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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #50 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
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #51 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
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #52 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

"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."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #53 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
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #54 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

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."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #55 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/

International Symposium on Sea Ice in a Changing Environment
International Glaciological Society; C3 Convention Centre, Hobart, Australia, 10 – 14 March 2014
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #56 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/
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #57 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

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."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #58 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

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."
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sidd

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #59 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

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #60 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


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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #61 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

"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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #62 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

"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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #63 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/
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #64 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


"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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #65 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

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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #66 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


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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #67 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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #68 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


"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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #69 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

"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."

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #70 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

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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #71 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

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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #72 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

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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #73 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

"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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #74 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
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #75 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



What do they mean by this:
“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?
Open other end.

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #76 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
« Last Edit: May 10, 2014, 05:13:55 PM by AbruptSLR »
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #77 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
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #78 on: May 10, 2014, 09:12:08 AM »
Certainly a mistake AbruptSLR you wrote 80.000 years...I guess you meant 800.000 years...?

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #79 on: May 10, 2014, 05:14:52 PM »
Laurent,

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

Best,
ASLR
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #80 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?
« Last Edit: May 10, 2014, 10:31:14 PM by icefest »
Open other end.

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #81 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

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #82 on: May 10, 2014, 10:30:00 PM »
Sorry, typo.

I meant 1,000Kyr
Open other end.

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #83 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


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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #84 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

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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #85 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


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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #86 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

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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #87 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


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."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #88 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


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

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."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #89 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/

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

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #90 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
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #91 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
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #92 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

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

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #93 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/
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #94 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

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

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

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

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #95 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


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

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #96 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.
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anyone can help me with the numerical formulas ? :) :)

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #97 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.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #98 on: March 27, 2015, 04:23:54 PM »
Thanks a lot.
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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?"
                                   

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #99 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

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

See: 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

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

See: 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://go.owu.edu/~chjackso/Climate/papers/Glen_1958_The%20flow%20law%20of%20ice.pdf
                                 
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson