Arctic Sea Ice : Forum

Cryosphere => Antarctica => Topic started by: AbruptSLR on May 16, 2013, 08:44:40 PM

Title: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 16, 2013, 08:44:40 PM
I am opening this new thread both to provide some discussion of glaciology basic concepts/terminology for basic readers, and also to create a forum to illustrate situations where traditional thinking on glaciology may result in imposing unnecessary risks/uncertainties on the global society with regard to abrupt sea level rise, ASLR.

First I provide the following definitions:

Mass balance is the change in the mass of a glacier or ice body, or part thereof, over a stated span of time.  The term mass budget is a synonym. The span of time is often a year or a season. A seasonal mass balance is nearly always either a winter balance or a summer balance. The (cumulative) mass balance, b, is the sum of accumulation, c, and ablation, a (the ablation
is defined here as negative). The symbol, b (for point balances) and B (for glacierwide balances)
has traditionally been used in studies of surface mass balance of valley glaciers.  Mass balance is often treated as a rate, b or B dot.

Accumulation
1. All processes that add to the mass of the glacier.
2. The mass gained by the operation of any of the processes of sense, expressed as a positive
number.
Components:
- Snow fall (usually the most important).
- Deposition of hoar (a layer of ice crystals, usually cup-shaped and facetted, formed by
vapor transfer (sublimation followed by deposition) within dry snow beneath the snow
surface), freezing rain, solid precipitation in forms other than snow (re-sublimation
composes 5-10% of the accumulation on Ross Ice Shelf, Antarctica).
- Gain of windborne blowing snow and drifting snow
- Avalanching
- Basal freeze-on (usually beneath floating ice)
- Internal accumulation.
Note: Unless it freezes, rainfall does not constitute accumulation, and nor does the addition of
debris by avalanching, ashfall or similar processes.

Ablation
1. All processes that reduce the mass of the glacier.
2. The mass lost by the operation of any of the processes of sense, expressed as a negative
number.
Components:
- Melting (usually the most important on land-based glaciers. Melt water that re-freezes
onto another part of the glacier is not referred to as ablation).
- Calving (or, when the glacier nourishes an ice shelf, ice discharge across the grounding
line): Calving is iceberg discharge into seas or lakes; important, for example, in
Greenland and Antarctica, where approximately 50% and 90%, respectively, of all
ablation occurs via calving.
- Loss of windborne blowing snow and drifting snow
- Avalanching
- Sublimation (important, for example, in dry climates, and on blue-ice zones in Antarctica; is a function of vapor pressure)
Note the difference between a) precipitation (includes solid precipitation and rain) and surface accumulation (does not include rain).  Note, that in contrast to what is natural in dynamic glaciology and glacial geomorphology, for mass-balance purposes the glacier consists only of frozen water. Sediment carried by the glacier is deemed to be outside the glacier. Meltwater in transit or in storage, for example in supraglacial lakes or subglacial cavities, is also regarded as being outside the glacier.
b) Meltwater and Meltwater runoff (A portion of melt may refreeze; the latter refers to the
meltwater that does not refreeze)
c) Meltwater runoff and Runoff (the latter includes rain or any other source of water other than
meltwater.
d) Accumulation and Net accumulation (the latter is a balance, i.e. accumulation plus ablation.
It is identical to the mass balance in case the balance is positive. It equals zero in case the
balance is negative).

Next, I briefly discuss the attached image showing some basic glaciological concepts for a marine terminating glacier contribution ice mass to SLR.  This figure shows how mass balance "dot b" integrated over area "A" gives the input of ice mass "Q" into the upstream end of the glacial flow (also "Q" but for U integrated over the glacier's cross section), thus causing a gravity force that is partially resisted by basal (and side) friction (and an allowance for energy dissipation from internal work of deforming and internal melting of the glacial ice as it flows down hill).  The value of Qcalving is intended in this image to represent the discharge (ice volume per unit time) of ice mass contributing to SLR associated with glacial flow velocity "U".  However, for simplicity this figure does not illustrate ice mass contribution to SLR from: (a) ice surface melting and run-off; (b) basal meltwater discharge, and (c) grounding line retreat due to advective melting of the grounded ice.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 16, 2013, 10:03:56 PM
I provide the following definitions of traditional glaciology terms related to glacial movement, not because I use them very much, but because they reflect the fact that most glaciologists were trained to think about mountain glaciers; and also because these term illustrate the complex/dynamic nature of glacial ice movement including: (a) both brittle and viscous behavior; (b) surface, internal, and basal ice melting; (c) ice - geo. interaction; (d) ice - atmos. interaction; and (e) ice - ocean/lake interaction:

Traditional Terms Related to Glacial Movement:
Ablation
wastage of the glacier through sublimation, ice melting and iceberg calving.
Ablation zone
Area of a glacier in which the annual loss of ice through ablation exceeds the annual gain from precipitation.
Arête
an acute ridge of rock where two cirques abut.
Bergshrund
crevasse formed near the head of a glacier, where the mass of ice has rotated, sheared and torn itself apart in the manner of a geological fault.
Cirque, corrie or cwm
bowl shaped depression excavated by the source of a glacier.
Creep
adjustment to stress at a molecular level.
Flow
movement (of ice) in a constant direction.
Fracture
brittle failure (breaking of ice) under the stress raised when movement is too rapid to be accommodated by creep. It happens for example, as the central part of a glacier moves faster than the edges.
Horn
spire of rock, also known as a pyramidal peak, formed by the headward erosion of three or more cirques around a single mountain. It is an extreme case of an arête.
Plucking/Quarrying
where the adhesion of the ice to the rock is stronger than the cohesion of the rock, part of the rock leaves with the flowing ice.
Tarn
a post-glacial lake in a cirque.
Tunnel valley
The tunnel that is formed by hydraulic erosion of ice and rock below an ice sheet margin. The tunnel valley is what remains of it in the underlying rock when the ice sheet has melted.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 17, 2013, 03:47:38 PM
My prior two posts in this thread focus on the "Traditional" glaciological thinking that chararterized IPCC AR4; which focused on Mountain glacier contributions to SLR and assumed that any ice mass loss from both the GIS and the AIS would be offset by an increase of snowfall in Antarctica. 

This post focuses on the "Neo-Traditional" glaciological thinking that will be characterized by IPCC AR5 (based on the SOD); which acknowledges a limited amount of SLR from both the GIS and the AIS based on the "Neo-Traditional" glacier models presented here, with the first attached image showing a comparison of an Antarctic type marine glacier (such as the PIG, which rests on the seafloor) vs a Greenland type marine terminating glacier (such as the Petermann Glacier, which terminates on the seafloor but rests primarily on the land).  This first image (a re-post) also shows the importance of interaction with warm ocean water, subglacial hydrology, and the ice shelf (or ice melange).

The second image (a re-post) illustrates the importance of the advective "saline pump" action on Antarctic marine glaciers, where the warm CDW approaches the grounding line, GL, through a trough and melts some of the ice there both causing the GL to retreat, and producing relative light (low salinity) meltwater that floats up along the underside of the ice shelf (causing more ice melt) further driving the advective inflow of more warm CDW.  This process thins both the ice shelf and the downstream edge of the marine glacier causing the ice flow velocity to accelerate, resulting in more iceberg calving from the ice shelf (leading to reduced buttressing from the ice shelf).

The third image (also a re-post) from Willis and Church, 2012, summarizes many additional  features of the "neo-traditional" conceptual models for the Antarctic type marine glacier (think PIG) including: (a) the fingerprint effect of the local sea elevation dropping due to a reducing of gravitational attraction associated with ice mass loss; (b) a change in the sea ice formation and associate saltwater rejection due to the increasing icemelt water at the surface, which reduces the formation of Antarctic Bottom Water, AABW, which is warming bottom water temperatures around the world; (c) Katabatic winds that can blow fresh fallen snow into the ocean; and (d) changes in circumpolar winds and associated currents that is affecting upwelling of warm circumpolar deepwater, CDW, which is currently accelerating the advective process shown in the first, second and third attached images.  While looking at this third image, I would like to note that in cases such as the PIG with a relatively narrow glacial valley the advective process can melt glacial ice faster than the glacier can thin sufficiently to float, in which case a subglacial cavity can extend beneath the glacial ice (which rests on the seafloor by arching across the subglacial cavity from one side of the glacial valley to the other).  I would also like to know while the Thwaites Glacier gateway trough currently also allows for such arches across a subglacial cavity; in the future when the east-side (PIG side) shear/buttress action is degraded, it will no longer be possible for the glacial ice to arch across a subglacial cavity, which in equilibrium cases will require the ice shelf to be sufficiently thin to float above the submerged mount on the east-side of the Thwaites Glacier gateway (see the PIG/Thwaites 2012 - 2060 thread).

The fourth attached image presents a conceptual model of how the inherent gravitational instability of Antarctic type marine glacier can cause the ice discharge, q, to increase when the GL retreats down the negative slope of the seafloor. In the two panels of this image, a. In a steady state, the groundling-line discharge, q (red curve), which is dependent on the thickness of the grounding line, must match the balance flux, ax (blue line), which in this 2D example is the product of an accumulation rate, a, and the upstream catchment length, x.  Steady state is achieved where q=ax, which occurs at three points in this example (indicated by green, yellow and red vertical lines), which also correspond to the ice-sheet steady-state profiles shown in b.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 18, 2013, 01:50:11 AM
As the neo-traditional (AR5) methodology is based on the same principles as the traditional (AR4) methodology, therefore this, and my next series of, posts will address some basic glacier movement/motion/flux principles.

The first image provides a visual presentation for a land-based glacier of many of the terms that I presented in my second post in this thread.

The second image shows how ice flow can scour and pluck out material from the glacier bed.

The third image shows the ice particle flow trajectories for a valley glacier, together with conceptual particle velocities and zones of extending and compressive flow relative to the equilibrium line (SMB).

The fourth image shows the balance rate and total balance rate along the length of a valley glacier.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 18, 2013, 02:16:24 AM
The first image presents a summary of some of the key glacial motion response characteristics and parameters (including shear, viscosity, temperature, slope, geometry, etc) governing typical land-based ice flow.

The second image illustrates the mechanical relationships of the response shown in the first image.

The third image illustrates the relationship of shear stress and strain with ice flow velocity distribution with the depth of the land-based glacier.

The fourth image illustrates how shear stress typically varies with glacier depth and slope.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 18, 2013, 02:27:15 AM
Regarding constituative properties:

The first image shows ice velocity as a function of viscosity.

The second image shows ice viscosity as a function of shear stress.

The third images shows the temperature dependence of ice flow rate.

The fourth image shows typical creep behavior of ice.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 18, 2013, 02:36:36 AM
The first image gives a freebody of flux(discharge) relationships for a valley glacier.

The second image give the relationship of glacial weight on magma in the mantle which creates a need for the glacial isostatic adjustment as glaciers accrete and recede.

The third image gives an idea of this influence of glacial ice mass loss on nearby active faults.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 18, 2013, 06:51:12 PM
Discussion (and downloads) related to the neo-traditional (AR5) ice sheet model:  Ice Sheet System Model (ISSM) and the associated Design Analysis Kit for Optimization and Terascale Applications (DAKOTA), can be found at:
http://issm.jpl.nasa.gov/ (http://issm.jpl.nasa.gov/)

ISSM is the result of a collaboration between the Jet Propulsion Laboratory and University of California at Irvine. Its purpose is to tackle the challenge of modeling the evolution of the polar ice caps in Greenland and Antarctica.  These tools are used to focus current IceBridge investigation, and their mathematical sophistication is too detailed to present here but they include: Viscous flow, elastic response, brittle behavior, basal friction, momentum, boundary conditions, thermo-mechanical properties of ice, grounding line migration, sensitivity analyses, error investigations, etc.

While I agree that such neo-traditional models have done a good job of modeling the extant behavior of Antarctic ice sheet behavior; efforts such as those presented in the "RCM" thread by Gladestone et al 2013 indicate that still more sophisticated models capturing local/regional circulation models (which are still in developmental stages) are needed to reasonably project the probabilities of future ice mass loss scenarios.

Therefore on the topic of "risks and uncertainties", examples of factors that such ice sheet models must do a better job of before we can rely upon them to accurately estimate the risks of say the Thwaites Glacier collapsing this century include:
- A better understanding of ocean-atmosphere-ice-land interaction (as is currently being developed by the Bisicles and CISM software, see the RCM thread for discussion).  This is particularly important to capture: (a)  the accelerated warming of the CDW during the El Nino hiatus period; (b) indications of future increased upwelling around the Antarctic coasts with increased global warming (including the influence of the rapid accumulation of CH4 over the Antarctic continent, see the "Antarctic Methane" thread); (c) the strong influence of glacial meltwater on the formation of Antarctic sea ice and ocean water circulation patterns; and (d) the influence of the projected early break-up of Antarctic sea ice beginning circa 2050-2060.
- Consideration that for the past 13 years the El Nino hiatus period has limited the onshore winds being driven into the ASE from the Amundsen Sea Low (see the discussing in the "Weather and Meteorology" thread) that occurs during strong El Nino periods, which should drive considerable more warm CDW water into the troughs leading to both the PIG and Thwaites Glacier (as did occur during last period of strong El Nino events in the 1990's).
- Better modeling of the influence of subglacial hydrological systems (particularly the influence of activation of the newly identified subglacial lake within the extensive Thwaites Glacier, TG, subglacial hydrological system).
- The influence of future surface meltwater (note that the onshore wind from an Amundsen Sea Low system is warm can could promote extensive surface melting in the TG basin within the next ten years) in the ASE area carrying heat from the surface directly down to the basal zone through ice fissures, thus reducing basal friction and increasing the influence of the subglacial hydrological system on ice flow.
- The influence of the recently identified high geothermal heat input into the base of Byrd Subglacial Basin.
- Possible increase of snowfall at the upstream end of the TG prior to 2050 that could maintain steep ice surface gradients that could help to trigger a "Jacobshaven Effect" type of rapid ice mass loss in the TG due to local gravitational instabilities of the calving surface, if future buttressing action of the TG ice shelf is reduced (see the PIG/Thwaites 2012 to 2060" thread discussion).
- Possible future isostatic rebound triggered earthquakes (see the images in my last post), as most ice is lost from the ASE glaciers.
- A probably loss of boundary restraint on the east (PIG side) of the Thwaites Gateway (possibly due to advective processes through the Thwaites trough, see the PIG/Thwaites 2012 to 2060 thread), resulting in a rapid acceleration of the ice flux from the TG.
- Reductions in ice viscosity, and an increase in internal ice melting, as the ice streams accelerate due to various mechanisms (including enthapy modeling).
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on June 22, 2013, 10:13:52 PM
The accompanying three images come from:

Mass Balance of West Antarctic Ice Sheet from ICESat Measurements
By H. Jay Zwally, Jun Li, John Robbins, Jack L. Saba, Donghui Yi
WAIS Workshop, Colorado, September 22, 2011

The first image shows that ICESat measures changes in surface elevation, and the figure illustrates the considerations required to determine the quantity of interest: ice Mass Balance.

The second image shows ICESat measurement for the West Antarctic for the period from 2003 to 2008.

The third image shows the specific considerations that must be considered when interpreting the ICESat measurement, including: firn compaction, bed elevation changes, ice elevation changes, ablation, and accumulation.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: icebgone on June 23, 2013, 12:21:29 AM
Excellent post ASLR.  PIG and TG have potential to destabilize quickly if conditions warrant.  In-depth measurements and analysis point to a need for installation of long-term self-sustaining field equipment in the critical zones.  I understand better now why you fear their early collapse before the end of this century.  The more so if we begin experiencing strong El Nino events before the end of this decade.  I fear any strong El Nino because it can create havoc above and beyond what has already been happening.  My glaciology knowledge is a bit old but still useful.  I wonder if it would be possible to reduce development time and money by borrowing existing measurements and technologies from pavement strain experiments used by Engineers to build highways and bridges in various environments? 
There is nothing like watching and hearing glaciers flow.  I recommend everyone to do this at least once in their life.  Each glacier also has a unique geochemistry taking place between the glacier, basal objects carried by the glacier and the underlying rock on which it flows.  It is difficult for me to believe that within a few lifetimes glaciers, the insatiable ice dragons of the earth, could be destroyed by the fires of man is sad indeed.     
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on June 23, 2013, 04:11:57 AM
icebgone,

Thank you for your thoughful words.  I trust that the Antarctic field researchers make the best use possible of their limited research funds, and that they continue make excellent progress.  My biggest concern is not with the researchers themselves but with the administrators who are reticient to acknowlege the true risks of rapid sea level rise contributions from Antarctica; and that by the time sufficient data is accumulated to overcome this reticience that thermal inertia will result in unacceptable levels of SLR.  I sincerely hope that I am wrong; but I hope to shine what limited daylight that I can on the relevant new findings (most of which seem to be supporting my concerns) as I find them, in the hope that some decision makers will that this matter more seriously in the future.

Best,
ASLR
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: Gray-Wolf on June 24, 2013, 02:09:43 PM
My biggest concern remains the stability of Ross? Comparable periods of earth history with similar CO2/temps show us that we can expect to lose ross at some point and see East and West Antarctic become separated by a channel from Ross to Weddell?

I remember some research (BAS?) in the early noughties that show a 'ruck' in the ice around Ross as if the momentum of a fast flowing ice mass was interupted by the formation of the 'buttress' that ross now is. I have to worry that this structure shows us what will happen to the feed glaciers for the embayment once that 'buttress' has been removed?

I am awaiting the next major calve from ross and have been eyeing the largr fissure from Roosevelt Island to the central section of the shelf for over 4 years now. when i first became interested in this feature I contact Bob Grumbine to see what he knew and he informed me that they had just installed sensors along the length of the feature to monitor changes. Currently ,were this to fail, it would be a berg 4 times larger than the last major loss from the shelf.

With the Arrival of warm bottom waters into the area ( worked around from PIG) I also worry that the 'grounding line' will now rapidly retreat putting more of a strain onto this feature and lead to it's early demise?
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on June 24, 2013, 05:10:16 PM
Gray-Wolf,

I believe that your concerns are all too real as I discuss in the "FRIS - RIS 2012 to 2060" thread.  Certainly, I believe that calving from the Ross Ice Shelf, RIS, will probably accelerate as the face of the ice shelf thins over the coming decades; and I believe that around 2050 to 2060 warm circumpolar deep water, CDW, will enter the Ross embayment, which would change RSI from a cold ice shelf to a warm ice shelf; which I believe will lead to the collapse of the RIS before the end of the century; which in turn will accelerate ice mass loss (VAF) to contribute to SLR.

That said, I still believe that the situations around the Amundsen Sea Embayment, ASE, and the Filchner Ronne Ice Shelf, FRIS, are even more critical than that for the RIS; but in any case it is likely that all of these areas may contribute to the collapse of the WAIS in rapid sequence (in one order or another) to each other.

Best,
ASLR
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on July 11, 2013, 01:47:06 AM
For anyone who wants to know some of the latest thinking about Antarctic firn densification, the following reference provided input into the Ice2sea program:

An improved semi-empirical model for the densification of Antarctic firn
by: S. R. M. Ligtenberg, M. M. Helsen, and M. R. van den Broeke; The Cryosphere, 5, 809–819, 2011; www.the-cryosphere.net/5/809/2011/; (http://www.the-cryosphere.net/5/809/2011/;) doi:10.5194/tc-5-809-2011

"Abstract. A firn densification model is presented that simulates steady-state Antarctic firn density profiles, as well as the temporal evolution of firn density and surface height. The model uses an improved firn densification expression that is tuned to fit depth-density observations. Liquid water processes (meltwater percolation, retention and refreezing) are also included. Two applications are presented. First, the steady-state model version is used to simulate the strong spatial variability in firn layer thickness across the Antarctic ice sheet. Second, the time-dependent model is run for 3 Antarctic locations with different climate conditions. Surface height changes are caused by a combination of accumulation, melting and firn densification processes. On all 3 locations, an upward trend of the surface during autumn, winter and spring is present, while during summer there is a more rapid lowering of the surface. Accumulation and (if present) melt introduce large inter-annual variability in surface height trends, possibly hiding ice dynamical thickening and thinning."
Title: Re: better GIA models for Antarctica
Post by: sidd on July 18, 2013, 08:03:21 PM
http://www.the-cryosphere-discuss.net/7/3497/2013/tcd-7-3497-2013.html (http://www.the-cryosphere-discuss.net/7/3497/2013/tcd-7-3497-2013.html)

" A range of different GRACE gravity models were evaluated, as well as a new ICESat surface height trend map computed using an overlapping footprint approach. When the GIA models created from the combination approach were compared to in-situ GPS ground station displacements, the vertical rates estimated showed consistently better agreement than existing GIA models. In addition, the new empirically derived GIA rates suggest the presence of strong uplift in the Amundsen Sea and Philippi/Denman sectors, as well as subsidence in large parts of East Antarctica."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on July 18, 2013, 11:33:27 PM
Sidd,

This is a great find and the conclusions for the ASE by Ligtenberg et al 2013 seem to match those present by Groh et al, using preliminary GPS data in:

An investigation of Glacial Isostatic Adjustment over the Amundsen Sea sector, West Antarctica
by: A. Groh; H. Ewert, M. Scheinert, M. Fritsche, A. Rülke, A. Richter, R. Rosenau, R. Dietrich
http://dx.doi.org/10.1016/j.gloplacha.2012.08.001 (http://dx.doi.org/10.1016/j.gloplacha.2012.08.001)

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on July 20, 2013, 03:10:08 PM
The following information about calving is taken from:

http://en.wikipedia.org/wiki/Ice_calving (http://en.wikipedia.org/wiki/Ice_calving)

"Ice calving, also known as glacier calving or iceberg calving, is the breaking off of chunks of ice at the edge of a glacier. It is a form of ice ablation or ice disruption. It is the sudden release and breaking away of a mass of ice from a glacier, iceberg, ice front, ice shelf, or crevasse. The ice that breaks away can be classified as an iceberg, but may also be a growler, bergy bit, or a crevasse wall breakaway.
Calving of glaciers is often accompanied by a loud cracking or booming sound before blocks of ice up to 60 metres (200 ft) high break loose and crash into the water. The entry of the ice into the water causes large, and often hazardous waves. The waves formed in locations like Johns Hopkins Glacier can be so large that boats cannot approach closer than 3 kilometres (1.9 mi). These events have become major tourist attractions in locations such as Alaska.
Many glaciers terminate at oceans or freshwater lakes which results naturally with the calving of large numbers of icebergs. Calving of Greenland's glaciers produce 12,000 to 15,000 icebergs each year alone.
Calving of ice shelves is usually preceded by a rift. These events are not often observed.
Etymologically, calving is cognatic with calving as in birthing a calf.

Causes:

It is useful to classify causes of calving into first, second, and third order processes. First order processes are responsible for the overall rate of calving at the glacier scale. The first order cause of calving is longitudinal stretching, which controls the formation of crevasses. When crevasses penetrate the full thickness of the ice, calving will occur. Longitudinal stretching is controlled by friction at the base and edges of the glacier, glacier geometry and water pressure at the bed. These factors, therefore, exert the primary control on calving rate.
Second and third order calving processes can be considered to be superimposed on the first order process above, and control the occurrence of individual calving events, rather than the overall rate. Melting at the waterline is an important second order calving process as it undercuts the subaerial ice, leading to collapse. Other second order processes include tidal and seismic events, buoyant forces and melt water wedging.
When calving occurs due to waterline melting, only the subaerial part of the glacier will calve, leaving a submerged 'foot'. Thus, a third order process is defined, whereby upward buoyant forces cause this ice foot to break off and emerge at the surface. This process is extremely dangerous, as it has been known to occur, without warning, up to 300m from the glacier terminus.

Calving Law:

Though many factors that contribute to calving have been identified, a reliable predictive mathematical formula is still under development. Data is currently being assembled from ice shelves in Antarctica and Greenland to help establish a 'calving law'. Variables used in models include properties of the ice such as thickness, density, temperature, c-axis fabric, impurity loading, though 'ice front normal spreading stress', is likely the most important variable, however it is usually not measured.
There are currently several concepts upon which to base a predictive law. One theory states that the calving rate is primarily a function of the ratio of tensile stress to vertical compressive stress, i.e., the calving rate is a function of the ratio of the largest to smallest principle stress.  Another theory, based on preliminary research, shows that the calving rate increases as a power of the spreading rate near the calving front.

Major Calving Events:

Filchner-Ronne Ice Shelf
In October, 1988, the A-38 iceberg broke away from the Filchner-Ronne Ice Shelf. It was about 150 km x 50 km, a mass of ice bigger than the area of Delaware. A second calving occurred in May 2000 and created an iceberg 167 km x 32 km.

Amery Ice Shelf
A major calving event occurred in 1962 to 1963. Currently, there is a section at the front of the shelf referred to as the 'loose tooth'. This section, about 30 km by 30 km is moving at about 12 meters per day and is expected to eventually calve away."

This post may help readers to better understand the recent iceberg calving event for the Pine Island Ice Shelf, PIIS; and to better understand the risks of potential future calving events, particularly for PIIS and the Filchner Ice Shelf. Understanding ice calving is fundamental to better understanding the risks of abrupt SLR.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on August 20, 2013, 08:13:18 PM
For those interested in the historical glacial data that the National Snow and Ice Data Center compiles on Antarctica please check-out their new beta interface to this data at the following weblink:

http://nsidc.org/agdc/acap/ (http://nsidc.org/agdc/acap/)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on August 21, 2013, 06:47:53 PM
The following reference concludes that "… basal sliding is widespread beneath the Antarctic Ice Sheet"; which implies that only a very small fraction of the AIS needs to warm-up in order to significantly accelerate ice mass loss from the AIS (particularly the WAIS); which could occur either due to increased basal friction as the ice velocity increases and/or due to an increase in basal meltwater volume:

http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20125/abstract (http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20125/abstract)

Inversion of basal friction in Antarctica using exact and incomplete adjoints of a higher-order model; M. Morlighem, H. Seroussi, E. Larour, & E. Rignot; 2013; Journal of Geophysical Research: Earth Surface; DOI: 10.1002/jgrf.20125

Abstract:
"Basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics. Here, we use a complete map of surface velocity of the Antarctic Ice Sheet to infer the basal friction over the entire continent by combining these observations with a three-dimensional, thermo-mechanical, higher-order ice sheet numerical model from the Ice Sheet System Model (ISSM) open source software. We demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of the cost function and of the scheme of regularization, at a spatial resolution as high as 3 km along the coastline. We compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation. The results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80% of the ice sheet. The basal friction coefficient, which relates basal friction to basal velocity, is however significantly heterogeneous: it is low on fast moving ice and high near topographic divides. Areas with low values extend far out into the interior, along glacier and ice stream tributaries, almost to the flanks of topographic divides, suggesting that basal sliding is widespread beneath the Antarctic Ice Sheet."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on August 22, 2013, 01:18:36 AM
The linked reference on the high sensitivity of tidewater outlet glacier dynamics to shape, while theoretical, is of high importance for glaciers such as the Thwaites Glacier, that meets all of the papers criteria for high instability.  The link provides a free pdf:


http://www.the-cryosphere.net/7/1007/2013/tc-7-1007-2013.html (http://www.the-cryosphere.net/7/1007/2013/tc-7-1007-2013.html)


Enderlin, E. M., Howat, I. M., and Vieli, A.: High sensitivity of tidewater outlet glacier dynamics to shape, The Cryosphere, 7, 1007-1015, doi:10.5194/tc-7-1007-2013, 2013

"Abstract. Variability in tidewater outlet glacier behavior under similar external forcing has been attributed to differences in outlet shape (i.e., bed elevation and width), but this dependence has not been investigated in detail. Here we use a numerical ice flow model to show that the dynamics of tidewater outlet glaciers under external forcing are highly sensitive to width and bed topography. Our sensitivity tests indicate that for glaciers with similar discharge, the trunks of wider glaciers and those grounded over deeper basal depressions tend to be closer to flotation, so that less dynamically induced thinning results in rapid, unstable retreat following a perturbation. The lag time between the onset of the perturbation and unstable retreat varies with outlet shape, which may help explain intra-regional variability in tidewater outlet glacier behavior. Further, because the perturbation response is dependent on the thickness relative to flotation, varying the bed topography within the range of observational uncertainty can result in either stable or unstable retreat due to the same perturbation. Thus, extreme care must be taken when interpreting the future behavior of actual glacier systems using numerical ice flow models that are not accompanied by comprehensive sensitivity analyses."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on August 23, 2013, 08:15:09 PM
The following linked reference provides insight about glacial earthquakes of the Whillans Ice Stream:

http://onlinelibrary.wiley.com/doi/10.1002/grl.50130/abstract (http://onlinelibrary.wiley.com/doi/10.1002/grl.50130/abstract)


Winberry J. P., S. Anandakrishnan, D. A. Wiens, and R. B. Alley (2013), Nucleation and seismic tremor associated with the glacial earthquakes of Whillans Ice Stream, Antarctica, Geophys. Res. Lett., 40, 312–315, doi:10.1002/grl.50130.


Abstract:

"The ability to monitor transient motion along faults is critical to improving our ability to understand many natural phenomena such as landslides and earthquakes. Here, we usedata from a GPS and seismometer network that were deployed to monitor the regularly repeating glacial earthquakes of Whillans Ice Stream, West Antarctica to show that a unique pattern of precursory slip precedes complete rupture along the bed of the ice stream. Additionally, we show that rupture can be independently tracked by increased levels of microseismic activity, including harmonic tremor, that are coincident with the onset of slip at any location, thus providing a remote means of monitoring stress and rupture propagation during the glacial earthquakes."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on August 23, 2013, 11:00:26 PM
I attach fig 5 from Morlighem (2013). The blue areas indicate basal sliding, extending fingers deep into the ice. To me this implies that ice melt at the edge will be replaced with ice from the thinning interior. And we have seen surface melt quite deep into Antarctica also.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on August 23, 2013, 11:05:23 PM
Enderlin (2013) notes :

"... glaciers with the widest outlets reach flotation above the basal depression, triggering
a much larger retreat and discharge increase."

"Further, we suggest that similar sensitivity analyses should be completed using two- or three-dimensional models in order to assess the influence of glacier shape on grounding line stability for glaciers and ice streams with strong lateral convergence along their trunks."

"... glaciers with wider steady-state grounding lines and those with deeper basal depressions will tend to be closer to flotation in the depression than narrower or shallow glaciers, and thus less dynamic thinning will be required to bring the ice within the depression to flotation."

Each of these sentences reminded me strongly of Thwaites.

sidd

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on August 24, 2013, 05:34:18 PM
Sidd,


The Morlighem et al (2013) figure that you posted, and the Enderlin et al (2013) quotes that you cite, bring into sharp focus the seriousness of the risk for ASLR that the world faces circa 2050 by which time the ocean water advection and the acceleration of basal sliding has had a chance to destablize large portions of the WAIS.  I find it particularly disturbing that some of the dark blue basal sliding fingers are starting to converge from different sides of the WAIS (eg the PIG fingers with those from the Weddell Sea basin, and the Ferrigno finger with the PIG fingers).

Best,
ASLR
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on August 24, 2013, 07:32:28 PM
Agreed on convergence of the sliding streams. For some detail compare surface velocities in the attached figure 1 from  Rignot(2008, doi:10.1038/ngeo102).

In addition to the ones pointed out already, I note the Siple coast area streams reaching towards  Thwaites under Mercer, Whillans, Kamb, Bindschadler, MacAyeal and Echelmeyer. The fingers behind Byrd and David facing the Siple coast shock me as they extend behind the Transantarctic ridge. Cook, Ninnis, Moscow U and Totten are disturbing. I don't even want to look at Amery.

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on August 25, 2013, 02:57:03 AM
Sidd,

When I think about the 118.1k years ago spike in sea level that O'Leary et al 2013 provide evidence for (see the discussion in the "Timing" thread and the "Paleo-evidence" thread); I can only conclude that if O'Leary et al 2013 are correct then the portions of the EAIS that you highlight in you post must be primed for collapse shortly after the collapse of the WAIS otherwise it would be impossible to reach the 9m eustatic SLR value cited by O'Leary et al 2013 in the timeframe that they discuss.  I am not certain whether an equable climate would be needed to quickly trigger the marine-terminating portions of the EAIS that you cite; but it is disturbing to think about this possibility (such as a 1m surge of sea level by 2055 due to a partial collapse of the WAIS forcing warm Pacific ocean water into the Arctic Ocean, thus rapidly extending the ice free season for the Arctic Ocean, leading to a rapid build-up of specific humidity in the Arctic atmosphere, leading to a rapid transition to an equable climate, leading to a rapid collapse of the marine-terminating glaciers/ice streams shortly after 2100.

I do not mean to sound alarmist, but I do not have any confidence that because the current models do not show a meaninful risk of such a scenario, that society is actual safe from such an occurrence.

Best,
ASLR
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: nukefix on August 25, 2013, 04:23:08 PM
Agreed on convergence of the sliding streams. For some detail compare surface velocities in the attached figure 1 from  Rignot(2008, doi:10.1038/ngeo102).
Rignot's group has produced a new map with much improved coverage, see here:

http://nsidc.org/data/nsidc-0484.html (http://nsidc.org/data/nsidc-0484.html)
http://www.sciencemag.org/content/333/6048/1427 (http://www.sciencemag.org/content/333/6048/1427)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on August 25, 2013, 09:30:02 PM
As nukefix points out, Rignot(2011) has a better picture, which I have attached
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 01, 2013, 12:15:55 AM
The following linked reference provides insight into the glaciofluvial processes in the Garwood Valley, Antarctica:

http://gsabulletin.gsapubs.org/content/125/9-10/1484.abstract (http://gsabulletin.gsapubs.org/content/125/9-10/1484.abstract)


Garwood Valley, Antarctica: A new record of Last Glacial Maximum to Holocene glaciofluvial processes in the McMurdo Dry Valleys;
by: Joseph S. Levy, Andrew G. Fountain, Jim E. O’Connor, Kathy A. Welch and W. Berry Lyons; June 7, 2013, doi: 10.1130/B30783.1 v. 125 no. 9-10 p. 1484-1502


"Abstract
We document the age and extent of late Quaternary glaciofluvial processes in Garwood Valley, McMurdo Dry Valleys, Antarctica, using mapping, stratigraphy, geochronology, and geochemical analysis of sedimentary and ice deposits. Geomorphic and stratigraphic evidence indicates damming of the valley at its Ross Sea outlet by the expanded Ross Sea ice sheet during the Last Glacial Maximum. Damming resulted in development of a proglacial lake in Garwood Valley that persisted from late Pleistocene to mid-Holocene time, and in the formation of a multilevel delta complex that overlies intact, supraglacial till and buried glacier ice detached from the Ross Sea ice sheet. Radiocarbon dating of delta deposits and inferred relationships between paleolake level and Ross Sea ice sheet grounding line positions indicate that the Ross Sea ice sheet advanced north of Garwood Valley at ca. 21.5 ka and retreated south of the valley between 7.3 and 5.5 ka. Buried ice remaining in Garwood Valley has a similar geochemical fingerprint to grounded Ross Sea ice sheet material elsewhere in the southern Dry Valleys. The sedimentary sequence in Garwood Valley preserves evidence of glaciofluvial interactions and climate-driven hydrological activity from the end of the Pleistocene through the mid-Holocene, making it an unusually complete record of climate activity and paleoenvironmental conditions from the terrestrial Antarctic."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 03, 2013, 05:16:58 PM
The following linked reference (with a free pdf) discusses the effects of entrained debris on the basal sliding stability of glaciers:

http://www3.geosc.psu.edu/~cjm38/papers_talks/ZoetJGR2013.pdf (http://www3.geosc.psu.edu/~cjm38/papers_talks/ZoetJGR2013.pdf)


Zoet, L. K., B. Carpenter, M. Scuderi, R. B. Alley, S. Anandakrishnan, C. Marone, and M. Jackson (2013), The effects of entrained debris on the basal sliding stability of a glacier, J. Geophys. Earth Surf., 118, doi:10.1002/jgrf.20052.

Abstract:
"New laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick-slip motion of debris-laden ice over rock, with implications for glacier behavior. Friction between a rock substrate and clasts in ice generates heat, which melts nearby ice to produce lubricating water. An increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production. Unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase. Low initial lubrication can result from cold or drained conditions, whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads.  Strengthening of the interface (healing) during “stick” intervals between slip events occurs primarily through meltwater refreezing. When healing and unstable sliding are taken together, the experiments reported here suggest that stick-slip behavior is common from motion of debris-laden glacier ice over bedrock."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 03, 2013, 05:24:44 PM
The linked reference discusses accelerated subglacial erosion in response to stick-slip motion:


http://geology.gsapubs.org/content/41/2/159.short (http://geology.gsapubs.org/content/41/2/159.short)



Accelerated subglacial erosion in response to stick-slip motion; by: L.K. Zoet, R.B. Alley, S. Anandakrishnan and K. Christianson; 2012; Geology; v. 41 no. 2 p. 159-162; doi: 10.1130/G33624.1


"Abstract
Subglacial stick-slip motion speeds erosion by hydrofracturing and in other ways, as determined from analysis of the growing body of field data. Microearthquake monitoring commonly detects subglacial earthquakes, likely mostly from stick-slip motion of debris-laden ice over bedrock. Source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps. We calculate that the resulting expansion of higher-pressure water in nearby cracks promotes hydrofracturing, with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior. This mechanism also helps generate the step-like topography favoring block plucking. This stick-slip glacier-erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress, ice-flow velocity, and water supply."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 05, 2013, 12:52:01 AM
The following link presents research that West Antarctica began to glaciate about 32 million years ago, when more of this area was at a higher elevation than at the present time:

http://phys.org/news/2013-09-west-antarctica-ice-sheet-million.html (http://phys.org/news/2013-09-west-antarctica-ice-sheet-million.html)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 07, 2013, 09:27:59 PM
The following abstracts are taken from the proceedings of the following IGSOC sponsored symposia, and they all relate to fundamental glaciology and the mechanics of associated ice shelves:


International Symposium on Changes in Glaciers and Ice Sheets: observations, modelling and environmental interactions; 28 July–2 August; Beijing, China; Contact: Secretary General, International Glaciological Society


http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htm (http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htm)


A particle-based simulation model for glacier dynamics
J.C. MOORE, J.A. ÅSTRÖM, T. RIIKILÄ, T. TALLINEN, T. ZWINGER, D. BENN, J. TIMONEN
Corresponding author: J.C. MOORE
Corresponding author e-mail:
"A particle-based computer simulation model has been developed with the objective of investigating the dynamics of glaciers. To describe ice dynamics in a realistic fashion, a model must include elastic deformation, granular flow and fracture of large ice bodies. In spite of several simplifications, which include restriction to two dimenions only and simplified rheology for ice and water, the present model is able to reproduce iceberg and small debris size distributions observed in calving. The observed size distributions are well approximated by universal scaling laws. On a moderate slope a large ice-block (here 50 m high and 200 m long) is stable as long there is enough friction with the substrate. This is a quiescent state. At a critical extent of frictional contact there is an onset of global sliding and the model glacier begin to surge. During the surge, the glacier is fragmented into small pieces. In this case the fragment size distribution has the shape that is typically observed for grinding processes."


A new calving law based on continuous damage and fracture mechanics
J. KRUG, J. WEISS, G. DURAND, O. GAGLIARDINI
Corresponding author: J. Krug
Corresponding author e-mail: jean.krug@ujf-grenoble.fr
"A number of studies have shown that mechanical ice loss through calving is responsible for most of the ice discharge from glaciers and ice sheets. However calving processes are complex and still poorly understood. Representation of calving in ice-sheet models is still limited and the estimation of future ice loss from the Greenland and Antarctic ice sheets is therefore inaccurate. This is the reason why the last IPPC report asked for a better understanding and representation of calving processes. Several approaches are necessary to represent at the same time the slow deformation of the ice, the initiation of crevasses and their rapid propagation preceding the calving event. First, the effect of damage and small-scale fracturing on this slow viscous deformation can be represented by continuous damage mechanics (CDM). CDM describes the evolution of damage in the ice from a state in which the ice has no defect to the appearance of a macro-crack. This evolution depends on the stress field and is advected with the flow of ice. Second, the fast propagation of pre-existing crevasses into the media can be satisfyingly described using linear elastic fracture mechanics (LEFM) for which ice is considered as an elastic medium. This approach allows us to deal with the stress concentration at the tip of the crack and so differs from the traditionally used Nye’s criterion. Together, they may propose a complete and versatile calving law that covers first-order processes (related to longitudinal stretching and surface velocity gradients) as well as second-order processes occurring at the glacier front (subaqueous melting, force imbalance, etc.). These two approaches are combined and implemented into the Elmer/Ice full-Stokes ice-flow model. An initial grounded terminated glacier is perturbed by an increase in frontal subaqueous melting. This process, called undercutting, results in a block of ice overhanging the sea, followed by the calving of the aerial part. Strength of the model to various physical parameters is tested, such as the ratio between water depth and ice thickness, the inlet velocity and the temperature of the ice. The shape of the bedrock is also investigated."



Dynamics of meltwater plumes under ice shelves: frictional geostrophy and melt-channelling instability
Felix NG, Adrian JENKINS
Corresponding author: Felix Ng
Corresponding author e-mail: f.ng@sheffield.ac.uk
"The flow of buoyant meltwater plumes under several ice shelves has been reproduced in numerical simulations that represent such plumes as a well-mixed layer, and researchers have begun using these simulations to explore how ocean warming could cause irreversible melting and retreat of ice shelves. Here we present a new mathematical theory for the coupled ice-shelf–meltwater-plume system to illuminate two aspects of this problem. The first aspect concerns the two-dimensional flow field of the plume and how it determines the spatial distribution of the rate of sub-ice-shelf basal melt. By starting with the equations used in the simulations, we derive a simplified model of plume physics that explains how sub-shelf friction and Coriolis force conspire to govern the plume water flux and deflection angle. This leads us to explain why the plumes always transition from friction-dominated flow near the grounding line towards geostrophic flow farther out under the shelf. This theory identifies plume buoyancy, the Coriolis parameter and shelf-base slope as key factors of the transition and elucidates how they impact the sub-shelf melt rate. The second aspect of the problem concerns the origin of basal melt channels that have been observed under many ice shelves, including the floating tongue of Petermann Glacier in Greenland and the ice shelf fed by Pine Island Glacier in Antarctica. By conducting a linear stability analysis of our model, we find that with typical parameters the coupled shelf–plume system is unstable to perturbations so that incipient channels with spacing of the order of kilometres form across the ice-shelf base. This analysis establishes how model parameters control the incipient-channel spacing and orientation and pinpoints the mechanisms behind the instability. These results inform future studies that seek to understand how fully developed basal channels affect how an ice shelf evolves."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 07, 2013, 10:57:04 PM
The following abstracts are taken from the proceedings of the following IGSOC sponsored symposia, and they all relate remote sensing of information needed for fundamental glaciology in Antarctica:

International Symposium on Changes in Glaciers and Ice Sheets: observations, modelling and environmental interactions; 28 July–2 August; Beijing, China; Contact: Secretary General, International Glaciological Society


http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htm (http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htm)

Mass balance of Antarctic ice sheet 1992–2008 from ERS and ICESat: gains exceed losses
H. Jay ZWALLY, Jun LI, John ROBBINS, Jack L. SABA, Donghui YI, Anita BRENNER
Corresponding author: H. Jay ZWALLY
Corresponding author e-mail: zwally@icesat2.gsfc.nasa.gov
"During 2003–2008, the mass gain of the Antarctic ice sheet from snow accumulation exceeded the loss from ice discharge by 73±23 Gt a–1 (3.7% of input), as derived from ICESat laser altimetry. The 131 Gt a–1 gain in East Antarctica (EA) and the 70 Gt a–1 gain in four drainage systems (DS) of West Antarctic (WA2) exceeded combined losses of 98 Gt a–1 from three coastal DS of West Antarctic (WA1) and 28 Gt a–1 from the Antarctic Peninsula (AP). Re-analysis of ERS radar-altimeter data, including a new post-glacial-rebound correction, indicates an even larger overall gain of 120 ± 51 Gt a–1 during 1992–2001. In WA2 and EA, persistent dynamic thickening (deficiency of ice flow relative to long-term accumulation) contributed more than 200 Gt a–1 to the net positive balance in both periods. Consistent with observed outlet-glacier accelerations, loss increases of 38 Gt a–1 in WA1 and 21 Gt a–1 in AP from increased dynamic thinning dominated a gain increase of 9 Gt a–1 from positive accumulation anomalies in WA1 and AP. These decadal-scale changes are small relative to the long-term dynamic thickening in EA and WA2, which may buffer additional dynamic thinning for several decades."

Modeling dynamic thickening in East Antarctica as observed from ICESat
Weili WANG, H. Jay ZWALLY, Jun LI
Corresponding author: Weili WANG
Corresponding author e-mail: weili.wang@nasa.gov
"Mass changes of the Antarctic ice sheet derived from ICESat laser altimetry show that during 2003–08 mass gains from snow accumulation exceeded losses from ice discharge by 73 Gt a–1 (0.20 mm a–1 sea level depletion). Results from ERS radar altimetry give a similar net gain of 120 Gt a–1 for 1992–2001. In East Antarctica and four West Antarctic drainage systems, most of the net mass gain is caused by persistent dynamic thickening (excess of long-term accumulation relative to ice flow) at a rate of 207 Gt a–1, and not by contemporaneous increases in snowfall. To investigate the dynamic thickening rate, we apply a 3-D ice-sheet model to the Antarctic ice sheet for the sensitivity experiments with climate change. The model results indicate that the East Antarctic ice sheet has been growing due to increased snowfall after the last ice age. The modeled thickening rate near Vostok is 2.5 cm a–1 for the present time, which is consistent with the observations from ICESat and ERS data. Overall, the model and observations indicate a long-term mass gain for East Antarctica and the interior of West Antarctic, which has been offsetting dynamic losses that have increased in the Antarctic Peninsula and West Antarctica during the last two decades."



Improved Antarctic surface mass-balance remote sensing using ASCAT
Alexander D. FRASER, Simon WOTHERSPOON, Hiroyuki ENOMOTO, Neal W. YOUNG
Corresponding author: Alexander D. Fraser
Corresponding author e-mail: adfraser@utas.edu.au
"Large-scale distribution of Antarctic surface mass balance (SMB) is currently poorly understood. High-quality in situ measurements of SMB are sparse, particularly in the interior of the continent. Remote sensing can be used to guide interpolation between in situ measurements. Previously, passive microwave polarization ratio, which is sensitive to the density of horizons of different dielectric properties in the upper snowpack (a proxy for SMB), has been used to guide interpolation of SMB points in Antarctica. We present evidence that maps of alternative parameters may be more suitable maps upon which to base interpolated fields. These maps come from the EUMETSAT Advanced Scatterometer (ASCAT) C-band scatterometer, which was launched in 2007. In particular, we use the ‘A’ (isotropic component of backscatter, sensitive to grain size within the C-band penetration depth of ~20 m) and ‘B’ (linear component of backscatter dependence on incidence angle, sensitive to grain-size profile). Importantly, these maps are sensitive to recently mapped extensive areas of surface wind glaze, which are areas of near-zero net accumulation and thus are less prone to overestimation of SMB compared with earlier large-scale SMB maps. A further focus of this work is a comparison of several statistical interpolation methods, including a careful consideration of the statistical treatment of negative SMB values. A primary output of this work is a new SMB map of the Antarctic continent based on these improved fields."

Synoptic-timescale observations of Antarctic snowfall/wind redistribution events from scatterometer data
Alexander D. FRASER, Melissa A. NIGRO, John CASSANO, Neal W. YOUNG, Benoit LEGRESY, Hiroyuki ENOMOTO
Corresponding author: Alexander D. Fraser
Corresponding author e-mail: adfraser@utas.edu.au
"The orbit and swath configuration of the EUMETSAT Advanced Scatterometer (ASCAT) instrument gives C-band backscatter measurements from a wide range of azimuth and incidence angles over most of the Antarctic continent. A 5 day orbital subcycle combined with this excellent observation angle diversity means that complete maps of accumulation-sensitive parameters can be produced on a 5 day basis. Analysis of time series of these parameters reveals several abrupt changes in localized regions, particularly in the ‘A’ parameter (isotropic component of backscatter, which is sensitive to snow grain size) and the ‘M2’ parameter (the magnitude of the second-order Fourier term describing the near-bi-sinusoidal azimuthal response, which is an indicator of the presence/magnitude of sastrugi/other surface microrelief). Using 15 km grid spacing Antarctic Mesoscale Prediction System (AMPS) numerical weather prediction model data, we show these abrupt changes in the ‘A’ and ‘M2’ parameters are associated with snowfall events arising from incursions of air from lower latitudes. Both the ‘A’ and ‘M2’ parameters show a complex response to precipitation events, with both the sign and magnitude of the response depending on wind reworking/redistribution. This observation of changes in near-surface snowpack conditions complements recent results from other authors using GRACE-derived gravity and CloudSat-derived snowfall observations to detect similar snowfall events in East Antarctica."



How accurately can radar altimetry contribute to estimate the Antarctic ice sheet volume and mass balance?
B. LEGRÉSY, M. HORWATH, S.R.M. LIGTENBERG, M.R. VAN DEN BROEKE, F. BLAREL
Corresponding author: B. Legresy
Corresponding author e-mail: benoit.legresy@legos.obs-mip.fr
"Knowing the interannual variations of the Antarctic ice sheet net snow accumulation, or surface mass balance (SMB), is essential for analyzing and interpreting present-day observations. For example, accumulation events like the one in East Antarctica in 2009 challenge our ability to interpret observed decadal-scale trends in terms of long-term changes versus natural fluctuations. We developed a higher accuracy time series of radar altimetry with ERS2 and Envisat data from 1995 to 2010. We will present the surface topography variations, the internal error levels for both altimeters and the radar echo and ground miss-repeat corrections made. We show that a different echo correction has to be applied to ERS2 and Envisat as the firn changed in between the two periods of observation. Therefore the possibility to correct the radar altimetry data for echo shape changes from the echo shape is limited, limiting the attainable accuracy of volume change estimates. We illustrate the great potential and limitations of radar altimetry by internal assessment and by comparing with other changes estimates as temporal gravity variations and atmospheric modeling of firn densification. We evaluate the limits of techniques depending on the temporal and spatial scales of interest. SMB variations cause changes in the firn density structure, which need to be accounted for when converting volume trends from satellite altimetry into mass trends. Recent assessments of SMB and firn volume variations mainly rely on atmospheric modeling and firn densification modeling. The modeling results need observational validation, which has been limited until now. Geodetic observations by satellite altimetry and satellite gravimetry reflect interannual firn volume and mass changes, among other signals like changes in ice-flow dynamics. Therefore, these observations provide a means of validating modeling results over the observational period. We present comprehensive comparisons between seasonal and interannual volume variations from radar altimetry and firn densification modeling, and between interannual mass variations from SMB."


Estimated ICESat inter-campaign bias and its impact on the determination of ice-sheet mass balance
Donghui YI, H. Jay ZWALLY, John W. ROBBINS, Jun LI, Jack L. SABA, Jinlun ZHANG
Corresponding author: Donghui Yi
Corresponding author e-mail: donghui.yi@nasa.gov
"ICESat operated for 18 campaign periods from March 2003 to October 2009. Most of the operational periods were between 34 and 38 days long. Because of laser failure and orbit transition from 8 day to 91 day orbit, there were four periods lasting 57, 16, 23 and 12 days. Owing to laser characteristic changes (three different lasers, laser energy decreasing with time, the changes in laser pulse shape and beam pattern, etc.), there are range biases (D) between ICESat campaign periods. The long-term trend of the inter-campaign biases (dD/dt) directly affects the derived ice-sheet mass-balance results. In this study, we used the ICESat measured mean sea level over the sea-ice-covered Arctic Ocean to estimate ICESat inter-campaign biases and evaluate the impact of the inter-campaign biases on ice-sheet mass balance. The mean sea level was calculated by averaging the elevation of the leads (open water and thin ice) within the Arctic Ocean sea-ice pack, with waveform saturation correction, inverse barometer correction, dry and wet troposphere corrections, and tidal corrections applied. The ocean dynamic topography effect was also evaluated. We adjusted the derived D by a trend of 0.31 ± 0.07 cm a–1 to account for the current rate of sea-level rise. The resulting mean inter-campaign bias trend (dDsl/dt) from September 2003 to November 2008 (the four full year period of ICESat’s 91 day orbit operation) is –1.60 ± 0.77 cm a–1. Converting this to a volume change rate (dV/dt), we get about 28 km3 a–1 for Greenland and about 198 km3 a–1 for Antarctica. Comparing ICESat elevation profiles over Lake Vostok, Antarctica, with ERS elevation profile over the same region, the bias-corrected ICESat profiles show more consistency than the profiles with no bias correction. The ICESat data used in this study are release version 633 with transmitted pulse Gaussian/Centroid peak location correction (G-C) applied."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 08, 2013, 03:16:41 AM
A free access pdf of the referenced paper is available at the following link.  This paper presents theoretical work that is applicable to the Thwaites Glacier, TG, and indicates that mechanisms such as: ocean-ice interaction, crevasses, basal meltwater,  ice shelf collapse, and/or surface melting is required to destabilize the TG, and that simple geometry is most likely not sufficient:


http://www.the-cryosphere.net/7/647/2013/tc-7-647-2013.pdf (http://www.the-cryosphere.net/7/647/2013/tc-7-647-2013.pdf)


Ice-shelf buttressing and the stability of marine ice sheets
by: G. H. Gudmundsson; The Cryosphere, 7, 647–655, 2013; www.the-cryosphere.net/7/647/2013/; (http://www.the-cryosphere.net/7/647/2013/;) doi:10.5194/tc-7-647-2013


Abstract. Ice-shelf buttressing and the stability of marine type ice sheets are investigated numerically. Buttressing effects are analysed for a situation where a stable grounding line is located on a bed sloping upwards in the direction of flow. Such grounding-line positions are known to be unconditionally unstable in the absence of transverse flow variations.  It is shown that ice-shelf buttressing can restore stability under these conditions. Ice flux at the grounding line is, in general, not a monotonically increasing function of ice thickness.  This, possibly at first somewhat counterintuitive result, is found to be fully consistent with recent theoretical work.  Grounding lines on retrograde slopes are conditionally stable, and the stability regime is a non-trivial function of bed and ice-shelf geometry. The stability of grounding lines cannot be assessed from considerations of local bed slope only."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 08, 2013, 03:30:10 AM
A free access pdf of the referenced paper is available at the following link.  This paper presents both theoretical and field evidence that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses:


http://www.the-cryosphere.net/7/407/2013/tc-7-407-2013.pdf (http://www.the-cryosphere.net/7/407/2013/tc-7-407-2013.pdf)



Surface undulations of Antarctic ice streams tightly controlled by bedrock topography;
by: J. De Rydt, G. H. Gudmundsson, H. F. J. Corr, and P. Christoffersen; The Cryosphere, 7, 407–417, 2013; www.the-cryosphere.net/7/407/2013/; (http://www.the-cryosphere.net/7/407/2013/;) doi:10.5194/tc-7-407-2013


"Abstract. Full Stokes flow-line models predict that fast flowing ice streams transmit information about their bedrock topography most efficiently to the surface for basal undulations with length scales between 1 and 20 times the mean ice thickness. This typical behaviour is independent of the precise values of the flow law and sliding law exponents, and should be universally observable. However, no experimental evidence for this important theoretical prediction has been obtained so far, hence ignoring an important test for the physical validity of current-day ice flow models. In our work we use recently acquired airborne radar data for the Rutford Ice Stream and Evans Ice Stream, and we show that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses. The sensitivity depends on the slip ratio, i.e. the ratio between mean basal sliding velocity and mean deformational velocity.  We find that higher values of the slip ratio generally lead to a more efficient transfer, whereas the transfer is significantly dampened for ice that attains most of its surface velocity by creep. Our findings underline the importance of bedrock topography for ice stream dynamics on spatial scales up to 20 times the mean ice thickness. Our results also suggest that local variations in the flow regime and surface topography at this spatial scale cannot be explained by variations in basal slipperiness."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 08, 2013, 04:25:07 AM
The following link leads to a free pdf related to an Antarctic ice sheet model that has matched ice mass balance trends observed by the GRACE satellite:

http://link.springer.com/content/pdf/10.1007%2Fs00382-012-1464-3.pdf (http://link.springer.com/content/pdf/10.1007%2Fs00382-012-1464-3.pdf)


Verification of model simulated mass balance, flow fields and tabular calving events of the Antarctic ice sheet against remotely sensed observations;
by: Diandong Ren, Lance M. Leslie, & Mervyn J. Lynch; Climate Dynamics; June 2013, Volume 40, Issue 11-12, pp 2617-2636

"Abstract
The Antarctic ice sheet (AIS) has the greatest potential for global sea level rise. This study simulates AIS ice creeping, sliding, tabular calving, and estimates the total mass balances, using a recently developed, advanced ice dynamics model, known as SEGMENT-Ice. SEGMENT-Ice is written in a spherical Earth coordinate system. Because the AIS contains the South Pole, a projection transfer is performed to displace the pole outside of the simulation domain. The AIS also has complex ice-water-granular material-bedrock configurations, requiring sophisticated lateral and basal boundary conditions. Because of the prevalence of ice shelves, a ‘girder yield’ type calving scheme is activated. The simulations of present surface ice flow velocities compare favorably with InSAR measurements, for various ice-water-bedrock configurations. The estimated ice mass loss rate during 2003–2009 agrees with GRACE measurements and provides more spatial details not represented by the latter. The model estimated calving frequencies of the peripheral ice shelves from 1996 (roughly when the 5-km digital elevation and thickness data for the shelves were collected) to 2009 compare well with archived scatterometer images. SEGMENT-Ice’s unique, non-local systematic calving scheme is found to be relevant for tabular calving. However, the exact timing of calving and of iceberg sizes cannot be simulated accurately at present. A projection of the future mass change of the AIS is made, with SEGMENT-Ice forced by atmospheric conditions from three different coupled general circulation models. The entire AIS is estimated to be losing mass steadily at a rate of ~120 km3/a at present and this rate possibly may double by year 2100."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 08, 2013, 02:24:27 PM
The following reference presents methodology for improving ice-sheet modeling:

Indirect inversions;
by: Olga Sergienko; Geophysical Research Abstracts; Vol. 15, EGU2013-5301, 2013; EGU General Assembly 2013

Abstract:
"Since Doug MacAyeal’s pioneering studies of the ice-stream basal traction optimizations by control methods, inversions for unknown parameters (e.g., basal traction, accumulation patterns, etc) have become a hallmark of the present-day ice-sheet modeling. The common feature of such inversion exercises is a direct relationship between optimized parameters and observations used in the optimization procedure. For instance, in the standard optimization for basal traction by the control method, ice-stream surface velocities constitute the control data.  The optimized basal traction parameters explicitly appear in the momentum equations for the ice-stream velocities (compared to the control data). The inversion for basal traction is carried out by minimization of the cost (or objective, misfit) function that includes the momentum equations facilitated by the Lagrange multipliers. Here, we build upon this idea, and demonstrate how to optimize for parameters indirectly related to observed data using a suite of nested constraints (like Russian dolls) with additional sets of Lagrange multipliers in the cost function.  This method opens the opportunity to use data from a variety of sources and types (e.g., velocities, radar layers, surface elevation changes, etc.) in the same optimization process."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 08, 2013, 02:40:18 PM
The following link provides a free pdf of the linked reference which discusses the topic of icequakes as related to global warming induced ice stream velocities:

http://www.igsoc.org/annals/54/64/a64A033.pdf (http://www.igsoc.org/annals/54/64/a64A033.pdf)


Deformation in Rutford Ice Stream, West Antarctica: measuring shear-wave anisotropy from icequakes
by: S.R. HARLAND, J.-M. KENDALL, G.W. STUART, G.E. LLOYD, A.F. BAIRD,
A.M. SMITH, H.D. PRITCHARD, and A.M. BRISBOURNE; Annals of Glaciology 54(64) 2013 doi:10.3189/2013AoG64A033

"ABSTRACT. Ice streams provide major drainage pathways for the Antarctic ice sheet. The stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which informs ice-flow modelling as to how ice masses respond to external changes such as global warming.  Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shearwave splitting from three-component icequake seismograms to characterize ice deformation via crystalpreferred orientation. Over 110 high-quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude of the splitting ranges from 2 to 80ms and suggests a maximum of 6% shear-wave splitting. The fast shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model).  Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI component is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream, as predicted by ice fabric models."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 08, 2013, 07:19:39 PM
The following abstracts come from the linked sources and are relevant to the glaciology:

www.igsoc.org/symposia/2013/kansas/proceedings/procsfiles/procabstracts_63.htm (http://www.igsoc.org/symposia/2013/kansas/proceedings/procsfiles/procabstracts_63.htm)
Contact: Secretary General, International Glaciological Society


67A025
Integrating RES and remotely sensed ice surface data to enhance survey design, mapping and characterization of the ice-sheet bed
Neil ROSS, Martin J. SIEGERT, Stewart S.R. JAMIESON, Hugh CORR, David RIPPIN, Fausto FERRACCIOLI, Rob G. BINGHAM, Tom A. JORDAN, Anne LE BROCQ, Kathryn ROSE
Corresponding author: Neil Ross
Corresponding author e-mail: neil.ross@ncl.ac.uk
Although commonly used to identify the locations and spatial extent of subglacial lakes and to map past and present ice-flow regimes, ice surface imagery is typically an oft-overlooked resource when major aerogeophysical surveys are designed, acquired and interpreted. Satellite-derived ice velocity data are now often used in the design of major airborne geophysical campaigns and the gridding of the resultant data (e.g. using mass conservation approaches). We propose and advocate that the careful targeted analysis and application of ice-sheet surface imagery can also provide considerable useful information at both pre- and post-survey stages. Better integration of remote-sensing imagery with radio-echo sounding (RES) data can increase the efficiency of, and enhance the scientific output from, both local and large-scale geophysical surveys of sub-ice conditions. Combining MODIS Mosaic of Antarctica and/or RADARSAT imagery with ground- and airborne RES data, we show that remote-sensing products that characterize the ice-sheet surface contain important, spatially continuous information on bed topography, sub-ice geology and basal conditions. We will outline the opportunities, benefits and limitations of the integration of ice-sheet surface imagery within the design and interpretation of major aerogeophysical campaigns, describing methods through which the information extracted from ice surface imagery can be enhanced and quantitatively analysed. We illustrate the importance of these data with examples from Antarctica and Greenland, discussing the present and past glaciological implications of our findings. Improved methodologies for the analysis of ice-sheet surface data products may unlock the potential for these high-resolution spatially contiguous datasets to enhance gridding of subglacial topography from sparse RES measurements and as input data for numerical ice-sheet models.

67A053
Sounding of subglacial nunatak ridges in West Antarctica using a UHF radar
Cameron LEWIS, Howard CONWAY, John STONE, Perry SPECTOR, John PADEN, Prasad GOGINENI
Corresponding author: Cameron Lewis
Corresponding author e-mail: cameronlewis@ku.edu
We developed an ultra-high-frequency (UHF) radar that operates over the frequency range of 600–900 MHz for surface-based measurements with a virtual antenna array of 16 elements. We used this radar to collect data on ice-covered nunataks in the Pirrit Hills of West Antarctica during the 2012/13 Antarctic summer season. These data were collected to generate fine-resolution 3-D bedrock topography maps for identifying the position and depth of the subglacial ridges of the Harter and John nunataks in preparation for future drilling campaigns. Data were collected over a grid for two areas: one approximately 0.6 km &mult; 0.3 km and the other approximately 1.2 km &mult; 0.5 km. We completed preliminary processing of the collected data, which included coherent integration, pulse compression and geo-synchronization. The processed data were used to generate radar echograms that revealed bedrock features at depths between 100 and 400 m below the surface. We also observed the presence of significant range hyperbolae indicating that a long aperture can be synthesized to obtain fine along-track resolution. Virtual phase center (tomographic) techniques will be applied to obtain fine resolution in the cross-track direction for producing 3-D maps of the bedrock topography. The combination of the radar system and data-processing techniques demonstrates the ability to image the ice–bedrock interface with fine resolution. This radar can also be used for fine-resolution imaging of the ice–water interface of ice shelves. We have successfully sounded 500–600 m thick ice shelves with an airborne version of the radar operating with a single receiver. Time-separated in situ measurements can provide single point analysis of ice-shelf basal melt rates; however, continuous wide-area coverage is needed to accurately evaluate the integrity of an ice shelf. Application of the virtual antenna array to the airborne version of the radar using the aforementioned data-processing techniques, with time-separated data collection campaigns, helps fill this data gap.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on September 08, 2013, 10:39:31 PM
Ren et al. (2012)

Clim Dyn (2013) 40:2617–2636 DOI 10.1007/s00382-012-1464-3 "Verification of model simulated mass balance, flow fields and tabular calving events of the Antarctic ice sheet against remotely sensed observations"
Ren et al.

I have issues:

1) "For example, the central parts of the Eastern Antarctic Ice Sheet typically cannot find a path way to ocean, which also is the case for the central section of the West Antarctic Ice Sheet (WAIS)."

I dispute that.

2)They admit:
"... ice shelf melting is enhanced by intrusions of warm circumpolar deep water onto the continental shelf and down into deep troughs carved into the sea floor during past ice ages (P. Molnar and J. Chen, personal communications, 2009). With 5-km resolution data (*3 km in longitudinal direction at this latitude), the ice model cannot fully resolve the troughs."

But we know CDW intrusion is a huge player.

3)They estimate a doubling of the 120GT/yr AIS mass loss by 2100. I think they are far,far, too low

sidd
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 09, 2013, 01:23:55 AM
Sidd,

Thanks for your comments (all of which I agree with).  I feel remiss, in that I have not taken the time to comment sufficiently to point out the many limitations (and to highlight the positives) of the various references that I am providing.  All readers should think critically of any published model projections of ice mass loss though 2100, as no computer program now (or probably for 20 to 30 years from now) can adequately capture all of the various lightly synergistic and nonlinear mechanisms that are likely to lead to AIS negative ice mass balance by 2100.  Therefore, when we read such projections, we should not think that we are reading a prediction (like we get from well estabished science), but rather we should think: "What behavior is this model capturing and what is it leaving out."

Certainly, any model claiming to make a +/- 25% estimate of AIS ice mass loss by 2100 would need to accurately model not only the full influence of warm CDW (probably for RCP 8.5 95%CL) but also the full influence of:

(a) surface and basal crevasses; (b) subglacial hydrological systems; (c) suface meltwater influences on both ice shelves and ice sheets; (d) basal friction and geothermal basal effects; (e) changes in ocean current circulation patterns; (f) storm and infragravity wave effects; (g) SAM, ENSO, PDO, etc oscillational effects; (h) geometric and local instability effects similar to the "Jakobshavn Effect"; plus all of the other effects that I have referred to in the various threads in the Antarctic folder.

A big problem is that the public is use to science having solved so many problems to a degree of accuracy that the public can feel comfortable relying on the scientific "answers" that the public is not use to being given a partial answer by scientists.  It is also a problem that if the scientist does cite all the limitations of a model projection (e.g. AR4 SLR projections clearly stated they they did not include dynamic ice mass loss contributions); then either the public ignores the limits cited by the scientist, or they discount out of hand all of the hardwork that the scientist did to produce the partial answer.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 10, 2013, 02:09:56 AM
The linked reference has a free pdf, and I agree with Drouet et al that:
"Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections."

http://www.the-cryosphere.net/7/395/2013/tc-7-395-2013.html (http://www.the-cryosphere.net/7/395/2013/tc-7-395-2013.html)

Drouet, A. S., Docquier, D., Durand, G., Hindmarsh, R., Pattyn, F., Gagliardini, O., and Zwinger, T.: Grounding line transient response in marine ice sheet models, The Cryosphere, 7, 395-406, doi:10.5194/tc-7-395-2013, 2013.


"Abstract. Marine ice-sheet stability is mostly controlled by the dynamics of the grounding line, i.e. the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated within the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice-sheet models has been compared. The models differ in the physics implemented (full Stokes and shallow shelf approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be broadly consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, still small differences appear for these latter variables, and they can lead to large discrepancies (> 100%) observed in terms of ice sheet contribution to sea level when cumulated over time. Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 13, 2013, 10:58:01 PM
As Antarctic glacial speeds accelerate their basal ice will tend to become more debris-rich; which implies that the following research on anisotropy of magnetic susceptibility (AMS) will become increasingly useful in monitoring strains around the margins of accelerating Antarctic Ice Streams such as the Thwaites Glacier:

http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20144/abstract (http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20144/abstract)

Magnetic fabrics in the basal ice of a surge-type glacier;
by: Edward J. Fleming, Harold Lovell, Carl T.E. Stevenson, Michael S. Petronis, Douglas I. Benn, Michael J. Hambrey, Ian J. Fairchild; 2013; Journal of Geophysical Research: Earth Surface; DOI: 10.1002/jgrf.20144


"Abstract
Anisotropy of magnetic susceptibility (AMS) has been shown to provide specific useful information regarding the kinematics of deformation within subglacially deformed sediments. Here we present results from debris-rich basal glacier ice to examine deformation associated with glacier motion. Basal ice samples were collected from Tunabreen, a polythermal surge-type glacier in Svalbard. The magnetic fabrics recorded show strong correlation with structures within the ice, such as sheath folds and macroscopic stretching lineations. Thermomagnetic, low-temperature susceptibility, varying field susceptibility and isothermal remanent magnetism (IRM) acquisition experiments reveal that the debris-rich basal ice samples have a susceptibility and anisotropy dominated by paramagnetic phases within the detrital sediment. Sediment grains entrained within the basal ice are inferred to have rotated into a preferential alignment during deformation associated with flow of the glacier. An up-glacier directed plunge of magnetic lineations and subtle deviation from bulk glacier flow at the margins highlight the importance of non-coaxial strain during surge propagation. The results suggest that AMS can be used as an ice petrofabric indicator for investigations of glacier deformation and interactions with the bed."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 14, 2013, 02:28:51 AM
The linked reference (with a pdf) indicates the importance of accounting for the influence of tidal action on Antarctic glacial ice velocities when interpreting satellite measurements:


http://www.the-cryosphere.net/7/1375/2013/tc-7-1375-2013.html (http://www.the-cryosphere.net/7/1375/2013/tc-7-1375-2013.html)


Marsh, O. J., Rack, W., Floricioiu, D., Golledge, N. R., and Lawson, W.: Tidally induced velocity variations of the Beardmore Glacier, Antarctica, and their representation in satellite measurements of ice velocity, The Cryosphere, 7, 1375-1384, doi:10.5194/tc-7-1375-2013, 2013

Abstract. Ocean tides close to the grounding line of outlet glaciers around Antarctica have been shown to directly influence ice velocity, both linearly and non-linearly. These fluctuations can be significant and have the potential to affect satellite measurements of ice discharge, which assume displacement between satellite passes to be consistent and representative of annual means. Satellite observations of horizontal velocity variation in the grounding zone are also contaminated by vertical tidal effects, the importance of which is highlighted here in speckle tracking measurements. Eight TerraSAR-X scenes from the grounding zone of the Beardmore Glacier are analysed in conjunction with GPS measurements to determine short-term and decadal trends in ice velocity. Diurnal tides produce horizontal velocity fluctuations of >50% on the ice shelf, recorded in the GPS data 4 km downstream of the grounding line. This variability decreases rapidly to <5% only 15 km upstream of the grounding line. Daily fluctuations are smoothed to <1% in the 11-day repeat pass TerraSAR-X imagery, but fortnightly variations over this period are still visible and show that satellite-velocity measurements can be affected by tides over longer periods. The measured tidal displacement observed in radar look direction over floating ice also allows the grounding line to be identified, using differential speckle tracking where phase information cannot be easily unwrapped."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on October 24, 2013, 01:47:57 AM
This article indicates that due to the highly non-linear response of marine terminating glaciers w.r.t. ice mass loss, that researchers should do a more thorough job on sensitivity analysis where presenting related research results:

http://www.the-cryosphere.net/7/1579/2013/tc-7-1579-2013.html (http://www.the-cryosphere.net/7/1579/2013/tc-7-1579-2013.html)

Enderlin, E. M., Howat, I. M., and Vieli, A.: The sensitivity of flowline models of tidewater glaciers to parameter uncertainty, The Cryosphere, 7, 1579-1590, doi:10.5194/tc-7-1579-2013, 2013

Abstract. Depth-integrated (1-D) flowline models have been widely used to simulate fast-flowing tidewater glaciers and predict change because the continuous grounding line tracking, high horizontal resolution, and physically based calving criterion that are essential to realistic modeling of tidewater glaciers can easily be incorporated into the models while maintaining high computational efficiency. As with all models, the values for parameters describing ice rheology and basal friction must be assumed and/or tuned based on observations. For prognostic studies, these parameters are typically tuned so that the glacier matches observed thickness and speeds at an initial state, to which a perturbation is applied. While it is well know that ice flow models are sensitive to these parameters, the sensitivity of tidewater glacier models has not been systematically investigated. Here we investigate the sensitivity of such flowline models of outlet glacier dynamics to uncertainty in three key parameters that influence a glacier's resistive stress components. We find that, within typical observational uncertainty, similar initial (i.e., steady-state) glacier configurations can be produced with substantially different combinations of parameter values, leading to differing transient responses after a perturbation is applied. In cases where the glacier is initially grounded near flotation across a basal over-deepening, as typically observed for rapidly changing glaciers, these differences can be dramatic owing to the threshold of stability imposed by the flotation criterion. The simulated transient response is particularly sensitive to the parameterization of ice rheology: differences in ice temperature of ~ 2 °C can determine whether the glaciers thin to flotation and retreat unstably or remain grounded on a marine shoal. Due to the highly non-linear dependence of tidewater glaciers on model parameters, we recommend that their predictions are accompanied by sensitivity tests that take parameter uncertainty into account.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on November 22, 2013, 04:38:02 PM
The following links leads to an website, explaining that the IceBridge Antarctic Mission is underway in an expanded format (operating from McMurdo instead of Chile); so that the airborne science mission can partially fill the void between the defunct ICESat satellite and the planned ICESat-2, which is slated to launch in 2016.


http://www.nbcnews.com/science/nasas-icebridge-mission-back-action-over-antarctica-2D11624553 (http://www.nbcnews.com/science/nasas-icebridge-mission-back-action-over-antarctica-2D11624553)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on November 26, 2013, 04:35:46 PM
The following link and summary cites NSF funded research about how cracks along the edges of the ice shelves in the Amundsen Sea Sector may provide a positive feedback factor for ice mass loss from this area (as I have discussed previously):

https://www.research.gov/research-portal/appmanager/base/desktop;jsessionid=1JyLSXcYvTKLnT6mLffJJjRtvsvnBK0tTyjB2hSY8tYtQP2pnJtL (https://www.research.gov/research-portal/appmanager/base/desktop;jsessionid=1JyLSXcYvTKLnT6mLffJJjRtvsvnBK0tTyjB2hSY8tYtQP2pnJtL)!-1826466010!941996275?_nfpb=true&_windowLabel=awardSummary_1&_urlType=action&wlpawardSummary_1_id=%2FresearchGov%2FAwardHighlight%2FPublicAffairs%2F23322_WestAntarcticiceshelvesslip-slidingaway.html&wlpawardSummary_1_action=selectAwardDetail

Research Summary:

"Nearly 40 years of satellite imagery suggests that west Antarctic ice shelves floating in the Amundsen Sea are steadily losing their grip on adjacent bay walls. This pattern of retreat could potentially amplify an already accelerating loss of ice to the sea, according to glaciologists at The University of Texas at Austin.
The record created in this study will give scientists a better understanding of the recent evolution of west Antarctica's ice shelves. Knowing why these changes have occurred is critical for predicting future changes. Previously, most computer models have neglected this specific pattern of ice-shelf retreat, partly because it involves fracture, but also because no comprehensive record of this pattern existed.
The Amundsen Sea Embayment is one of the few places in Antarctica with good long-term satellite coverage of its coastline. This comprehensive record shows clearly that the ice shelves changed substantially between the beginning of the Landsat satellite record in 1972 and late 2011. These changes were especially rapid during the past decade, and the affected ice shelves include the floating extensions of the rapidly thinning Thwaites and Pine Island glaciers.
Normally, the ice shelves grip onto rocky bay walls or slower ice masses at their very edge. However, as that grip continues to loosen, these already-thinning ice shelves will be even less able to hold back ice upstream. The fractured edges are retreating inland, resembling a cracked mirror in satellite imagery until the detached icebergs finally drift out to the open sea. This pattern is believed to be a symptom, rather than a trigger, of the recent glacier acceleration in this region. However, it could also generate additional acceleration."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on December 12, 2013, 12:58:48 AM
The following link leads to an interesting article about the recently completed 2013 Antarctic IceBridge campaign:

http://phys.org/news/2013-12-icebridge-successful-antarctic-campaign.html (http://phys.org/news/2013-12-icebridge-successful-antarctic-campaign.html)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 08, 2014, 12:01:01 AM
The following link leads to an interesting article about the formation of blue-ice areas in Eastern Antarctica:

http://www.igsoc.org/journal/60/219/t13J116.html (http://www.igsoc.org/journal/60/219/t13J116.html)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 08, 2014, 12:17:21 AM
The following link leads to an article about the first application of a drone for aerial sensing in Antarctica:

http://spectrum.ieee.org/tech-talk/robotics/aerial-robots/small-drone-probes-antarctic-ice-with-radar (http://spectrum.ieee.org/tech-talk/robotics/aerial-robots/small-drone-probes-antarctic-ice-with-radar)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 15, 2014, 11:02:11 PM
The following link leads to a pdf of the XXXIII SCAR Biennial 2014 meeting from August 22nd to September 3rd in New Zealand including the 2014 Open Science Conference focused on Antarctica and the Southern Ocean.  The program looks great and the presentations should include a lot of new insights:

http://www.scar2014.com/assets/Session_titles_and_Descriptions4.pdf (http://www.scar2014.com/assets/Session_titles_and_Descriptions4.pdf)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 27, 2014, 09:57:10 AM
I missed posting this reference a few months ago, but better late than never:

“A 308 year record of climate variability in West Antarctica,” by Elizabeth R. Thomas, Thomas J. Bracegirdle, John Turner, Eric W. Wolff published in Geophysical Research Letters. Article first published online: 18 OCT 2013 DOI: 10.1002/2013GL057782
http://onlinelibrary.wiley.com/doi/10.1002/2013GL057782/abstract (http://onlinelibrary.wiley.com/doi/10.1002/2013GL057782/abstract)

"Abstract
We present a new stable isotope record from Ellsworth Land which provides a valuable 308 year record (1702–2009) of climate variability from coastal West Antarctica. Climate variability at this site is strongly forced by sea surface temperatures and atmospheric pressure in the tropical Pacific and related to local sea ice conditions. The record shows that this region has warmed since the late 1950s, at a similar magnitude to that observed in the Antarctic Peninsula and central West Antarctica; however, this warming trend is not unique. More dramatic isotopic warming (and cooling) trends occurred in the mid-nineteenth and eighteenth centuries, suggesting that at present, the effect of anthropogenic climate drivers at this location has not exceeded the natural range of climate variability in the context of the past ~300 years."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 29, 2014, 07:10:25 PM
The following link leads to an interesting pdf about the British Antarctic Survey's aerial survey techniques/equipment:

http://www.antarctica.ac.uk/images/news/2487/p100-101_dr_fausto_ferraccioli.pdf (http://www.antarctica.ac.uk/images/news/2487/p100-101_dr_fausto_ferraccioli.pdf)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 30, 2014, 05:22:08 PM
In the short-term the information at the following link about microbes on ice as climate amplifiers, may be of most concern for the Greenland Ice Sheet; however, as Antarctica continues to warm, such microbes will likely become more of an amplifier for ice mass loss from the southern continent:

http://climatica.org.uk/microbes-ice-climate-amplifiers (http://climatica.org.uk/microbes-ice-climate-amplifiers)

Extract:
"Ice surface microbes – implications for climate
Clearly, glacier surfaces represent an active microbial biome. It has been suggested that this biome could be a climate amplifier, meaning the action of supraglacial microbes might exacerbate climate changes. The primary mechanism for this is thought to be related to ice surface albedo (reflectivity). Albedo is a phenomenon with which we are all familiar: when we wear black clothing we feel hotter than when we wear white because dark surfaces more efficiently absorb solar radiation. Where sediment exists on a glacier surface, the albedo is much lower than where ice is clean and bare, so melting under dark material is enhanced. This means that cryoconite dust that covers a larger area of the ice surface, also further reduces its albedo and ability to reflect radiation. The presence of microbes in and around cryoconite grains makes them darker still. Warmer temperatures might encourage these microbes to grow and proliferate, enhancing their effect to lower albedo and accelerating glacier melting.

Faster melting reduces the amount of reflective ice covering Earth’s surface and promotes absorption of solar radiation, exacerbating temperature rise. Therefore, microbial processes on glacier surfaces might be an important amplifier of temperature changes. Furthermore, there could be complex feedbacks associated with the release of atmospheric carbon by microbial respiration, and the drawdown and fixation of atmospheric carbon by photosynthesis by glacier surface microbes, which could be climatically significant at least at the regional scale.
 
The future
We know that the climate is changing for the warmer, and the response of ice’s microbial communities is currently uncertain. For example, if faster melt brings more nutrients will microbes fix more carbon? Will increased biomass enhance further reduce the albedo and amplify the warming? How do these processes feed back into the climate system? Answering these questions, amongst others, will play a significant role in understanding the response of glacier and ice sheet surfaces to future climate change.

Glacier microbiologists are adopting increasingly sophisticated techniques (flow cytometry, microscopy, infra-red gas analysis and fluorescence) to examine the response of microbes to climate change. However, very recently advanced molecular methods have been employed to great effect. For example, Arwyn Edwards and his team recently used a metagenomic approach to take a ‘snapshot’ of all the genes present within an Alpine cryoconite hole. We might see greater incorporation of sophisticated molecular techniques in the near future as we begin to tackle the remaining big questions regarding icy life and climate."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 30, 2014, 07:03:38 PM
I would just like to provide a link to the upcoming International Symposium on Sea Ice in a Changing Environment, in Hobart, Australia (and this location will guarantee discussion of Antarctic sea ice):

http://seaice.acecrc.org.au/igs2014/ (http://seaice.acecrc.org.au/igs2014/)

International Symposium on Sea Ice in a Changing Environment
International Glaciological Society; C3 Convention Centre, Hobart, Australia, 10 – 14 March 2014
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 02, 2014, 08:03:15 PM
When released in 2015 the linked atlas of submarine glacial landforms should help modelers to estimate the stability of marine terminating and marine glaciers:

http://www.submarineglacialatlas.com/ (http://www.submarineglacialatlas.com/)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 04, 2014, 10:51:18 PM
The linked article indicates that the Multiple Altimeter Beam Experimental Lidar (MABEL) instrument to be carried on the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission (scheduled for launch in late 2015), will be capable of measuring slopes on ~ 90-m spatial scales, a measurement that will be fundamental to deconvolving the effects of surface slope from the ice-sheet surface change for both Antarctica and Greenland:

Brunt, K.M. ; Neumann, T.A. ; Walsh, K.M. ; & Markus, T. (2014), "Determination of Local Slope on the Greenland Ice Sheet Using a Multibeam Photon-Counting Lidar in Preparation for the ICESat-2 Mission", Geoscience and Remote Sensing Letters, IEEE, Volume:11, Issue: 5,  May 2014, Page(s): 935 – 939, ISSN : 1545-598X, INSPEC Accession Number: 13970147;  Digital Object Identifier, doi: 10.1109/LGRS.2013.2282217

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6623082 (http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6623082)

Abstract: "The greatest changes in elevation in Greenland and Antarctica are happening along the margins of the ice sheets where the surface frequently has significant slopes. For this reason, the upcoming Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission utilizes pairs of laser altimeter beams that are perpendicular to the flight direction in order to extract slope information in addition to elevation. The Multiple Altimeter Beam Experimental Lidar (MABEL) is a high-altitude airborne laser altimeter designed as a simulator for ICESat-2. The MABEL design uses multiple beams at fixed angles and allows for local slope determination. Here, we present local slopes as determined by MABEL and compare them to those determined by the Airborne Topographic Mapper (ATM) over the same flight lines in Greenland. We make these comparisons with consideration for the planned ICESat-2 beam geometry. Results indicate that the mean slope residuals between MABEL and ATM remain small ( 0.05 °) through a wide range of localized slopes using ICESat-2 beam geometry. Furthermore, when MABEL data are subsampled by a factor of 4 to mimic the planned ICESat-2 transmit-energy configuration, the results are indistinguishable from the full-data-rate analysis. Results from MABEL suggest that ICESat-2 beam geometry and transmit-energy configuration are appropriate for the determination of slope on ~ 90-m spatial scales, a measurement that will be fundamental to deconvolving the effects of surface slope from the ice-sheet surface change derived from ICESat-2."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 05, 2014, 06:58:09 PM
The linked reference indicates that some of the approximations currently used in Shallow Ice Approximations maybe problematic, indicating that we may need to wait for more advanced models before we should believe projections from such Shallow Ice models:

Ahlkrona, J., N. Kirchner, and P. Lötstedt, 2013. A Numerical Study of Scaling Relations for Non-Newtonian Thin-film Flows with Applications in Ice Sheet Modelling, Quarterly Journal Of Mechanics And Applied Mathematics, 66(4), 417-435, doi:10.1093/qjmam/hbt009.

http://qjmam.oxfordjournals.org/content/66/4/417 (http://qjmam.oxfordjournals.org/content/66/4/417)

Abstract: "This article treats the viscous, non-Newtonian thin-film flow of ice sheets, governed by the Stokes equations, and the modelling of ice sheets with asymptotic expansion of the analytical solutions in terms of the aspect ratio, which is a small parameter measuring the shallowness of an ice sheet. An asymptotic expansion requires scalings of the field variables with the aspect ratio. There are several, conflicting, scalings in the literature used both for deriving simplified models and for analysis. We use numerical solutions of the Stokes equations for varying aspect ratios in order to compute scaling relations. Our numerically obtained results are compared with three known theoretical scaling relations: the classical scalings behind the Shallow Ice Approximation, the scalings originally used to derive the so-called Blatter–Pattyn equations, and a non-uniform scaling which takes into account a high viscosity boundary layer close to the ice surface. We find that the latter of these theories is the most appropriate one since there is indeed a boundary layer close to the ice surface where scaling relations are different than further down in the ice. This boundary layer is thicker than anticipated and there is no distinct border with the inner layer for aspect ratios appropriate for ice sheets. This makes direct application of solutions obtained by matched asymptotic expansion problematic."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on February 05, 2014, 09:41:28 PM
Full Stokes is not used in most models because of computational cost, although this factor is improving. The shallow shelf and shallow ice approximations combines with MacAyeal treatment of ice streams seems reasonable. To my mind, the main gap is 1)basal hydrology, including transition to temperate bed and 2)Especially in Greenland, neglect of latent heat transport by melt water("cryo hydrologic warming"). In regard to the latter, see Phillips(2013) doi:10.1002/jgrf.20079 about which i posted in the latest unforced variations threat at real climate. An interesting fact:

"For every 1% by ice sheet volume of water retained, the ultimate ice warming potential after full refreezing is ~1.8 C"

As i pointed out at realclimate , the discovery of a perennial water body in the firn pack on Greenland makes me wonder even more about heat transport into the ice mass in Greenland. The extensive subglacial hydrology in Antarctica make me further wonder about the role of basal melt in heat transport.

sidd
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 06, 2014, 12:08:07 AM
sidd,

Thanks for the comments.  For those who want the full Phillips et al 2013 reference, see:

Phillips, T., H. Rajaram, W. Colgan, K. Steffen, and W. Abdalati (2013), Evaluation of cryo-hydrologic warming as an explanation for increased ice velocities in the wet snow zone, Sermeq Avannarleq West Greenland, J. Geophys. Res. Earth Surf., 118, 1241–1256, doi:10.1002/jgrf.20079.


http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20079/abstract (http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20079/abstract)


Abstract: "Wintertime satellite-derived ice surface velocities, from 2001 through 2007, suggest an increase in ice velocity in the wet snow zone of Southwest Greenland. We present a thermomechanical model to evaluate the influence of surface meltwater runoff on englacial temperatures, via cryo-hydrologic warming (CHW), as a possible mechanism to explain this velocity increase at Sermeq Avannarleq. The model incorporates CHW through a previously published dual-column parameterization. We compare model simulations with (i) CHW active over the entire ice thickness (“base case CHW”), (ii) CHW active only in the surface 80 m of the ice sheet (“surface CHW”), and (iii) “no CHW” to represent a traditional thermomechanical model. The horizontal extent of CHW is prescribed based on equilibrium line altitude position and thus incorporates the upstream expansion of the ablation zone over the past decade. The base case CHW simulations reproduce the observed increase in inland ice velocity between 2001 and 2007 reasonably well. The no CHW and surface CHW simulations significantly underestimate observed ice surface velocities in both epochs. The higher ice velocities in the base case CHW simulations are attributable to both decreased basal ice viscosities associated with increased basal ice temperatures and an increase in the extent of basal sliding permitted by temperate bed conditions. Only the temperate bed extent predicted by the base case CHW simulation is consistent with independent observations of basal sliding. Based on our sensitivity analysis of CHW, we evaluate alternative explanations for an increase in inland ice velocity and suggest CHW is the most plausible mechanism."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 08, 2014, 01:42:51 AM
The linked reference (with a free access pdf) indicates that due to the influence on ice shelf freeboard of the sub-ice platelet layer, that past estimates of Antarctic ice shelf thickness have been overestimated by up to 19%.  Thinner ice shelves are weaker and more likely to break apart.  This may be one of the factors as to why the PIIS and the Thwaites Ice Shelf appear to be cracking and calving more rapidly than previously expected:

Price, D., Rack, W., Langhorne, P. J., Haas, C., Leonard, G., and Barnsdale, K.: The sub-ice platelet layer and its influence on freeboard to thickness conversion of Antarctic sea ice, The Cryosphere Discuss., 8, 999-1022, doi:10.5194/tcd-8-999-2014, 2014

http://www.the-cryosphere-discuss.net/8/999/2014/tcd-8-999-2014.html (http://www.the-cryosphere-discuss.net/8/999/2014/tcd-8-999-2014.html)

"Abstract. This is an investigation to quantify the influence of the sub-ice platelet layer on satellite measurements of total freeboard and their conversion to thickness of Antarctic sea ice. The sub-ice platelet layer forms as a result of the seaward advection of supercooled ice shelf water from beneath ice shelves. This ice shelf water provides an oceanic heat sink promoting the formation of platelet crystals which accumulate at the sea ice–ocean interface. The build-up of this porous layer increases sea ice freeboard, and if not accounted for, leads to overestimates of sea ice thickness from surface elevation measurements. In order to quantify this buoyant effect, the solid fraction of the sub-ice platelet layer must be estimated. An extensive in situ data set measured in 2011 in McMurdo Sound in the south-western Ross Sea is used to achieve this. We use drill-hole measurements and the hydrostatic equilibrium assumption to estimate a mean value for the solid fraction of this sub-ice platelet layer of 0.16. This is highly dependent upon the uncertainty in sea ice density. We test this value with independent Global Navigation Satellite System (GNSS) surface elevation data to estimate sea ice thickness. We find that sea ice thickness can be overestimated by up to 19%, with a mean deviation of 12% as a result of the influence of the sub-ice platelet layer. It is concluded that in close proximity to ice shelves this influence should be considered universally when undertaking sea ice thickness investigations using remote sensing surface elevation measurements."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 08, 2014, 10:54:35 AM
The linked reference paper (with a free access pdf) presents a box model of basal melting beneath six Antarctic ice shelves: Amery Ice Shelf (AMY), Pine Island Ice Shelf (PIIS), Ross (ROS), Fimbulisen (FIM), Ronne (RON), and Larsen C (LAR) ice shelves:

Olbers, D., Hellmer, H. H., and Buck, F. F. J. H.: A data-constrained model for compatibility check of remotely sensed basal melting with the hydrography in front of Antarctic ice shelves, The Cryosphere Discuss., 8, 919-951, doi:10.5194/tcd-8-919-2014, 2014.

http://www.the-cryosphere-discuss.net/8/919/2014/tcd-8-919-2014.html (http://www.the-cryosphere-discuss.net/8/919/2014/tcd-8-919-2014.html)

"Abstract. The ice shelf caverns around Antarctica are sources of cold and fresh water which contributes to the formation of Antarctic bottom water and thus to the ventilation of the deep basins of the World Ocean. While a realistic simulation of the cavern circulation requires high resolution, because of the complicated bottom topography and ice shelf morphology, the physics of melting and freezing at the ice shelf base is relatively simple. We have developed an analytically solvable box model of the cavern thermohaline state, using the formulation of melting and freezing as in Olbers and Hellmer (2010). There is high resolution along the cavern's path of the overturning circulation whereas the cross-path resolution is fairly coarse. The circulation in the cavern is prescribed and used as a tuning parameter to constrain the solution by attempting to match observed ranges for outflow temperature and salinity at the ice shelf front as well as of the mean basal melt rate. The method, tested for six Antarctic ice shelves, can be used for a quick estimate of melt/freeze rates and the overturning rate in particular caverns, given the temperature and salinity of the inflow and the above mentioned constrains for outflow and melting. In turn, the model can also be used for testing the compatibility of remotely sensed basal mass loss with observed cavern inflow characteristics."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 10, 2014, 05:38:38 PM
The attached image showing the location of Antarctic research stations (indicating that China is rapidly increasing its number of stations) is from the linked article:

http://qz.com/175325/why-china-just-built-this-lantern-shaped-research-base-in-antarctica/ (http://qz.com/175325/why-china-just-built-this-lantern-shaped-research-base-in-antarctica/)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 15, 2014, 04:01:02 AM
Research such as that in the linked (free access) paper, will help to project calving in marine terminating glaciers both in Greenland and Antarctica:


Krug, J., Weiss, J., Gagliardini, O., and Durand, G.: Combining damage and fracture mechanics to model calving, The Cryosphere Discuss., 8, 1111-1150, doi:10.5194/tcd-8-1111-2014, 2014


http://www.the-cryosphere-discuss.net/8/1111/2014/tcd-8-1111-2014.pdf (http://www.the-cryosphere-discuss.net/8/1111/2014/tcd-8-1111-2014.pdf)


"Abstract. Calving of icebergs is a major negative component of polar ice-sheet mass balance. We present a new calving modeling framework relying on both continuum damage mechanics and linear elastic fracture mechanics. This combination accounts for both the slow sub-critical surface crevassing and fast propagation of crevasses when calving occurs. First, damage of the ice occurs over long timescales and enhances the viscous flow of ice. Then brittle fracture propagation happens downward, over very short timescales, in ice considered as an elastic medium. The model is validated on Helheim Glacier, South-West Greenland, one of the most monitored fast-flowing outlet glacier. This allows to identify sets of model parameters giving a consistent response of the model and producing a dynamic equilibrium in agreement with observed stable position of the Helheim ice front between 1930 and today."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 15, 2014, 10:48:19 AM
The linked article indicates the importance of understanding subglacial hydrology dynamics:

L. H. Beem, S. M. Tulaczyk, M. A. King, M. Bougamont, H. A. Fricker, and P. Christoffersen, (2014), "Variable deceleration of Whillans Ice Stream, West Antarctica", Journal of Geophysical Research: Earth Surface, DOI: 10.1002/2013JF002958

http://onlinelibrary.wiley.com/doi/10.1002/2013JF002958/abstract (http://onlinelibrary.wiley.com/doi/10.1002/2013JF002958/abstract)

Abstract: "The Whillans Ice Stream Ice Plain (WIP) has been slowing since at least 1963. Prior constraints on this slowdown were consistent with a constant long-term deceleration rate. New observations of ice velocity from 11 continuous and 3 seasonal GPS sites indicate the deceleration rate varies through time including on interannual time scales. Between 2009 and 2012 WIP decelerated at a rate (6.1 to 10.9 ± 2 m/yr2) that was double the multidecadal average (3.0 to 5.6 ± 2 m/yr2). To identify the causes of slowdown, we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher-order inverse model. All model results support the conclusion that the observed deceleration of WIP is caused by an increase in basal resistance to motion at a rate of 10 to 40 Pa/yr. Subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on, changes in subglacial hydrology, or increases in the area of resistant basal substrate through differential erosion. The observed variability in WIP deceleration rate suggests that dynamics in subglacial hydrology, plausibly driven by basal freeze on and/or activity of subglacial lakes, plays a key role in modulating basal resistance to ice motion in the region."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 15, 2014, 11:09:16 AM
The linked article finds "… a statistically significant correlation between the arrival of tsunamis and propagation of front-initiated rifts in eight ice shelves.":

C. C. Walker, J. N. Bassis, H. A. Fricker, and R. J. Czerwinski, (2013), "Structural and environmental controls on Antarctic ice shelf rift propagation inferred from satellite monitoring", Journal of Geophysical Research: Earth Surface, Volume 118, Issue 4, pages 2354–2364, DOI: 10.1002/2013JF002742.


http://onlinelibrary.wiley.com/doi/10.1002/2013JF002742/abstract (http://onlinelibrary.wiley.com/doi/10.1002/2013JF002742/abstract)


Abstract: "Iceberg calving from ice shelves accounts for nearly half of the mass loss from the Antarctic Ice Sheet, yet our understanding of this process is limited. The precursor to iceberg calving is large through-cutting fractures, called “rifts,” that can propagate for decades after they have initiated until they become iceberg detachment boundaries. To improve our knowledge of rift propagation, we monitored the lengths of 78 rifts in 13 Antarctic ice shelves using satellite imagery from the Moderate Resolution Imaging Spectroradiometer and Multiangle Imaging Spectroradiometer between 2002 and 2012. This data set allowed us to monitor trends in rift propagation over the past decade and test if variation in trends is controlled by variable environmental forcings. We found that 43 of the 78 rifts were dormant, i.e., propagated less than 500 m over the observational interval. We found only seven rifts propagated continuously throughout the decade. An additional eight rifts propagated for at least 2 years prior to arresting and remaining dormant for the rest of the decade, and 13 rifts exhibited isolated sudden bursts of propagation after 2 or more years of dormancy. Twelve of the fifteen active rifts were initiated at the ice shelf fronts, suggesting that front-initiated rifts are more active than across-flow rifts. Although we did not find a link between the observed variability in rift propagation rate and changes in atmospheric temperature or sea ice concentration correlated with, we did find a statistically significant correlation between the arrival of tsunamis and propagation of front-initiated rifts in eight ice shelves. This suggests a connection between ice shelf rift propagation and mechanical ocean interaction that needs to be better understood."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 16, 2014, 03:42:11 AM
Although the following abstract is not yet published, I like the topic that iron fertilization from ice melt water in Antarctica gives a good idea of just how much ice is melting in the various areas, and particularly in the Amundsen Sea:


Sherrell, R. M., Lagerström, M., Stammerjohn, S., Yager, P. L., and Schofield, O., (2014) WORKINGS OF AN INTENSE NATURAL IRON FERTILIZATION REGION DURING CLIMATE WARMING: BIOACTIVE METAL DYNAMICS IN AMUNDSEN SEA POLYNYA, WEST ANTARCTICA, Ocean Science Meeting February 23-28 2014 Conference in Hawaii

Abstract: "An ocean-ice interaction region of global importance, the Amundsen Sea, West Antarctica, has experienced large increases of glacial meltwater in recent years. The Amundsen Sea Polynya (ASP) is bounded by rapidly thinning ice shelves, and hosts an extremely productive phytoplankton bloom lasting for 10 weeks. Macronutrients are replete in these waters, so the bloom depends on the continuous input of the limiting micronutrient iron (Fe), in dissolved and particulate forms, largely carried by glacial meltwater-laden seawater emerging from under the Dotson Ice Shelf (DIS). Warm Circumpolar Deepwater flows 10’s of km under the DIS, melts the ice shelf near the grounding line, and emerges at the western end of the shelf as an Fe-rich and particle-rich subsurface flow at 200-300 m. This buoyant, meltwater-laden water, labeled with oxygen isotopes, advects northward, mixes, and shoals to 50 m at the center of the ASP, bringing Fe to the euphotic zone, aided by wind- and iceberg-driven vertical mixing. During ASPIRE 2010-11, we determined dissolved and particulate distributions of Fe, Mn, Zn, Cu, Ni, and Co, and measured strongly decreasing vertical fluxes with depth, using a trace-metal clean CTD-rosette and a 3-depth drifting sediment trap array. We explore the controls on primary productivity in the polynya, and the influence of biological processes on the export and remineralization of metals on the Amundsen shelf vs. the open Southern Ocean, in the context of ongoing climate warming."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on February 21, 2014, 01:50:26 AM
The conclusions of the linked reference (with a free access pdf) states:

"Ice dynamics were also recorded thanks to the measurement of ice velocities and ice thickness. Maximum ice velocity values of 34.6ma−1  were obtained. Near equilibrium conditions were calculated at the BIA, where mean velocities of 20ma−1 were measured. The snow accumulation among the studied stakes outside BIAs showed values of up to 0.2m w.eq. a−1 (near 0.5ma−1 of snow). At the BIA, a local negative mass balance was detected as expected, with mean ablation rates of 0.1m w.eq. a−1."

Rivera, A., Zamora, R., Uribe, J. A., Jaña, R., and Oberreuter, J.: Union Glacier: a new exploration gateway for the West Antarctic Ice Sheet, The Cryosphere Discuss., 8, 1227-1256, doi:10.5194/tcd-8-1227-2014, 2014.

http://www.the-cryosphere-discuss.net/8/1227/2014/tcd-8-1227-2014.html (http://www.the-cryosphere-discuss.net/8/1227/2014/tcd-8-1227-2014.html)


"Abstract. Union Glacier (79°46' S/83°24' W) in the West Antarctic Ice Sheet (WAIS), has been used by the private company Antarctic Logistic and Expeditions (ALE) since 2007 for their landing and commercial operations, providing a unique logistic opportunity to perform glaciological research in a vast region, including the Ice divide between Institute and Pine Island glaciers and the Subglacial Lake Ellsworth. Union glacier is flowing into the Ronne Ice Shelf, where future migrations of the grounding line zone (GLZ) in response to continuing climate and oceanographic changes have been modelled. In order to analyse the potential impacts on Union glacier of this scenario, we installed an array of stakes, where ice elevation, mass balance and ice velocities have been measured since 2007, resulting in near equilibrium conditions with horizontal displacements between 10 and 33 m yr−1. GPS receivers and three radar systems have been also used to map the subglacial topography, the internal structure of the ice and the presence of crevasses along surveyed tracks. The resulting radar data showed a subglacial topography with a minimum of 858 m below sea level, much deeper than estimated before. The below sea level subglacial topography confirms the potential instability of the glacier in foreseen scenarios of GLZ upstream migration during the second half of the XXI century."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 01, 2014, 02:15:06 AM
While I applaud the linked (with a free access pdf) researchers' efforts to calibrate Antarctic snow fall rates; I point out that increasing snowfall rates will accelerate glacier ice mass loss and also snow scour rates (both of which will need to be considered when projecting Antarctic contributions to SLR):


Palerme, C., Kay, J. E., Genthon, C., L'Ecuyer, T., Wood, N. B., and Claud, C.: How much snow falls on the Antarctic ice sheet?, The Cryosphere Discuss., 8, 1279-1304, doi:10.5194/tcd-8-1279-2014, 2014

http://www.the-cryosphere-discuss.net/8/1279/2014/tcd-8-1279-2014.html (http://www.the-cryosphere-discuss.net/8/1279/2014/tcd-8-1279-2014.html)

"Abstract. Climate models predict Antarctic precipitation to increase during the 21st century, but their present day Antarctic precipitation differs. A fully model-independent climatology of the Antarctic precipitation characteristics, such as snowfall rates and frequency, is needed to assess the models, but was not available so far. Satellite observation of precipitation by active spaceborne sensors has been possible in the polar regions since the launch of CloudSat in 2006. Here we use CloudSat products to build the first multi-year model-independent climatology of Antarctic precipitation. The mean snowfall rate from August 2006 to April 2011 is 171 mm yr−1 over the Antarctic ice sheet north of 82° S. The ECMWF ERA Interim dataset agrees well with the new satellite climatology."

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 02, 2014, 11:12:05 PM
The linked research indicates that there are several negative feedback mechanisms inhibiting the acceleration of basal slip due to subglacial hydrology.  This is not to say that subglacial hydrology does not accelerate basal sliding, only that it is somewhat self-limiting:

Hoffman, M. J., and S. Price (2014), Feedbacks between coupled subglacial hydrology and glacier dynamics, J. Geophys. Res. Earth Surf., 119, doi:10.1002/2013JF002943

http://onlinelibrary.wiley.com/doi/10.1002/2013JF002943/abstract (http://onlinelibrary.wiley.com/doi/10.1002/2013JF002943/abstract)

Abstract: "On most glaciers and ice sheet outlets the majority of motion is due to basal slip, a combination of basal sliding and bed deformation. The importance of basal water in controlling sliding is well established, with increased sliding generally related to high basal water pressure, but the details of the interactions between the ice and water systems has not received much study when there is coupling between the systems. Here we use coupled subglacial hydrology and ice dynamics models within the Community Ice Sheet Model to investigate feedbacks between the ice and water systems. The dominant feedback we find is negative: sliding over bedrock bumps opens additional cavity space, which lowers water pressure and, in turn, sliding. We also find two small positive feedbacks: basal melt increases through frictional heat during sliding, which raises water pressure, and strain softening of basal ice during localized speedup causes cavities to close more quickly and maintain higher water pressures. Our coupled modeling demonstrates that a sustained input of surface water to a distributed drainage system can lead to a speedup event that decays even in the absence of channelization, due to increased capacity of the system through opening of cavities, which is enhanced through the sliding-opening feedback. We find that the negative feedback resulting from sliding-opening is robust across a wide range of parameter values. However, our modeling also argues that subglacial channelization is required to terminate speedup events over timescales that are commensurate with observations of late summer slowdown on mountain glaciers."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 02, 2014, 11:18:18 PM
The linked reference (with a free access pdf) provide glaciology about slip-stick events of the Whillians Ice Stream; which should help understand related behavior in other marine glaciers:

Pratt, M. J., J. P. Winberry, D. A. Wiens, S. Anandakrishnan, and R. B. Alley (2014), Seismic and geodetic evidence for grounding-line control of Whillans Ice Stream stick-slip events, J. Geophys. Res. Earth Surf., 119, doi:10.1002/2013JF002842.

http://onlinelibrary.wiley.com/doi/10.1002/2013JF002842/abstract (http://onlinelibrary.wiley.com/doi/10.1002/2013JF002842/abstract)

Abstract: "The tidally modulated, stick-slip events of Whillans Ice Stream in West Antarctica produce seismic energy from three locations near the grounding line. Using ice velocity records obtained by combining time series from colocated broadband seismometers and GPS receivers installed on the ice stream during the 2010–2011 and 2011–2012 austral summers, along with far-field seismic recordings of elastic waves, we locate regions of high rupture velocity and stress drop. These regions, which are analogous to “asperities” in traditional seismic fault studies, are areas of elevated friction at the base of the ice stream. Slip events consistently initiate at one of two locations: near the center of the ice stream, where events associated with the Ross Sea high tide originate, or a grounding-line spot, where events associated with the Ross Sea low tide initiate, as well as occasional high-tide events following a skipped low-tide event. The grounding-line site, but not the central site, produces Rayleigh waves observable up to 1000 km away, through fast expansion of the slip area. Grounding-line initiation events also show strong directivity in the downstream direction, indicating initial rupture propagation at 1.5 km/s, compared to an average of 0.150 km/s for the entire slip event. Following slip initiation, additional seismic energy is produced from two sources located near the grounding line: first at the downstream end of Subglacial Lake Engelhardt and second toward the farthest downstream extent of the ice stream. This evidence suggests that the stronger, higher-friction material along the grounding line controls motion throughout the stick-slip region."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 02, 2014, 11:41:14 PM
The linked reference (with a free access pdf), uses a numerical model of the rapid retreat of the paleo-ice stream in Marguerite Bay, Antarctic Peninsula, during the last deglaciation.  Lessons learned from this study include: (a) the concurrent application of multiple forcing mechanisms (SLR, warm ocean water, basal melting, etc.) can accelerate glacial retreat past a threshold condition into rapid retreat; (b) that ocean – ice interaction and bed topology are dominant factors in the rate of glacial retreat.  None of these findings are good news for the stability of the Thwaites Glacier.

Jamieson, S. S. R., A. Vieli, C. Ó Cofaigh, C. R. Stokes, S. J. Livingstone, and C.-D. Hillenbrand (2014), Understanding controls on rapid ice-stream retreat during the last deglaciation of Marguerite Bay, Antarctica, using a numerical model, J. Geophys. Res. Earth Surf., 119, doi:10.1002/2013JF002934

http://onlinelibrary.wiley.com/doi/10.1002/2013JF002934/abstract (http://onlinelibrary.wiley.com/doi/10.1002/2013JF002934/abstract)

Abstract: "Using a one-dimensional numerical model of ice-stream flow with robust grounding-line dynamics, we explore controls on paleo-ice-stream retreat in Marguerite Bay, Antarctica, during the last deglaciation. Landforms on the continental shelf constrain the numerical model and suggest that retreat was rapid but punctuated by a series of slowdowns. We investigate the sensitivity of ice-stream retreat to changes in subglacial and lateral topography and to forcing processes including sea-level rise, enhanced melting beneath an ice shelf, atmospheric warming, and ice-shelf debuttressing. Our experiments consistently reproduce punctuated retreat on a bed that deepens inland, with retreat-rate slowdowns controlled by narrowings in the topography. Sensitivity experiments indicate that the magnitudes of change required for individual forcing mechanisms to initiate retreat are unrealistically high but that thresholds are reduced when processes act in combination. The ice stream is, however, most sensitive to ocean warming and associated ice-shelf melting, and retreat was most likely in response to external forcing that endured throughout the period of retreat rather than to a single triggering “event.” Timescales of retreat are further controlled by the delivery of ice from upstream of the grounding line. Due to the influence of topography, modeled retreat patterns are insensitive to the temporal pattern of forcing evolution. We therefore suggest that despite regionally similar forcing mechanisms, landscape controls significant contrasts in retreat behavior between adjacent but topographically distinct catchments. Patterns of ice-stream retreat in the past, present, and future should therefore be expected to vary significantly."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 24, 2014, 08:19:00 PM
The linked reference presents an advanced analytical model for iceberg calving from marine terminating glaciers:

Krug, J., Weiss, J., Gagliardini, O., and Durand, G.: Combining damage and fracture mechanics to model calving, The Cryosphere Discuss., 8, 1631-1671, doi:10.5194/tcd-8-1631-2014, 2014.

http://www.the-cryosphere-discuss.net/8/1631/2014/tcd-8-1631-2014.html (http://www.the-cryosphere-discuss.net/8/1631/2014/tcd-8-1631-2014.html)

"Abstract. Calving of icebergs is a major negative component of polar ice-sheet mass balance. We present a new calving modeling framework relying on both continuum damage mechanics and linear elastic fracture mechanics. This combination accounts for both the slow sub-critical surface crevassing and fast propagation of crevasses when calving occurs. First, damage of the ice occurs over long timescales and enhances the viscous flow of ice. Then brittle fracture propagation happens downward, over very short timescales, in ice considered as an elastic medium. The model is validated on Helheim Glacier, South-West Greenland, one of the most monitored fast-flowing outlet glacier. This allows to identify sets of model parameters giving a consistent response of the model and producing a dynamic equilibrium in agreement with observed stable position of the Helheim ice front between 1930 and today."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 09, 2014, 07:25:09 PM
The linked article discusses a 2,000 yr old ice core drilled in the heart of Antarctica (in the EAIS) that could help to locate a future ice core that could reach back about a million years into the past:

http://www.japantimes.co.jp/news/2014/05/09/world/science-health-world/scientists-drill-2000-year-old-ice-core-in-antarctics-heart/#.U20Oo6Pn_IU (http://www.japantimes.co.jp/news/2014/05/09/world/science-health-world/scientists-drill-2000-year-old-ice-core-in-antarctics-heart/#.U20Oo6Pn_IU)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: icefest on May 10, 2014, 03:38:40 AM
The linked article discusses a 2,000 yr old ice core drilled in the heart of Antarctica (in the EAIS) that could help to locate a future ice core that could reach back about a million years into the past:

http://www.japantimes.co.jp/news/2014/05/09/world/science-health-world/scientists-drill-2000-year-old-ice-core-in-antarctics-heart/#.U20Oo6Pn_IU (http://www.japantimes.co.jp/news/2014/05/09/world/science-health-world/scientists-drill-2000-year-old-ice-core-in-antarctics-heart/#.U20Oo6Pn_IU)


What do they mean by this:
Quote
“Such an ice core would help us understand what caused a dramatic shift in the frequency of ice ages about 800,000 years ago, and further understand the role of carbon dioxide in climate change,” said Curran.

How does a 2,000 year core get records back to 800,000 years?
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 10, 2014, 05:15:41 AM
icefest,

It is my understanding that the 2,000-year old ice core does not provide any insight about the dramatic shift in frequency of the ice ages; it only offers insight on how in the EAIS the ice layers are laid-down (such as the rate of layering) so that the scientist can best select the site for the next (much longer) ice core hole that should reach back 1,000,000 years into the past. Any while I am not a scientist (I am an engineer), I believe that about 800,000 years ago the frequency of the ice ages increased, and I believe that polar amplification (including the influence of carbon dioxide and other feedback mechanisms) had a lot to do with the periodicity of these ice ages (and the future 1,000,000-year old ice core should help the scientists unravel the relationships of forcing (solar or otherwise) and climate sensitivity.

Best,
ASLR
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 10, 2014, 06:11:36 AM
icefest,

If you are more interested in the paleo-details then in the glaciology (of the integrity & thickness of the ice layers in the cores), then some of the information presented in the Paleo thread see the link below (including Reply #4 that explains the attached image from Hansen et al of the frequency of ice ages and global temperatures with time):

http://forum.arctic-sea-ice.net/index.php?topic=130.100 (http://forum.arctic-sea-ice.net/index.php?topic=130.100)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: Laurent on May 10, 2014, 09:12:08 AM
Certainly a mistake AbruptSLR you wrote 80.000 years...I guess you meant 800.000 years...?
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 10, 2014, 05:14:52 PM
Laurent,

Thanks for the catch.  I have corrected the original text to 800,000.

Best,
ASLR
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: icefest on May 10, 2014, 06:03:29 PM
Thanks for the detailed explanation ASLR, the explanation in combination with the day to think about it have worked and I'm pretty sure I understand it now.

I wonder who will get the first 100ky 1,000Kyr core.. Australia, US, China, Russia?
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on May 10, 2014, 09:39:00 PM
There are already 100Kyr and 400Kyr cores from GISP and Vostok. The planned one goes to 88Kyr.

sidd
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: icefest on May 10, 2014, 10:30:00 PM
Sorry, typo.

I meant 1,000Kyr
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 11, 2014, 02:02:38 AM
The following reference and the posts in the following link to the WAIS Divide Ice Core project discuss findings of this great study, reaching back 800,000 years:

http://forum.arctic-sea-ice.net/index.php/topic,333.0.html (http://forum.arctic-sea-ice.net/index.php/topic,333.0.html)


Edwards, J., Brook, E., Fegyveresi, J., Lee, J., Mitchell, L., Sowers, T., Alley, R., McConnell, J., Severinghaus, J. and Baggenstos, D. (2014), "Millennial and Sub-millennial Variability of Total Air Content from the WAIS Divide Ice Core", EGU General Assembly 2014, held 27 April - 02 May 2014 in Vienna, Austria, id. EGU2014-15368

Abstract: "The analysis of ancient air bubbles trapped in ice is integral to the reconstruction of climate over the last 800 ka. While mixing ratios of greenhouse gases along with isotopic ratios are being studied in ever increasing resolution, one aspect of the gas record that continues to be understudied is the total air content (TAC) of the trapped bubbles. Published records of TAC are often too low in temporal resolution to adequately capture sub-millennial scale variability.
Here we present a high-resolution TAC record (10-50 year sampling resolution) from the WAIS Divide ice core, measured at Oregon State and Penn State Universities. The records cover a variety of climatic conditions over the last 56 ka and show millennial variability of up to 10% and sub-millennial variability between 2.5 and 3.5%. We find that using the pore close off volume parameterization (Delomotte et al., J. Glaciology, 1999, v.45), along with the site temperature derived from isotopes, our TAC record implies unrealistically large changes in surface pressure or elevation. For example, the TAC decreases by ~10% between 19.5ka and 17.3ka, and would imply an elevation increase of nearly 800m. The total accumulation of ice over this period is just 280m (Fudge et al. Nature 2013), making the calculated elevation interpretation implausible.
To resolve this discrepancy, we investigate the millennial and sub-millennial variability in our TAC record as a function of changes in firn densification and particularly layering. The firn is the uppermost layer of an ice sheet where snow is compressed into ice, trapping ancient air. Thus firn processes are important for the interpretation of total air content as well as other gas records. We compare our TAC record with proxies for dust, temperature and accumulation to determine how processes other than elevation affect TAC."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 29, 2014, 11:25:05 PM
The following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm (http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm)

This research indicates that meltwater plumes could act as a positive feedback mechanisms to promote ice calving on a vertical glacier face:

70A0916
The effect of meltwater plumes on the melting of a vertical glacier face
Satoshi KIMURA, Paul HOLLAND, Adrian JENKINS, Matthew PIGGOTT
Corresponding author: Satoshi Kimura
Corresponding author e-mail: skimura04@gmail.com

Abstract: "Ice-sheet meltwater is commonly discharged into ocean fjords from the bottom of deep fjord-terminating glaciers. This meltwater forms upwelling plumes in front of the glacier calving face. We simulate the meltwater plumes using a non-hydrostatic ocean model with a mesh that is unstructured in three dimensions and subgrid mixing calibrated by comparison to established plume theory. The presence of an ice face reduces the entrainment of sea water into the meltwater plumes, so the plumes remain attached to the ice front, in contrast to previous simple models. Ice melting increases with height above the discharge, also in contrast to some simple models, and we speculate that this ‘overcutting’ may contribute to the tendency of icebergs to topple inwards toward the ice face upon calving. When two channels are located in close proximity, the meltwater plumes can coalesce and form a single plume. Such merged meltwater plumes ascend faster but occupy a smaller fraction of the ice face, so that the melt rate averaged over the glacier decreases. The overall melt rate is found to increase with discharge flux only up to a critical value, which depends on the channel size, and decrease thereafter. For a given discharge flux, the geometry of the plume source also significantly affects the melting, with higher melt rates obtained for a shallower, wider source. We speculate that the melt rate per unit discharge decreases as the ice-sheet melting season progresses and the subglacial system becomes more channelized. The melt rate is not a simple function of the subglacial discharge flux, as assumed by many previous studies."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 30, 2014, 02:05:13 AM
The following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm (http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm)


O'Leary 2014 presents a practicable web-based approach to ice-sheet models for education:

70A1104
An interactive ice-sheet model for education
Martin O’LEARY
Corresponding author: Martin O’Leary
Corresponding author e-mail: m.e.w.oleary@gmail.com

Abstract: "Much of modern glaciology is focused on the develoment and application of large-scale numerical models. These models incorporate a wide range of processes at a high level of fidelity. Consequently, in order to work with these models, users often require considerable computational resources, as well as a high level of technical skill. These factors reflect the priorities of a research environment. In an educational context, or for simple experiments designed to build one’s intuitions, the priorities are very different. Ease of use is much more important, as is direct access to a range of outputs. Simple representations of individual processes may be more useful than a more complex model of interactions. These priorities lead to a very different kind of model, many of which exist, but few of which are publically available. Here we present a web-based interactive model of the Greenland and East Antarctic ice sheets, based on the popular GRANTISM model by Frank Pattyn. Using Javascript, the model code runs in the user’s web browser, allowing for a great deal of interactivity. Both the model forcings and state can be inspected and modified in ‘real time’, and on even modest modern computers and handheld devices, simulations of tens of thousands of years of evolution run in a matter of seconds. Because of the web-based approach, no technical knowledge is required to operate the model, but the results are easily exported for further analysis using more traditional tools."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on May 30, 2014, 07:55:29 PM
The following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm (http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm)

When the Thwaites Glacier starts exhibiting Jakobshavn type behavior in the future, the impact of melange on glacier dynamics will be critical:

70A1060
Investigating the impact of melange on glacier dynamics
Jean KRUG, Gaël DURAND, Olivier GAGLIARDINI, Jérôme WEISS
Corresponding author: Jean Krug
Corresponding author e-mail: jean.krug@ujf-grenoble.fr

Abstract: "Ice melange is a heterogeneous mixture of sea ice, wind-blown snow, fragments of marine ice and calved icebergs which can be found in the fjords of outlet glaciers in Greenland, or in Antarctica, inside the rifts of larges ice shelves. Its behaviour is highly dependant on the season: in winter, freezing sea ice rigidifies the melange by binding together its component; in summer, melting of ice weakens the melange, allowing each of its components to move independently from the others. In the Greenlandic fjords, observations have shown that the seasonal cycles of advance and retreat of the outlets glaciers are correlated with the state of the melange, with an advancing front in winter preceding a rapid retreat of the glacier in the late spring, when the melange weakens. Previous studies suggest that the back force applied by the rigid melange layer in winter may prevent calved icebergs from rotating away from the glacier front. As a response, the glacier front advances and slows down. On the contrary, the abrupt disintegration of melange in spring releases the back pressure, allowing icebergs to detach from the glacier, leading to a front retreat and an increase of ice velocity. Here, we study this process using a new calving law based on both continuum damage mechanics and fracture mechanics. This framework is implemented in the Elmer/Ice full-Stokes finite-element model, and thus allows for a reliable representation of processes occurring at the front. Several experiments are carried out, investigating the response of a synthetic outlet glacier to different parameters, such as the melange thickness, the applied back force and the glacier size. At last, the impact of a seasonal variability of the strength of the melange layer on the behaviour of the glacier over several years is investigated."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on July 11, 2014, 06:18:04 PM
As policy makers will not fully recognize the risks of SLR until model projections can capture the correct ice mass loss response from at least the WAIS, the following linked reference focused on the modeling efforts for WAIS provides valuable insight on the historical modeling progress already made, and the future challenges yet to be over-come:

William Thomas, (2014), "Research agendas in climate studies: the case of West Antarctic Ice Sheet research", Climate Change, Volume 122, Issue 1-2, pp 299-311

http://link.springer.com/article/10.1007/s10584-013-0981-3#page-1 (http://link.springer.com/article/10.1007/s10584-013-0981-3#page-1)


Abstract: "Concern over anthropogenic climatic change has been the major driver behind the rapid expansion in climate studies in recent decades. However, research agendas revolving around other intellectual or practical problems motivate much of the work that contributes to scientific understanding of present changes in climate. Understanding these agendas and their historical development can help in planning research programs and in communicating results, and it can often elucidate the sources of disagreements between scientists pursuing differing agendas. This paper focuses on research agendas relating to the possible glaciological instability of the West Antarctic Ice Sheet (WAIS). For much of the history of this research, which dates back to International Geophysical Year traverses, WAIS instability was thought of as innate rather than climatically triggered, even as a growing program of intensive field research was heavily motivated by tentative links drawn between WAIS instability and concerns over anthropogenic climatic change. Meanwhile, climate models for many years did not countenance instability mechanisms. It is only over the past fifteen years that field glaciological research has been integrated with other forms of empirical research, and that empirical studies of WAIS have been more closely integrated with the broader body of climate studies."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 13, 2014, 12:50:09 AM
First, the first linked reference provides information on how to consider the influence of Atmospheric River events on snowfall in East Antarctica:

Irina V. Gorodetskaya, Maria Tsukernik, Kim Claes, Martin F. Ralph, William D. Neff and Nicole P. M. Van Lipzig, (2014), "The role of atmospheric rivers in anomalous snow accumulation in East Antarctica", Geophysical Research Letters, DOI: 10.1002/2014GL060881


http://onlinelibrary.wiley.com/doi/10.1002/2014GL060881/abstract (http://onlinelibrary.wiley.com/doi/10.1002/2014GL060881/abstract)


Abstract: "Recent, heavy snow accumulation events over Dronning Maud Land (DML), East Antarctica, contributed significantly to the Antarctic ice sheet surface mass balance (SMB). Here, we combine in-situ accumulation measurements and radar-derived snowfall rates from Princess Elisabeth station (PE), located in the DML escarpment zone, along with the ECMWF Interim re-analysis to investigate moisture transport patterns responsible for these events. In particular, two high-accumulation events in May 2009 and February 2011 showed an atmospheric river (AR) signature with enhanced integrated water vapor (IWV), concentrated in narrow long bands stretching from subtropical latitudes to the East Antarctic coast. Adapting IWV-based AR threshold criteria for Antarctica (by accounting for the much colder and drier environment), we find that it was four-five ARs reaching the coastal DML that contributed 74-80% of the outstanding SMB during 2009 and 2011 at PE. Therefore, accounting for ARs is crucial for understanding East Antarctic SMB."


Second, the following linked reference provides an open access pdf of a paper providing guidance on how much snow falls on the AIS:

Palerme, C., Kay, J. E., Genthon, C., L'Ecuyer, T., Wood, N. B., and Claud, C., (2014), "How much snow falls on the Antarctic ice sheet?", The Cryosphere, 8, 1577-1587, doi:10.5194/tc-8-1577-2014.

http://www.the-cryosphere.net/8/1577/2014/tc-8-1577-2014.html (http://www.the-cryosphere.net/8/1577/2014/tc-8-1577-2014.html)

Abstract: "Climate models predict Antarctic precipitation to increase during the 21st century, but their present day Antarctic precipitation differs. A model-independent climatology of the Antarctic precipitation characteristics, such as snowfall rates and frequency, is needed to assess the models, but it is not yet available. Satellite observations of precipitation by active sensors has been possible in the polar regions since the launch of CloudSat in 2006. Here, we use two CloudSat products to generate the first multi-year, model-independent climatology of Antarctic precipitation. The first product is used to determine the frequency and the phase of precipitation, while the second product is used to assess the snowfall rate. The mean snowfall rate from August 2006 to April 2011 is 171 mm year−1 over the Antarctic ice sheet, north of 82° S. While uncertainties on individual precipitation retrievals from CloudSat data are potentially large, the mean uncertainty should be much smaller, but cannot be easily estimated. There are no in situ measurements of Antarctic precipitation to directly assess the new climatology. However, distributions of both precipitation occurrences and rates generally agree with the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim data set, the production of which is constrained by various in situ and satellite observations, but does not use any data from CloudSat. The new data set thus offers unprecedented capability to quantitatively assess Antarctic precipitation statistics and rates in climate models."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 26, 2014, 12:38:57 AM
Per the linked reference, models are making progress in simulating ice shelf flow dynamics (focused on Antarctica).

E. Larour, A. Khazendar, C. P. Borstad, H. Seroussi, M. Morlighem, E. Rignot, (2014), "Representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics: Application to Brunt/Stancomb-Wills Ice Shelf, Antarctica", DOI: 10.1002/2014JF003157

http://onlinelibrary.wiley.com/enhanced/doi/10.1002/2014JF003157/ (http://onlinelibrary.wiley.com/enhanced/doi/10.1002/2014JF003157/)

Abstract: "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."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on October 17, 2014, 11:28:35 PM
Per the following NASA link, the sixth year of the Operation IceBridge flights in Antarctica began yesterday on Oct 16 2014 and will run through late November 2014:

http://www.nasa.gov/press/2014/october/nasa-begins-sixth-year-of-airborne-antarctic-ice-change-study-0/#.VEDr0fnF92A (http://www.nasa.gov/press/2014/october/nasa-begins-sixth-year-of-airborne-antarctic-ice-change-study-0/#.VEDr0fnF92A)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on October 30, 2014, 01:36:10 AM
I have already comments on may of the relevant abstracts at the following link leaded website with abstract from the IGSOC 2014 Chamonix Symposium earlier this year; however, as there are many topic cove that in that symposium that I did not comment on earlier, I provide the following link to those that are interested in looking for themselves:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/proceedings.html (http://www.igsoc.org/symposia/2014/chamonix/proceedings/proceedings.html)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 12, 2015, 11:22:43 PM
The linked reference (with an open access pdf) indicates that marine glaciers are sensitive to short-time scale cyclical perturbations (such as El Nino/La Nina events) and that particularly West Antarctica-type of marine glaciers are susceptible to accelerated ice mass loss due to such forcing.

Aykutlug, E. and K. Dupont, T. , (2015), "A sensitivity study of fast outlet glaciers to short timescale cyclical perturbations", The Cryosphere Discuss., 9, 223-250, doi:10.5194/tcd-9-223-2015.

http://www.the-cryosphere-discuss.net/9/223/2015/tcd-9-223-2015.html (http://www.the-cryosphere-discuss.net/9/223/2015/tcd-9-223-2015.html)

Abstract. The dynamic response of outlet glaciers on short (annual to decadal) timescales is affected by various external forcings, such as basal or oceanic conditions. Understanding the sensitivity of the dynamic response to such forcings can help assess more accurate ice volume projections. In this work, we investigate the spatiotemporal sensitivity of outlet glaciers to fast cyclical forcings using a one-dimensional depth and width-averaged heuristic model. Our results indicate that even on such short timescales, nonlinearities in ice dynamics may lead to an asymmetric response, despite the forcing functions being symmetric around each reference value. Results also show that such short-timescale effects become more pronounced as glaciers become closer to flotation. While being qualitatively similar for both downsloping and upsloping bed geometries, the results indicate higher sensitivity for upsloping ("West Antarctica-like") beds. The range in asymmetric response for different configurations motivate parameterizing or including short-timescale effects in models while investigating the dynamic behavior of outlet glaciers.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 22, 2015, 01:48:05 AM
The linked University of Washington article discusses the first deep ice core taken at the South Pole:


http://www.washington.edu/news/2015/01/20/scientists-drilling-first-deep-ice-core-at-the-south-pole/ (http://www.washington.edu/news/2015/01/20/scientists-drilling-first-deep-ice-core-at-the-south-pole/)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 17, 2015, 06:50:56 PM
The linked reference indicates that Antarctica is warming so rapidly that the warmer air holds more precipitation which is increasing snow fall that could reduce the amount of SLR by 2115 by about 3cm; however, this will also trigger increased calving of glacial ice mass loss.

Frieler, K., Clark, P.U., He, F., Buizert, C., Reese, R., Ligtenberg, S.R.M., van den Broeke, M.R., Winkelmann, R., Levermann, A. (2015), "Consistent evidence of increasing Antarctic accumulation with warming", Nature Climate Change, doi: 10.1038/nclimate2574

http://www.nature.com/articles/nclimate2574.epdf?referrer_access_token=zcVCkk2YskeU2qFuarv4_9RgN0jAjWel9jnR3ZoTv0OomwGMYEHQXIw07hboCIJ1SX43Rq67EUUxuZAjoUU_wHJ70bZTf6FCezp21MtW3g7tH1F7FNr6s-70_-FvsvjUcZ22Z_iv4mXf9qUEbGor2wIGw38Ny_NMou_s13prYCfmuUbQq6GosUbd4MTdsAAaEK-IgNxXLru4Ae8WRcaG366d-aSFhIpgxBnE4kF9h-c%3D&tracking_referrer=www.spacedaily.com (http://www.nature.com/articles/nclimate2574.epdf?referrer_access_token=zcVCkk2YskeU2qFuarv4_9RgN0jAjWel9jnR3ZoTv0OomwGMYEHQXIw07hboCIJ1SX43Rq67EUUxuZAjoUU_wHJ70bZTf6FCezp21MtW3g7tH1F7FNr6s-70_-FvsvjUcZ22Z_iv4mXf9qUEbGor2wIGw38Ny_NMou_s13prYCfmuUbQq6GosUbd4MTdsAAaEK-IgNxXLru4Ae8WRcaG366d-aSFhIpgxBnE4kF9h-c%3D&tracking_referrer=www.spacedaily.com)

Abstract: "Projections of changes in Antarctic Ice Sheet (AIS) surface mass balance indicate a negative contribution to sea level because of the expected increase in precipitation due to the higher moisture holding capacity of warmer air. Observations over the past decades, however, are unable to constrain the relation between temperature and accumulation changes because both are dominated by strong natural variability. Here we derive a consistent continental-scale increase in accumulation of approximately 5 ± 1% K−1, through the assessment of ice-core data (spanning the large temperature change during the last deglaciation, 21,000 to 10,000 years ago), in combination with palaeo-simulations, future projections by 35 general circulation models (GCMs), and one high-resolution future simulation. The ice-core data and modelling results for the last deglaciation agree, showing uniform local sensitivities of ~6% K−1. The palaeo-simulation allows for a continental-scale aggregation of accumulation changes reaching 4.3% K−1. Despite the different timescales, these sensitivities agree with the multi-model mean of 6.1 ± 2.6% K−1 (GCM projections) and the continental-scale sensitivity of 4.9% K−1 (high-resolution future simulation). Because some of the mass gain of the AIS is offset by dynamical losses induced by accumulation, we provide a response function allowing projections of sea-level fall in terms of continental-scale accumulation changes that compete with surface melting and dynamical losses induced by other mechanisms."

See also:
http://www.enn.com/climate/article/48347 (http://www.enn.com/climate/article/48347)

Extract: " What they found was that Antarctica warmed an average of 5 to 10 degrees (Celsius) during that period – and for every degree of warming, there was a 5 percent increase in snowfall.

“The additional weight of the snow would have increased the ice flow into the ocean offsetting some of the limiting effect on sea level rise,” said Katja Frieler, a climatologist at the Potsdam Institute for Climate Impact Research in Germany and the lead author of the study. “It’s basic ice physics.”"

http://www.spacedaily.com/reports/Global_warming_brings_more_snow_to_Antarctica_999.html (http://www.spacedaily.com/reports/Global_warming_brings_more_snow_to_Antarctica_999.html)

Extract: "Snow piling up on the ice is heavy and presses down - the higher the ice, the more pressure. Because additional snowfall elevates the grounded ice-sheet on the Antarctic continent but less so the floating ice shelves at its shore, the ice flows more rapidly into the ocean and contributes to sea level," co-author Ricarda Winkelmann from PIK explains.

Accounting for this effect a 5-percent increase in snowfall on Antarctica would mean a calculative drop in sea-level of about 3 cm after 100 years. Other processes, however, will effect a rise in sea-level in the end. For instance, already rather little warming of the ocean could cause ice at the Antarctic shore to break off more easily, hence more ice mass from the continent would flow out and discharge into the ocean."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 24, 2015, 11:59:41 PM
The linked reference (with an open access pdf) does not consider cliff-failures or hydrofracturing of marine glaciers and thus projects relatively low WAIS contributions to SLR by 2100:

Cornford, S. L., Martin, D. F., Payne, A. J., Ng, E. G., Le Brocq, A. M., Gladstone, R. M., Edwards, T. L., Shannon, S. R., Agosta, C., van den Broeke, M. R., Hellmer, H. H., Krinner, G., Ligtenberg, S. R. M., Timmermann, R., and Vaughan, D. G. (2015), "Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate", The Cryosphere Discuss., 9, 1887-1942, doi:10.5194/tcd-9-1887-2015.

http://www.the-cryosphere-discuss.net/9/1887/2015/tcd-9-1887-2015.html (http://www.the-cryosphere-discuss.net/9/1887/2015/tcd-9-1887-2015.html)


Abstract: "We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet. Each of the simulations begins with a geometry and velocity close to present day observations, and evolves according to variation in meteoric ice accumulation, ice shelf melting, and mesh resolution. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rates anomalies that remove most of the ice shelves over a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions, ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Sensitivity to mesh resolution is spurious, and we find that sub-kilometer resolution is needed along most regions of the grounding line to avoid systematic under-estimates of the retreat rate, although resolution requirements are more stringent in some regions – for example the Amundsen Sea Embayment – than others – such as the Möller and Institute ice streams."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna on March 26, 2015, 11:34:29 PM
I need help with flow of a glacier in a channel of rectangular, elliptic or parabolic cross-section.
.
.
.
.
anyone can help me with the numerical formulas ? :) :)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 27, 2015, 12:02:04 AM
I need help with flow of a glacier in a channel of rectangular, elliptic or parabolic cross-section.
.
.
.
.
anyone can help me with the numerical formulas ? :) :)

The attached text book chapter is the best that I can do.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna on March 27, 2015, 04:23:54 PM
Thanks a lot.
.
.
Sorry to say, i want to bother you guys lots.....I am reading the chapter that AbrusptSLR have forwarded me. From there I have some questions, very basics though....but need to understand...
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Can you please tell me " what is deviatoric part of the stress tensor?
                                    " what is strain rate" ? EQUATION 10.3 and 10.4.
                                    "what are those different notations used in the equation 10.3 & 10.4? "
                                    " In glen's law: what is second stress invariant?"
                                     "  -do-           : what is summation convention?"
                                   
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 27, 2015, 04:59:51 PM

Can you please tell me " what is deviatoric part of the stress tensor?

See: http://en.wikipedia.org/wiki/Cauchy_stress_tensor (http://en.wikipedia.org/wiki/Cauchy_stress_tensor)

                                    " what is strain rate" ? EQUATION 10.3 and 10.4.

See: http://en.wikipedia.org/wiki/Strain_rate (http://en.wikipedia.org/wiki/Strain_rate)

                                    "what are those different notations used in the equation 10.3 & 10.4? "

See: http://en.wikipedia.org/wiki/Strain_rate (http://en.wikipedia.org/wiki/Strain_rate)

                                    " In glen's law: what is second stress invariant?"

See: http://en.wikipedia.org/wiki/Cauchy_stress_tensor (http://en.wikipedia.org/wiki/Cauchy_stress_tensor)

                                     "  -do-           : what is summation convention?"

See: http://glaciers.gi.alaska.edu/sites/default/files/Notes_Icdynamics_Aschwanden.pdf (http://glaciers.gi.alaska.edu/sites/default/files/Notes_Icdynamics_Aschwanden.pdf)
&
http://go.owu.edu/~chjackso/Climate/papers/Glen_1958_The%20flow%20law%20of%20ice.pdf (http://go.owu.edu/~chjackso/Climate/papers/Glen_1958_The%20flow%20law%20of%20ice.pdf)
                                 
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna on March 27, 2015, 05:39:37 PM
Thank you for the help.
.
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Once tried to understand the Wikipedia things, but....couldn't get those. Anyways, will try again and let you know !!
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna 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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna 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
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna on March 27, 2015, 06:04:32 PM
Why is it called Newtonian fluid ?
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: Laurent 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
Quote
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 ?
Quote
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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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).
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna on March 27, 2015, 06:45:19 PM
Anyways, thanks for the help. Along with that, sorry for asking silly questions.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: rituparna 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
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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://dx.doi.org/10.3189/2015JoG14J048)

http://www.ingentaconnect.com/content/igsoc/jog/2015/00000061/00000225/art00003 (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."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: wili on July 13, 2015, 04:54:26 PM
Study finds surprisingly high geothermal heating beneath West Antarctic Ice Sheet

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

(Apologies if this was already posted. Thanks to COBob at robertscribbler's blog for this.)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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.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://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 (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.""
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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.""


Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/ (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.

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: solartim27 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=87859)

http://earthobservatory.nasa.gov/IOTD/view.php?id=87657 (http://earthobservatory.nasa.gov/IOTD/view.php?id=87657)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/ (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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/ (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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/ (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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/ (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.”
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: A-Team on October 08, 2016, 03:12:45 PM
Quote
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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on October 08, 2016, 06:03:39 PM
Quote
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 (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/1/6/e1500093)
http://advances.sciencemag.org/content/advances/1/6/e1500093.full.pdf (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 (http://www.sciencedirect.com/science/article/pii/S0012821X14005780)

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/)
&
http://www.the-cryosphere-discuss.net/tc-2017-8/tc-2017-8.pdf (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)."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/ (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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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 (https://www.nature.com/articles/ncomms15228)
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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.

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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 (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."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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."

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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, (https://doi.org/10.5194/tc-2017-98,) in review, 2017.

http://www.the-cryosphere-discuss.net/tc-2017-98/ (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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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, (https://doi.org/10.5194/tc-2017-183,) in review, 2017.

https://www.the-cryosphere-discuss.net/tc-2017-183/ (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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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/abs/1704.05597)
https://arxiv.org/pdf/1704.05597.pdf (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 (http://nsidc.org/sites/nsidc.org/files/files/WAIS_SciPlanHMHF_final.pdf)

Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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 (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 (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."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR 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."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on December 04, 2017, 10:00:13 PM
The linked reference discusses how gas hydrates in the bed sediment beneath marine glaciers can cause 'sticky spots' that can regulate ice stream flow rates:

Winsborrow, M., K. Andreassen, A. Hubbard, A. Plaza-Faverola, E. Gudlaugsson and H. Patton (2016). "Regulation of ice stream flow through subglacial formation of gas hydrates." Nature Geosci 9(5): 370-374, DOI: 10.1038/NGEO2696

https://www.nature.com/articles/ngeo2696
&
http://www.nature.com/articles/ngeo2696.epdf?referrer_access_token=IHHHsNRUI3lD2eFpTMWvl9RgN0jAjWel9jnR3ZoTv0N6H6twa9eus1zouX_OVF0HHps81v4XTc0_11DCSpeGLDxz98tw1yul2mr16lbVJL4uOjHYggNVEvnorXQDpPb-4F8Dx03N10vp8xTpF1OSQUCQuGQbrx_agiKHwJMiE0Vb3p9RlZE1kgUDa_7CPZDbIHfa0-zC2RtwAc1-HEOzfwPw5ovCnEJWlCwr6K4nmQjxYGctlb4MLBBjUrGaOUBg&tracking_referrer=austhrutime.com

 Abstract: "Variations in the flow of ice streams and outlet glaciers are a primary control on ice sheet stability, yet comprehensive understanding of the key processes operating at the ice–bed interface remains elusive. Basal resistance is critical, especially sticky spots—localized zones of high basal traction—for maintaining force balance in an otherwise well-lubricated/high-slip subglacial environment. Here we consider the influence of subglacial gas-hydrate formation on ice stream dynamics, and its potential to initiate and maintain sticky spots. Geophysical data document the geologic footprint of a major palaeo-ice-stream that drained the Barents Sea–Fennoscandian ice sheet approximately 20,000 years ago. Our results reveal a ∼250 km sticky spot that coincided with subsurface shallow gas accumulations, seafloor fluid expulsion and a fault complex associated with deep hydrocarbon reservoirs. We propose that gas migrating from these reservoirs formed hydrates under high-pressure, low-temperature subglacial conditions. The gas hydrate desiccated, stiffened and thereby strengthened the subglacial sediments, promoting high traction—a sticky spot— that regulated ice stream flow. Deep hydrocarbon reservoirs are common beneath past and contemporary glaciated areas, implying that gas-hydrate regulation of subglacial dynamics could be a widespread phenomenon."

Also see:

Title: "Regulation of Ice Stream Flow Through Subglacial Formation of Gas Hydrates"

http://austhrutime.com/ice_stram_flow_regulation_subglacial_gas_hydrates.htm

Extract: "Based on the presence of extensive sedimentary basins and modelling studies (Wadham et al., 2012; Wallmann et al., 2012) it is proposed that abundant gas hydrate accumulations are present beneath the ice sheets of Greenland and Antarctica. Also, gas hydrates have been identified in ice core samples obtained from above the subglacial Lake Vostok in East Antarctica (Uchida et al., 1994). The role of potentially widespread gas hydrate reservoirs in the modification of the thermomechanical regime at the base of contemporary ice sheets, which makes them critically sensitive, as well as their impact on ice steam force balance and dynamics has, so far, not been recognised. This control that was previously unforeseen, given the current lack of knowledge with regard to the distribution of gas hydrate, represents a significant unknown in attempts to model the current and future discharge and evolution of contemporary ice sheets, as well as their contribution to rising global sea levels."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 02, 2018, 04:31:27 PM
The linked two-part open access reference discusses basal drag of an Antarctic Peninsula marine glacier:

Zhao, C., Gladstone, R. M., Warner, R. C., King, M. A., and Zwinger, T.: Basal drag of Fleming Glacier, Antarctica, Part A: sensitivity of inversion to temperature and bedrock uncertainty, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-241, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2017-241/

Abstract. Many glaciers in West Antarctica and the Antarctic Peninsula are now rapidly losing ice mass. Understanding of the dynamics of these fast-flowing glaciers, and their potential future behavior, can be improved through ice sheet modeling studies. Inverse methods are commonly used in ice sheet models to infer the basal shear stress, which has a large effect on the basal velocity and internal ice deformation. Here we use the full-Stokes Elmer/Ice model to simulate the Wordie Ice Shelf-Fleming Glacier system in the southern Antarctic Peninsula. With a control inverse method, we model the basal drag from the surface velocities observed in 2008. We propose a three-cycle spin-up scheme to remove the influence of initial temperature field on the final inversion. This is particularly important for glaciers with significant temperature-dependent internal deformation. We find that the Fleming Glacier has strong, temperature-dependent, deformational flow in the fast-flowing regions. Sensitivity tests using various bed elevation datasets and ice front boundary conditions demonstrate the importance of high-accuracy ice thickness/bed geometry data and precise location of the ice front boundary.

&

Zhao, C., Gladstone, R. M., Warner, R. C., King, M. A., and Zwinger, T.: Basal drag of Fleming Glacier, Antarctica, Part B: implications of evolution from 2008 to 2015, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-242, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2017-242/

Abstract. The Wordie Ice Shelf-Fleming Glacier system in the southern Antarctic Peninsula has experienced a long-term retreat and disintegration of its ice shelf in the past 50 years. Upstream glacier acceleration and dynamic thinning have been observed over the past two decades, especially after 2008 when only a little constraining ice shelf remained at the Fleming Glacier front. It is important to know whether the substantial speed up and surface draw-down of the glacier since 2008 is a direct response to increasing ocean forcing or driven by the feedback within an unstable marine-based glacier system or both. To explore the mechanism underlying the changes, we use a Stokes (full stress) model to simulate the basal shear stress of the Fleming system in 2008 and 2015. Recent observational studies have suggested the 2008–2015 velocity change was due to the ungrounding of the Fleming Glacier front. Our modelling shows that the fast flowing region of the Fleming Glacier shows a very low basal shear stress in 2008 but with a band of higher basal shear stress along the ice front. It indicates that the ungrounding process might have not started in 2008, which is consistent with the height above buoyancy calculation in 2008. Comparison of our inversions for basal shear stresses for 2008 and 2015 suggests the migration of the grounding line by ~ 9 km upstream from the grounding line position in 1996, a shift which is consistent with the change in floating area deduced from the height above buoyancy in 2015. The southern branch of the Fleming Glacier and the Prospect Glacier apparently have retreated by ~ 1–3 km from 2008 to 2015. The retrograde bed underneath the Fleming Glacier has promoted migration of the grounding line, which we suggest may be triggered by subglacial drainage as a response to the increased basal water supply through greater frictional heating at the ice-bedrock interface further upstream in the fast-flowing region. Improved knowledge of bed topography near the grounding line and further transient simulation is required to predict the future grounding line movement of the Fleming Glacier system precisely and subsequently understand better the ice dynamics and the its future contribution to sea level.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on January 02, 2018, 09:57:08 PM
The Wordie ice shelf was one of Mercer's canaries.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 03, 2018, 12:22:28 AM
The Wordie ice shelf was one of Mercer's canaries.

The next couple of decades may have those canaries on their last legs.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on January 03, 2018, 07:06:47 AM
Decades ? Wordie is already gone. The Mercer canary i watch now  is the midsummer 0C isotherm.

And  got a pet canary of my own. Rain. She looks unwell recently, with the rain on the Ross.

sidd
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: Csnavywx on January 03, 2018, 06:01:44 PM
Yeah, wasn't it the 0C January isotherm? If I'm recalling correctly, Larsen B didn't last very long after it crossed.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 03, 2018, 07:53:09 PM
And  got a pet canary of my own. Rain. She looks unwell recently, with the rain on the Ross.

sidd

I am sure that DeConto & Pollard are concerned about hydrofracturing of ice shelves due to rain (as illustrated by the attached image for last January).

ASLR
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on January 22, 2018, 11:53:35 PM
The linked reference studies the: "Effect of Topography on Subglacial Discharge and Submarine Melting During Tidewater Glacier Retreat"

J. M. Amundson & D. Carroll (9 January 2018), "Effect of Topography on Subglacial Discharge and Submarine Melting During Tidewater Glacier Retreat", JGR – Earth Surface, DOI: 10.1002/2017JF004376

http://onlinelibrary.wiley.com/doi/10.1002/2017JF004376/full

Title: "To first order, subglacial discharge depends on climate, which determines precipitation fluxes and glacier mass balance, and the rate of glacier volume change. For tidewater glaciers, large and rapid changes in glacier volume can occur independent of climate change due to strong glacier dynamic feedbacks. Using an idealized tidewater glacier model, we show that these feedbacks produce secular variations in subglacial discharge that are influenced by subglacial topography. Retreat along retrograde bed slopes (into deep water) results in rapid surface lowering and coincident increases in subglacial discharge. Consequently, submarine melting of glacier termini, which depends on subglacial discharge and ocean thermal forcing, also increases during retreat into deep water. Both subglacial discharge and submarine melting subsequently decrease as glacier termini retreat out of deep water and approach new steady state equilibria. In our simulations, subglacial discharge reached peaks that were 6–17% higher than preretreat values, with the highest values occurring during retreat from narrow sills, and submarine melting increased by 14% for unstratified fjords and 51% for highly stratified fjords. Our results therefore indicate that submarine melting acts in concert with iceberg calving to cause tidewater glacier termini to be unstable on retrograde beds. The full impact of submarine melting on tidewater glacier stability remains uncertain, however, due to poor understanding of the coupling between submarine melting and iceberg calving."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 20, 2018, 03:57:08 PM
Using statistics-based methodology (calibrated using observed data), the linked reference is slowly pushing ice-sheet models to adopt improved fracture models for both ice shelves and marine glaciers to improve iceberg calving events.  I note that DeConto & Pollard have used paleo-statistics in their ice-sheet models; so hopefully mainstream glaciologists will adopt improved fracture modeling techniques that will allow them to converge towards DeConto & Pollard's published findings:

Emetc, V., Tregoning, P., Morlighem, M., Borstad, C., and Sambridge, M.: A statistical fracture model for Antarctic ice shelves and glaciers, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-5, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-5/

Abstract. Antarctica and Greenland hold enough ice to raise sea level by more than 65 m if they were to melt completely. Predicting future ice sheet mass balance depends on our ability to model these ice sheets, which is limited by our current understanding of several key physical processes, such as iceberg calving. Large-scale ice flow models either ignore this process or represent it crudely. To model fracture formation, which is an important component of many calving models, Continuum Damage Mechanics as well as Linear Fracture Mechanics are commonly used. However, these methods applied across the Antarctic continent 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 this approach 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 84 % for grounded ice and 61 % for floating ice and mean overestimation error of 26 % and 20 %, respectively, thus providing the basis for modelling calving of ice shelves. We find that Antarctic ice shelves can be classified into groups based on the factors that control fracture location. The factors that trigger fracturing as well as sustain existing fractures advected from upstream vary from one ice shelf to another.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on March 22, 2018, 06:30:15 PM
Hopefully, the new radio-echo sounding (RES) diagnostic will be applied soon to better constrain the basal water beneath the Antarctic glaciers surveyed by Operation IceBridge (with over a decade worth of existing data ready to be analyzed).

Jordan, T. M., Williams, C. N., Schroeder, D. M., Martos, Y. M., Cooper, M. A., Siegert, M. J., Paden, J. D., Huybrechts, P., and Bamber, J. L.: A constraint upon the basal water distribution and basal thermal state of the Greenland Ice Sheet from radar bed-echoes, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-53, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-53/

Abstract. There is widespread, but often indirect, evidence that a significant fraction of the bed beneath the Greenland Ice Sheet is thawed (at or above the pressure melting point for ice). This includes the beds of major outlet glaciers and their tributaries and a large area around the NorthGRIP borehole in the ice-sheet interior. The ice-sheet scale distribution of basal water is, however, poorly constrained by existing observations. In principle, airborne radio-echo sounding (RES) enables the detection of basal water from bed-echo reflectivity, but unambiguous mapping is limited by uncertainty in signal attenuation. Here we introduce a new RES diagnostic for basal water that is associated with wet to dry transitions in bed material: bed-echo reflectivity variability. Importantly, this diagnostic is demonstrated to be attenuation-insensitive and the technique enables combined analysis of over a decade of Operation IceBridge survey data. The basal water predictions are compared with existing analyses for the basal thermal state (frozen and thawed beds) and geothermal heat flux. In addition to the outlet glaciers, we demonstrate widespread water storage in the northern and eastern interior. Notably, we observe a quasi-linear ‘corridor’ of basal water extending from NorthGRIP to Petermann glacier that spatially correlates with elevated heat flux predicted by a recent magnetic model. Finally, with a general aim to stimulate regional and process specific investigations, the basal water predictions are compared with bed topography, subglacial flow paths, and ice-sheet motion. The basal water distribution, and its relationship with the basal thermal state, provides a new constraint for numerical models.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on April 02, 2018, 07:37:12 PM
The linked reference provides field data between 2010 and 2016 about the grounding line location for all Antarctic marine glaciers:

Konrad et al. (2018), "Net retreat of Antarctic glacier grounding lines", Nature Geoscience, doi:10.1038/s41561-018-0082-z

http://www.nature.com/articles/s41561-018-0082-z

See also:

Title: "Antarctica retreating across the sea floor"

https://phys.org/news/2018-04-antarctica-retreating-sea-floor.html

Extract: "Research by the UK Centre for Polar Observation and Modelling (CPOM) at the University of Leeds has produced the first complete map of how the ice sheet's submarine edge, or "grounding line", is shifting. Most Antarctic glaciers flow straight into the ocean in deep submarine troughs, the grounding line is the place where their base leaves the sea floor and begins to float.

Their study, published today in Nature Geoscience, shows that the Southern Ocean melted 1,463 km2 of Antarctica's underwater ice between 2010 and 2016 - an area the size of Greater London."
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on June 19, 2018, 10:29:03 PM
Stearns et al. find that sliding laws for glaciers in error:

"Our results show that the relationship between basal drag and sliding velocity does not follow Weertman-style behavior"

"Our results show that there is a strong relationship between height-above-buoyancy (effective pressure) and ice velocity"

This is an important result, and if it holds up, will make modelling of glacier flow much easier.

doi: 10.1126/science.aat2217

https://phys.org/news/2018-06-gauging-ice-sheet-movement-sea-level-rise.html

sidd
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on July 16, 2018, 05:56:11 PM
The linked study indicates insufficiently fine ice sheet model mesh can lead to false positive indications of convergence to a stable grounding line location.  This indicates that future ice sheet models should use finer mesh spacing and that findings from early model projections are likely non-conservative with regard to human safety:

Gladstone, R., Xia, Y., and Moore, J.: Neutral equilibrium and forcing feedbacks in marine ice sheet modelling, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-124, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-124/

Abstract. Poor convergence with resolution of ice sheet models when simulating grounding line migration has been known about for over a decade. However, some of the associated numerical artifacts remain absent from the published literature.
In the current study we apply a Stokes-flow finite element marine ice sheet model to idealised grounding line evolution experiments. We show that with insufficiently fine model resolution, a region containing multiple steady state grounding line positions exists, with one steady state per node of the model mesh. This has important implications for the design of perturbation experiments used to test convergence of grounding line behaviour with resolution. Specifically, the design of perturbation experiments can be under-constrained, potentially leading to a "false positive" result. In this context a false positive is an experiment that appears to achieve convergence when in fact the model configuration is not close to its converged state. We demonstrate a false positive: an apparently successful perturbation experiment (i.e. reversibility is shown) for a model configuration that is not close to a converged solution. If perturbation experiments are to be used in the future, experiment design should be modified to provide additional constraints to the initialisation/spin up requirements.

This region of multiple locally stable steady state grounding line positions has previously been mistakenly described as neutral equilibrium. This distinction has important implications for understanding the impacts of discretizing a forcing feedback involving grounding line position and basal friction. This forcing feedback cannot, in general, exist in a region of neutral equilibrium, and could be the main cause of poor convergence in grounding line modelling.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: sidd on September 05, 2018, 08:21:04 AM
Meyer et al. outline a line of thinking on glacier sliding that might turn out to be very important:

Abstract:
--
Discharge from sliding outlet glaciers controls uncertainty in projections for future sea level.
Remarkably, over 90% of glacial area is subject to gravitational driving stresses below 150
kPa (median ∼70 kPa). Longstanding explanations that appeal to the shear-thinning rheology
of ice tend to overpredict driving stresses and are restricted to areas where ice sheets only
deform (roughly 50%). Over the more dynamic portions that slide, driving stresses must be
balanced by thermo-mechanical interactions that control basal strength. Here we show that
median bed strength is comparable to a threshold effective stress set by ice–liquid surface
energy and till pore size. Above this threshold, ice infiltrates sediment to produce basal layers
of debris-rich ice, even where net melting takes place. We demonstrate that the narrow range
of inferred bed strengths can be explained by the mechanical resistance to sliding where
roughness is enhanced by heterogeneous freeze-on.
--

From the paper:

"During such “soft-bedded” sliding, we have argued that surface energy and pore size combine to control the scale of effective stress (p_f ) beyond which obstacle growth increases the basal roughness enough to retard significant sliding."

"The scaling presented here gives a plausible explanation for why the range of driving stress remains so limited for sliding glaciers in general."

open access. read allaboutit.

DOI:10.1038/s41467-018-05716-1

sidd
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on September 08, 2018, 07:50:53 PM
The linked article discusses the scientific value of a new high resolution elevation model of Antarctica.  Such a tool could be very valuable in verifying/calibrating cliff failure and hydrofracturing models of ice sheets:

Title: "New map of Antarctica shows the icy continent in 'stunning detail'"

https://www.usatoday.com/story/tech/science/2018/09/07/antarctica-new-map-shows-icy-continent-stunning-detail/1224078002/

Extract: "Scientists from Ohio State University and the University of Minnesota have created what they say is the best, most complete and accurate map ever made of the frozen continent at the bottom of the world …

“Now we’ll be able to see changes in melting and deposition of ice better than ever before,” Morin said. “That will help us understand the impact of climate change and sea level rise. We’ll be able to see it right before our eyes.”"
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on October 31, 2018, 04:51:30 PM
The linked reference discusses basic mechanics of ice cliff failures, and the attached image illustrates the relationship of acceleration of calving rate vs freeboard and relative water depth of an ice cliff for a marine glacier and/or a marine-terminating glacier:

Tanja Schlemm and Anders Levermann (2018), "A simple stress-based cliff-calving law", The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-205

https://www.the-cryosphere-discuss.net/tc-2018-205/tc-2018-205.pdf

Abstract. Over large coastal regions in Greenland and Antarctica the ice sheet calves directly into the ocean. In contrast to ice-shelf calving, an increase in cliff calving directly contributes to sea-level rise and a monotonously increasing calving rate with ice thickness can constitute a self-amplifying ice loss mechanism that may significantly alter sea-level projections both of Greenland and Antarctica. Here we seek to derive a minimalistic stress-based parameterization for cliff calving. To this end we compute the stress field for a glacier with a simplified two-dimensional geometry from the two-dimensional Stokes equation. First we assume a constant yield stress to derive the failure region at the glacier front from the stress field within the ice sheet. Secondly, we assume a constant response time of ice failure due to exceedance of the yield stress. With this strongly constraining but very simple set of assumption we propose a cliff-calving law where the calving rate follows a power-law dependence on the freeboard of the ice with exponents between 2 and 3 depending on the relative water depth at the calving front. The critical freeboard below which the ice front is stable decreases with increasing relative water depth of the calving front. For a dry water front it is, for example, 75m. The purpose of this study is not to provide a comprehensive calving law, but to derive a particularly simple equation with a transparent and minimalistic set of assumptions.
Title: Re: Glaciology Basics and Risks - Uncertainties
Post by: AbruptSLR on November 08, 2018, 12:23:23 AM
nother tool for estimating iceberg calving from the face of a marine (or marine terminating) glacier:

Trevers, M., Payne, A. J., Cornford, S. L., and Moon, T.: Buoyant forces promote tidewater glacier iceberg calving through large basal stress concentrations, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-212, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-212/
&
https://www.the-cryosphere-discuss.net/tc-2018-212/tc-2018-212.pdf

Abstract. Iceberg calving parameterisations currently implemented in ice sheet models do not reproduce the full observed range of calving behaviours. For example, though buoyant forces at the ice front are known to trigger full-depth calving events on major Greenland outlet glaciers, a multi-stage iceberg calving event at Jakobshavn Isbræ is unexplained by existing models. To explain this and similar events, we propose a notch-triggered rotation mechanism whereby a relatively small subaerial calving event triggers a larger full-depth calving event due to the abrupt increase in buoyant load and the associated stresses generated at the ice-bed interface. We investigate the notch-triggered rotation mechanism by applying a geometric perturbation to the subaerial section of the calving front in a diagnostic flowline model of an idealised glacier snout, using the full-Stokes, finite element method code Elmer/Ice. Different sliding laws and water pressure boundary conditions are applied at the ice-bed interface. Water pressure has a big influence on the likelihood of calving, and stress concentrations large enough to open crevasses were generated in basal ice. Significantly, the location of stress concentrations produced calving events of approximately the size observed, providing support for future application of the notch-triggered rotation mechanism in ice-sheet models.