern Hemisphere westerly winds have migrated southward and strengthened due to increasing atmospheric CO2 concentrations and/or ozone depletion. These changes have been linked to thinning of Antarctic ice shelves and marine-terminating glaciers. Results of geologic drilling on Antarctica’s continental margins indicate late Neogene marine-based ice-sheet variability and numerical modeling indicates a fundamental role for oceanic heat in controlling this variability over at least the past 20 million years. While ice-sheet variability has been observed, sedimentologic sequences from the outer continental shelf are still required to evaluate the extent of past ice-sheet variability and the role of oceanic heat flux in controlling ice-sheet mass balance. IODP Expedition 374 is scheduled to be drilled in January 2018 and proposes a latitudinal and depth transect of sixdrill sites from the outer continental shelf and rise in the eastern Ross Sea to resolve the relationship between climatic/oceanic change and West Antarctic Ice Sheet evolution through the Neogene and Quaternary. This location was selected because numerical ice-sheet models indicate that it is highly sensitive to changes in ocean heat flux and sea level. The proposed drilling is designed for optimal data–model integration, which will enable an improved understanding of the sensitivity of Antarctic Ice Sheet mass balance during warmer-than-present climates (e.g. the early Pliocene and middle Miocene). Additionally, the proposed transect links ice proximal records from the inner Ross Sea continental shelf (e.g. ANDRILL sites) to deep southwest Pacific drilling sites/targets to obtain an ice proximal to far-field view of Neogene climate and Antarctic cryosphere evolution.

75A2402
An update on ice-shelf changes in Northern Greenland
Jérémie Mouginot, Eric Rignot, Bernd Scheuchl, Mathieu Morlighem, Ala Khazendar
Corresponding author: Jeremie Mouginot
Corresponding author e-mail: jmougino@uci.edu
Zachariæ Isstrøm, in northeast Greenland, is retreating and accelerating, most probably because of enhanced melting at its ice-shelf bottom followed by its break-up. Nioghalvfjerdsfjorden, its neighbor, is also showing signs of thinning close to its grounding line, as is Petermann Gletscher, located 800 km more to the west. Here, we investigate dynamic and geometrical changes of all the other glaciers located along the northern coast of Greenland, namely Humboldt Gletscher, Steensby Gletscher, Ryder Gletscher, Ostenfeld Gletscher, Marie Sophie Gletscher, Academy Gletscher and Hagen Bræ. Using satellite and airborne-based remote-sensing sensors, we reconstruct the time series of speed, grounding-line position, ice thickness and surface elevation changes since the 80s. We will provide an update of the glacier ice discharges and will discuss any large-scale pattern of enhanced melting of the northern Greenlandic ice shelves . We will conclude with the possibility of actual or future destabilization -or lack thereof- of the glaciers in this sector of Greenland.

75A2445
Rapid melting in the basal zone of a major Greenland outlet glacier
Poul Christoffersen, Tun Jan Young, Bryn Hubbard, Samuel Huckerby Doyle, Alun Hubbard, Marion Bougamont, Coen Hofstede, Keith Nicholls
Corresponding author: Poul Christoffersen
Corresponding author e-mail: pc350@cam.ac.uk
The Greenland ice sheet is losing mass and raising sea levels by 1 mm a–1. While melting of the ice sheet explains half of the net annual loss, the other half is caused by dynamic processes operating in the catchments of marine-terminating outlet glaciers. These processes are poorly understood because they are confined to the basal zone, which is often inaccessible. The Subglacial Access and Fast Ice Research Experiment (SAFIRE) is addressing this paucity of data by drilling to the bed of Store Glacier, the second-largest outlet glacier in West Greenland in terms of flux. Seven 600-m-deep boreholes were drilled to the base of the glacier, about 30 km inland from the calving terminus, at a location where ice flows at a rate of 700 m a–1. Sensors installed at the bed and within ice show that the glacier overrides a warm bed consisting of soft, water-saturated sediment. Basal motion comprised a combination of intense deformation of temperature basal ice as well as sliding. High basal water pressure with diurnal variations showed that water produced on the surface is transported subglacially in a distributed basal water system, which nevertheless was sufficiently efficient to cause rapid lowering of the water level in all seven boreholes, once the system was intercepted. To evaluate the quantify of heat transported from surface to bed, we measured rates of basal melting with a phase-sensitive, frequency-modulated continuous wave (FMCW) radar system installed autonomously at the borehole drill site. The radar captured internal and basal reflector ranges at high spatial (millimetre) and temporal (hourly) resolutions, producing a unique time series of ice deformation and basal melting, coincident with englacial and subglacial borehole measurements. Here, we show that the rate of basal melting was 3 m a–1 in winter, when heat at the bed is provided mainly by basal friction, and that it increases to 20 m a–1 in summer, when heat is also transported to the bed from the surface. Our measurements show that the flow of outlet glaciers from the Greenland Ice Sheet is influenced not only by their interaction with the ocean but equally by their interaction with the atmosphere, making them potentially more sensitive to climate change than thought so far.


75A2456
Future fate of the polar ice sheets and implications for global coastlines
Rob DeConto
Corresponding author: Rob DeConto
Corresponding author e-mail: deconto@geo.umass.edu
New climate and ice-sheet modeling, calibrated to past changes in sea level, is painting a stark picture of the future fate of the great polar ice sheets if greenhouse-gas emissions continue unabated. This is especially true for Antarctica, where a substantial fraction of the ice sheet rests on bedrock more than 500 m below sea level. Here, we will explore the sensitivity of the polar ice sheets to a warming atmosphere and ocean, using models that include previously underappreciated physical processes, including surface meltwater-driven hydrofracturing and structural failure of ice cliffs. Approaches to more precisely define the climatic thresholds capable of triggering rapid and potentially irreversible ice-sheet retreat will also be discussed, as will the potential for policy and aggressive mitigation strategies like those discussed at the 2015 Paris Climate Conference to substantially reduce the risk of extreme sea-level rise.

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #128 on: May 29, 2017, 04:32:45 PM »
The linked Climate Central article is entitled: "Waves Rippled Through Greenland’s Ice. That’s Ominous", just imagine what the future holds for the GIS, WAIS & EAIS:

http://www.climatecentral.org/news/waves-greenland-ice-melt-21488

Extract: "It’s the latest piece of bad news about Greenland’s ice. The ice sheet has been pouring roughly 270 megatons of ice a year into the ocean via the glaciers that stretch out from its hulking mass since 2000. That’s a big uptick compared to preceding decades.

The new research, published earlier this week in Geophysical Research Letters shows a new way that climate change is taking a toll. Scientists at the NASA Jet Propulsion Laboratory, led by Surendra Adhikari, were looking at data from a series of GPS stations set up around the various outlet glaciers that tumble from Greenland’s ice sheet to the sea. Ironically, they were looking at the GPS data to see if it was worth maintaining the network of stations that rings Greenland.

They found evidence of a never-before-observed phenomenon affecting Rink Glacier, a glacier on the western flank of Greenland. The glacier usually sends about 11 gigatons of ice into the ocean each summer melt season.

But 2012 was different. A fast-moving (by glacial standards), massive wave rumbled through the glacier’s interior, causing an extra 6.7 gigatons of ice and water to slosh into the sea. That’s the equivalent of 55 million blue whales, the largest animal on earth.

The wave — dubbed a solitary wave because of its singular nature — traveled at 2.5 miles per month in the summer, picking up to 7.5 miles per month in the fall. Rink Glacier typically only moves a mile or two in a normal year."

Edit, for the open access paper see:

S. Adhikari, E. R. Ivins & E. Larour (26 May 2017), "Mass transport waves amplified by intense Greenland melt and detected in solid Earth deformation", Geophysical Research Letters, DOI: 10.1002/2017GL073478

http://onlinelibrary.wiley.com/doi/10.1002/2017GL073478/abstract

Abstract: "The annual cycle and secular trend of Greenland mass loading are well recorded in measurements of solid Earth deformation. Horizontal crustal displacements can potentially track the spatiotemporal detail of mass changes with great fidelity. Our analysis of Greenland crustal motion data reveals that a significant excitation of horizontal amplitudes occurs during the intense melt years. We discover that solitary seasonal waves of substantial mass transport (1.67 ± 0.54 Gt/month) traveled at an average speed of 7.1 km/month through Rink Glacier in 2012. We deduce that intense surface melting enhanced either basal lubrication or softening of shear margins, or both, causing the glacier to thin dynamically in summer. The newly routed upstream subglacial water was likely to be both retarded and inefficient, thus providing a causal mechanism for the prolonged ice transport to continue well into the winter months. As the climate continues to produce increasingly warmer spring and summer, amplified seasonal waves of mass transport may become ever more present with important ramifications for the future sea level rise."
« Last Edit: May 29, 2017, 04:39:05 PM by AbruptSLR »
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #129 on: May 31, 2017, 12:11:20 AM »
The findings of the linked reference on glacial dynamics during the Pleistocene can be used to help calibrate current models:

Hans Engler, Hans G. Kaper, Tasso J. Kaper and Theodore Vo (May 19, 2017), "Dynamical systems analysis of the Maasch-Saltzman model for glacial cycles", arXiv:1705.06336v1

https://arxiv.org/pdf/1705.06336.pdf

Abstract: "This article is concerned with the internal dynamics of a conceptual model proposed by Maasch and Saltzman [J. Geophys. Res., 95;D2 (1990) 1955-1963] to explain central features of the glacial cycles observed in the climate record of the Pleistocene Epoch.  It is shown that, in most parameter regimes, the long-term system dynamics occur on certain intrinsic two-dimensional invariant manifolds in the three-dimensional state space. These invariant manifolds are slow manifolds when the characteristic time scales for the total global ice mass and the volume of North Atlantic Deep Water are well separated, and they are center manifolds when the characteristic time scales for the total global ice mass and the volume of North Atlantic Deep Water are comparable. In both cases, the reduced dynamics on these manifolds are governed by Bogdanov-Takens singularities, and the bifurcation curves associated to these singularities organize the parameter regions in which the model exhibits glacial cycles."

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #130 on: June 09, 2017, 05:38:34 PM »
Before cliff failures occur Antarctic ice shelves must calve away, and the linked reference discusses improved methodology for predicting such future events:

Emetc, V., Tregoning, P., and Sambridge, M.: A statistical fracture model for Antarctic glaciers, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-98, in review, 2017.

http://www.the-cryosphere-discuss.net/tc-2017-98/

Abstract. Antarctic and Greenland hold more than 99 % of all fresh water on Earth and, therefore, can significantly influence global sea level. Predicting future ice sheet mass balance depends upon ice sheet modelling, but it is limited by knowledge of a number of processes, some of which are still poorly understood. One such process is the calving of the ice shelves, where blocks of ice break off from the ice front. However, large scale ice flow models do not include an accurate representation of this process and the most commonly used damage mechanics and fracture mechanics methods have a large number of uncertainties. Here we present an alternative, statistics-based method to model the most probable zones of nucleation of fractures. We test our theory on all main ice shelf regions in Antarctica, including the Antarctic Peninsula. We can model up to 99 % of observed fractures, with an average rate of 77 % which represents a 50 % improvement over previously used damage-based approaches, thus providing the basis for modelling calving of ice shelves. We found that classifying Antarctic ice shelf regions based on the factors that controlled fracture formation led to grouping of ice shelves/glaciers with similar physical characteristics and geometry.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #131 on: September 05, 2017, 04:06:12 PM »
Hopefully, modelers will use these findings to upgrade their projections for tidewater glaciers:

Mercenier, R., Lüthi, M. P., and Vieli, A.: Calving relation for tidewater glaciers based on detailed stress field analysis, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-183, in review, 2017.

https://www.the-cryosphere-discuss.net/tc-2017-183/

Abstract. Ocean terminating glaciers in Arctic regions have undergone rapid dynamic changes in recent years, which have been related to a dramatic increase in calving rates. Iceberg calving is a dynamical process strongly influenced by the geometry at the terminus of tidewater glaciers. We investigate the effect of varying water level, calving front slope and basal sliding on the stress state and flow regime for an idealized grounded ocean-terminating glacier and scale these results with ice thickness and velocity. Results show that water depth and calving front slope strongly affect the stress state while the effect from variations in basal sliding is much smaller. An increased relative water level or a reclining calving front slope strongly decrease the stresses and velocities in the vicinity of the terminus and hence have a stabilizing effect on the calving front. We find that surface stress magnitude and distribution are determined by solely the water depth relative to ice thickness for simple geometries. Based on this scaled relationship for the stress peak at the surface, and assuming a critical stress for damage initiation, we propose a simple and new parametrization for calving rates for grounded tidewater glaciers that is in good agreement with observations.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #132 on: September 12, 2017, 04:24:07 PM »
For a better understanding of marine glaciers I provide the following:

Robert E. Kopp, Robert M. DeConto, Daniel A. Bader, Carling C. Hay, Radley M. Horton, Scott Kulp, Michael Oppenheimer, David Pollard, Benjamin H. Strauss (2017), "Implications of ice-shelf hydrofracturing and ice-cliff collapse mechanisms for sea-level projections", arXiv:1704.05597v1

https://arxiv.org/abs/1704.05597
https://arxiv.org/pdf/1704.05597.pdf

Abstract: "Probabilistic sea-level projections have not yet integrated insights from physical ice-sheet models representing mechanisms, such as ice-shelf hydrofracturing and ice-cliff collapse, that can rapidly increase ice-sheet discharge. Here, we link a probabilistic framework for sea-level projections to a small ensemble of Antarctic ice-sheet (AIS) simulations incorporating these physical processes to explore their influence on projections of global-mean sea-level (GMSL) and relative sea-level (RSL) change. Under high greenhouse gas emissions (Representative Concentration Pathway [RCP] 8.5), these physical processes increase median projected 21st century GMSL rise from ∼80  cm to ∼150  cm. Revised median RSL projections would, without protective measures, by 2100 submerge land currently home to >79  million people, an increase of ∼25  million people. The use of a physical model, rather than simple parameterizations assuming constant acceleration, increases sensitivity to forcing: overlap between the central 90\% of the frequency distributions for 2100 for RCP 8.5 (93--243 cm) and RCP 2.6 (26--98 cm) is minimal. By 2300, the gap between median GMSL estimates for RCP 8.5 and RCP 2.6 reaches >10  m, with median RSL projections for RCP 8.5 jeopardizing land now occupied by ∼900  million people (vs. ∼80  million for RCP 2.6). There is little correlation between the contribution of AIS to GMSL by 2050 and that in 2100 and beyond, so current sea-level observations cannot exclude future extreme outcomes. These initial explorations indicate the value and challenges of developing truly probabilistic sea-level rise projections incorporating complex ice-sheet physics."

For related planned WAIS research see also:

"How Much, How Fast?
A Decadal Science Plan
Quantifying the Rate of Change of the West Antarctic Ice
Sheet Now and in the Future"

http://nsidc.org/sites/nsidc.org/files/files/WAIS_SciPlanHMHF_final.pdf

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #133 on: October 06, 2017, 04:05:29 PM »
I get the impression that the cited open source software is best suited for relatively slow moving land dominated ice sheets such as in the GIS and or the EAIS.

Joseph H. Kennedy, et al (2017), "LIVVkit: An extensible, python-based, land ice verification and validation toolkit for ice sheet models", JAMES, DOI: 10.1002/2017MS000916

http://onlinelibrary.wiley.com/doi/10.1002/2017MS000916/full

Abstract: "To address the pressing need to better understand the behavior and complex interaction of ice sheets within the global Earth system, significant development of continental-scale, dynamical ice sheet models is underway. Concurrent to the development of the Community Ice Sheet Model (CISM), the corresponding verification and validation (V&V) process is being coordinated through a new, robust, Python-based extensible software package, the Land Ice Verification and Validation toolkit (LIVVkit). Incorporated into the typical ice sheet model development cycle, it provides robust and automated numerical verification, software verification, performance validation, and physical validation analyses on a variety of platforms, from personal laptops to the largest supercomputers. LIVVkit operates on sets of regression test and reference data sets, and provides comparisons for a suite of community prioritized tests, including configuration and parameter variations, bit-for-bit evaluation, and plots of model variables to indicate where differences occur. LIVVkit also provides an easily extensible framework to incorporate and analyze results of new intercomparison projects, new observation data, and new computing platforms. LIVVkit is designed for quick adaptation to additional ice sheet models via abstraction of model specific code, functions, and configurations into an ice sheet model description bundle outside the main LIVVkit structure. Ultimately, through shareable and accessible analysis output, LIVVkit is intended to help developers build confidence in their models and enhance the credibility of ice sheet models overall."

See also: "The software could help strengthen ice sheet models to provide a better basis for policy decisions."

https://eos.org/research-spotlights/open-source-tool-aims-to-boost-confidence-in-ice-sheet-models

Extract; "Despite these predictions, significant uncertainties plague computer models that simulate the behavior of ice sheets and how they interact with the global climate. To enhance the credibility of ice sheet models, Kennedy et al. are developing an open-source software package called the Land Ice Verification and Validation toolkit (LIVVkit) that analyzes models for correctness and performance."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #134 on: October 23, 2017, 05:03:08 PM »
The linked reference considers how a marine glacier retreating down a retrograde bedrock slope could become regrounded, where potential ice rises and pinning points are present (not that no such pinning point has been identified in the Thwaites gateway):

Jong, L. M., Gladstone, R. M., Galton-Fenzi, B. K., and King, M. A.: Simulated dynamic regrounding during marine ice sheet retreat, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-217, in review, 2017.

https://www.the-cryosphere-discuss.net/tc-2017-217/

Abstract. Marine terminating ice sheets are of interest due to their potential instability, making them vulnerable to rapid retreat. Modelling the evolution of glaciers and ice streams in such regions is key to understanding their possible contribution to sea level rise. The friction caused by the sliding of ice over bedrock, and the resultant shear stress, are important factors in determining the velocity of sliding ice. Many models use simple power-law expressions for the relationship between the basal shear stress and ice velocity or introduce an effective pressure dependence into the sliding relation in an ad hoc. manner. Sliding relations based on water-filled sub-glacial cavities are more physically motivated, with the overburden pressure of the ice included. Here we show that using a cavitation based sliding relation allows for the temporary regrounding of an ice shelf at a point downstream of the main grounding line of a marine ice sheet undergoing retreat across a retrograde bedrock slope. This suggests that the choice of sliding relation is especially important when modelling grounding line behaviour of regions where potential ice rises and pinning points are present and regrounding could occur.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #135 on: November 14, 2017, 05:57:55 PM »
For those who are interested, I note that ice mass loss from the Greenland Ice Sheet can stimulate ice mass loss from Antarctica via the bipolar seesaw mechanism:

Goelzer, H., Robinson, A., Seroussi, H. et al. (2017), "Recent Progress in Greenland Ice Sheet Modelling", Curr Clim Change Rep, https://doi.org/10.1007/s40641-017-0073-y


https://rd.springer.com/article/10.1007%2Fs40641-017-0073-y?utm_content=buffer00b68&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

Structured Abstract:

"Purpose of Review
This paper reviews the recent literature on numerical modelling of the dynamics of the Greenland ice sheet with the goal of providing an overview of advancements and to highlight important directions of future research. In particular, the review is focused on large-scale modelling of the ice sheet, including future projections, model parameterisations, paleo applications and coupling with models of other components of the Earth system.

Recent Findings
Data assimilation techniques have been used to improve the reliability of model simulations of the Greenland ice sheet dynamics, including more accurate initial states, more comprehensive use of remote sensing as well as paleo observations and inclusion of additional physical processes.

Summary
Modellers now leverage the increasing number of high-resolution satellite and air-borne data products to initialise ice sheet models for centennial time-scale simulations, needed for policy relevant sea-level projections. Modelling long-term past and future ice sheet evolution, which requires simplified but adequate representations of the interactions with the other components of the Earth system, has seen a steady improvement. Important developments are underway to include ice sheets in climate models that may lead to routine simulation of the fully coupled Greenland ice sheet–climate system in the coming years."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson