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steve s

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Re: Potential Collapse Scenario for the WAIS
« Reply #350 on: April 24, 2015, 11:29:10 PM »
Any mechanism that is not gradual is hard to model. Ice dam formation and release is at its base a catastrophic process capable of moving far greater masses of solids quickly down riverine channels than a gradual process using the same amount of fluids. 

We know the Bretz Floods happened; any such process in Antarctica would increase the centennial slr over the quantity without it given the same rainfall and heat flux. The pressurized ponds under the Thwaites might provide surprises when combined with cliff fracturing. Gradualism is not a safe religion, if mathematically convenient. The highest Pollard estimates would prove low.

sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #351 on: April 25, 2015, 06:02:00 AM »
does anyone have the total VAF (volume above flotation) for AIS handy ? i seem to recall it was estimated to be about 3.5m or so for WAIS, so the 10m/century thing couldn't go on too long, unless EAIS got involved ...

MWP1A was 5m/century for 500 years, a meter every score of years, but there was a lot more ice about then.

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #352 on: April 25, 2015, 12:18:05 PM »
does anyone have the total VAF (volume above flotation) for AIS handy ? i seem to recall it was estimated to be about 3.5m or so for WAIS, so the 10m/century thing couldn't go on too long, unless EAIS got involved ...

MWP1A was 5m/century for 500 years, a meter every score of years, but there was a lot more ice about then.

sidd,
While I asked & answered the same question in Reply #337; I provide the following extract from Wikipedia for the WAIS potential SLR contribution:

http://en.wikipedia.org/wiki/West_Antarctic_Ice_Sheet

Extract: "It has been hypothesised that this disintegration could raise sea levels by approximately 3.3 metres (11 ft). (If the entire West Antarctic Ice Sheet were to melt, this would contribute 4.8 m (16 ft) to global sea level.)"
Also, I provide the accompanying table (in meters) from Pfeffer 2011 for the potential SLR contributions for both the AIS & the GIS (GrIS); which concur with the figure in Reply #337.

Also, remember that the Pollard et al (2015) SLR projections include contributions from the EAIS (see my Reply #349).

Very Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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oren

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Re: Potential Collapse Scenario for the WAIS
« Reply #353 on: April 25, 2015, 02:24:19 PM »
When discussing the 10 meter SLR over a century there is also thermal expansion, and some other factors I am not an expert of. So I don't think the whole 10m is supposed to come from GIS WAIS and EAIS. This must have been discussed many times somewhere on the forum.

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #354 on: April 25, 2015, 05:35:54 PM »
When discussing the 10 meter SLR over a century there is also thermal expansion, and some other factors I am not an expert of. So I don't think the whole 10m is supposed to come from GIS WAIS and EAIS. This must have been discussed many times somewhere on the forum.

oren,

There are many different ways of thinking about the various different levels of plausible SLR contributions from different sources by 2100 or later, and you can find some of those discussions in the linked thread below:

http://forum.arctic-sea-ice.net/index.php/topic,874.300.html

The accompany four images are taken from that thread with the first image showing the AR5 way of thinking broken-down by source.  The second & third images are from an expert panel by Horton, showing at least one expert giving a chance of 7m of combined SLR by 2100.  The fourth image from O'Leary shows paleo-evidence of about 6m of SLR during the Eemian peak (MIS 5e) and given the over-lapping error bars this SLR could have happened in as much as 2,000 years to as little as 100 years (or less).

Best,
ASLR
“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: Potential Collapse Scenario for the WAIS
« Reply #355 on: April 25, 2015, 05:48:39 PM »
oren,

While I was scrolling through old SLR plots, I thought that you might be interested in the attached four plots, just to help calibrate recent SLR contributions, & potential total ice mass loss contributions, from different sources.

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

Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #356 on: April 25, 2015, 07:41:47 PM »
Also see fig.2a in Rohling et al 2013, which shows 10 cm/yr as a sort of probabilistic ultimate speed limit for SLR, based on paleo-records:
http://www.nature.com/srep/2013/131212/srep03461/fig_tab/srep03461_F2.html

But who knows, these records may have to be rewritten in the not too distant future?

Iceismylife

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Re: Potential Collapse Scenario for the WAIS
« Reply #357 on: April 25, 2015, 09:37:07 PM »
...

More seriously, ice flow at 8 miles/hr hasn't been seen yet., and i do not think the models come up with those speeds.  A better way to get heat into the ice is ... rain. Consider what hurricane Sandy would have done to the saddle in GIS at 67 N.

sidd
I'm not talking about how fast ice can flow over bedrock.  I'm talking about how fast icebergs floating in water can flow.  And that speed is just as fast as the water is flowing.  If you advance the calving face on the jacobshavn glacier 100 km inland past its current location then you would need an ice flow rate 1/200 of 8 miles/hr.  Or it would only have to double what the glacier is currently doing during its summertime peek.  Over a 200 km long calving face.  Just something to think about.

oren

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Re: Potential Collapse Scenario for the WAIS
« Reply #358 on: April 25, 2015, 11:12:23 PM »
Thanks a lot for these very detailed responses. Highly appreciated.
Focusing on the relative contribution of non-IS sources of SLR, looking at the 99.9% of the first table, 1.06m out of 2.45m (over 90 years) comes from glaciers and ice caps, land water storage and thermal expansion, which is over 40% of the total. Getting to 10m SLR / century in some worst case scenario will be proportionately more from IS than from other sources, so "only" about 8 meters need to come from GIS, WAIS and EAIS in such a scenario.

Iceismylife

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Re: Potential Collapse Scenario for the WAIS
« Reply #359 on: April 26, 2015, 12:24:50 AM »
In very simple terms.  The loss of 50% of Greenland's ice sheet in a 35 year time span would hit 10 meters per century rate of sea level rise.  Jakobshavn could reach that loss rate easily and I think it will. 

Zachariae isstrom calved icebergs all winter.  It has speed up 50% from 2012 until now.


Shared Humanity

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Re: Potential Collapse Scenario for the WAIS
« Reply #360 on: April 26, 2015, 05:21:27 AM »
In very simple terms.  The loss of 50% of Greenland's ice sheet in a 35 year time span would hit 10 meters per century rate of sea level rise.  Jakobshavn could reach that loss rate easily and I think it will. 

Zachariae isstrom calved icebergs all winter.  It has speed up 50% from 2012 until now.

Please. I've come to expect more rigorous discussion here. Let's take this kind of wild speculation somewhere else.

icefest

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Re: Potential Collapse Scenario for the WAIS
« Reply #361 on: April 26, 2015, 09:45:03 PM »
In very simple terms.  The loss of 50% of Greenland's ice sheet in a 35 year time span would hit 10 meters per century rate of sea level rise.  Jakobshavn could reach that loss rate easily and I think it will. 

Zachariae isstrom calved icebergs all winter.  It has speed up 50% from 2012 until now.



This would mean that zachariae would be losing 2% of the current Greenland ice volume a year.

2.8 million kmm3 as compared to ~17 km3. That's 5 orders of magnitude. Incredibly improbable.
Open other end.

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #362 on: April 29, 2015, 11:23:27 PM »
The linked reference (see attached image of the large range of various projections of cumulative PIG SLR contributions through 2050), indicates the large range of projected Antarctic ice mass loss by different models and discusses establishing minimum performance benchmarks before model projections should be considered for guidance.  This paper indicates to me that ice sheet modelers need to get their house in order before issuing any more SLR guidance documents that err embarrassingly far on the side of least drama.

Durand, G. and Pattyn, F.  (2015), "Reducing uncertainties in projections of Antarctic ice mass loss, The Cryosphere Discuss.", 9, 2625-2654, doi:10.5194/tcd-9-2625-2015.

http://www.the-cryosphere-discuss.net/9/2625/2015/tcd-9-2625-2015.html

Abstract: "Climate model projections are often aggregated into multi-model averages of all models participating in an Intercomparison Project, such as the Coupled Model Intercomparison Project (CMIP). A first initiative of the ice-sheet modeling community, SeaRISE, to provide multi-model average projections of polar ice sheets' contribution to sea-level rise recently emerged. SeaRISE Antarctic numerical experiments aggregate results from all models willing to participate without any selection of the models regarding the processes implemented in. Here, using the experimental set-up proposed in SeaRISE we confirm that the representation of grounding line dynamics is essential to infer future Antarctic mass change. We further illustrate the significant impact on the ensemble mean and deviation of adding one model with a known biais in its ability of modeling grounding line dynamics. We show that this biased model can hardly be discriminated from the ensemble only based on its estimation of volume change. However, tools are available to test parts of the response of marine ice sheet models to perturbations of climatic and/or oceanic origin (MISMIP, MISMIP3d). Based on recent projections of the Pine Island Glacier mass loss, we further show that excluding ice sheet models that do not pass the MISMIP benchmarks decreases by an order of magnitude the mean contribution and standard deviation of the multi-model ensemble projection for that particular drainage basin."
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #363 on: May 01, 2015, 05:07:25 PM »
While I believe that Lennart just alluded this reference in the EAIS thread, I repeat it here in the WAIS thread as very few people seem to remember what is going on in all of the multiple Antarctic threads:

Alley, R.B., S. Anandakrishnan. K. Christianson, H.J. Horgan, A. Muto, B.R. Parizek, D. Pollard and R.T. Walker (2015) "Oceanic forcing of ice-sheet retreat: West Antarctica and more", Ann. Rev. Earth Plan. Sci., 43, 7.1-7.25, doi:10.1146/annurev-earth-060614-105344.

http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-060614-105344?journalCode=earth


Abstract: "Ocean-ice interactions have exerted primary control on the Antarctic Ice Sheet and parts of the Greenland Ice Sheet, and will continue to do so in the near future, especially through melting of ice shelves and calving cliffs. Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing >3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence."

Expected final online publication date for the Annual Review of Earth and Planetary Sciences Volume 43 is May 30, 2015. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.

« Last Edit: May 01, 2015, 09:50:31 PM by AbruptSLR »
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #364 on: May 01, 2015, 06:58:14 PM »
The linked reference (using both gravimetry & altimetry, satellite data) indicates that ice mass loss from the AIS, & particularly from the WAIS, is continuing to accelerate (for example the WAIS ice mass loss increased from about 121 billion tons in 2008 to twice that in 2014).  The research partially links this AIS ice mass loss acceleration to the Antarctic Circumpolar Wave (ACW) via changes in precipitation. 

Additionally, as now the Tropical Pacific atmosphere is finally moving into a possibly more permanent El Nino configuration, the attached Earth Surface Wind & Mean Sea Level Pressure Map for May 1, 2015 shows that the ABSL and an un-named low pressure system off of Wilkes are now generating winds that are blowing warm CDW into both the ASE and underneath the Totten Ice Shelf.

A. Mémin , T. Flament, B. Alizier, C. Watson, F. Rémy (2015), "Interannual variation of the Antarctic Ice Sheet from a combined analysis of satellite gravimetry and altimetry data", Earth and Planetary Science Letters, Volume 422, 15 July 2015, Pages 150–156


http://www.sciencedirect.com/science/article/pii/S0012821X15001946


Abstract: "Assessment of the long term mass balance of the Antarctic Ice Sheet, and thus the determination of its contribution to sea level rise, requires an understanding of interannual variability and associated causal mechanisms. We performed a combined analysis of surface-mass and elevation changes using data from the GRACE and Envisat satellite missions, respectively. Using empirical orthogonal functions and singular value decompositions of each data set, we find a quasi 4.7-yr periodic signal between 08/2002 and 10/2010 that accounts for ∼15–30%∼15–30% of the time variability of the filtered and detrended surface-mass and elevation data. Computation of the density of this variable mass load corresponds to snow or uncompacted firn. Changes reach maximum amplitude within the first 100 km from the coast where it contributes up to 30–35% of the annual rate of accumulation. Extending the analysis to 09/2014 using surface-mass changes only, we have found anomalies with a periodicity of about 4–6 yrs that circle the AIS in about 9–10 yrs. These properties connect the observed anomalies to the Antarctic Circumpolar Wave (ACW) which is known to affect several key climate variables, including precipitation. It suggests that variability in the surface-mass balance of the Antarctic Ice Sheet may also be modulated by the ACW."

See also:
http://www.firstpost.com/world/antarcticas-ice-sheets-melting-twice-faster-92-billion-tonnes-per-year-2222562.html

Extract: "Since 2008, ice loss from West Antarctica's unstable glaciers doubled from an average annual loss of 121 billion tons of ice to twice that by 2014, the researchers found."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #365 on: May 01, 2015, 08:18:12 PM »
There is a paper out by Harig and Simons doi:10.1016/j.epsl.2015.01.029, latest GRACE data

"Ignoring GIA model uncertainty, over the period 2003–2014, West Antarctica has been losing ice mass at a rate of −121 ± 8 Gt/yr and has experienced large acceleration of ice mass losses along the Amundsen Sea coast of −18 ± 5 Gt/yr^2 , doubling the mass loss rate in the past six years. The Antarctic Peninsula shows slightly accelerating ice mass loss, with larger accelerated losses in the southern half of the Peninsula. Ice mass gains due to snowfall in Dronning Maud Land have continued to add about half the amount of West Antarctica’s loss back onto the continent over the last decade. We estimate the overall mass losses from Antarctica since January 2003 at −92 ± 10 Gt/yr."

sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #366 on: May 01, 2015, 09:40:09 PM »
A hat tip to Mercer, Strong Language about Thwaites, pessimism about model improvements:

doi:10.1146/annurev-earth-060614-105344

"Oceanic Forcing of Ice-Sheet Retreat: West Antarctica and More" Alley et al.

"Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing greater than3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario."

"Mercer (1968) cited the geological evidence of West et al. (1960) (incorrectly cited as West & Sparks 1961) showing that “rise in sea level to above present levels took place during the Sangamon Hypsithermal [MIS 5e] and was very rapid,” which “suggests possible catastrophic disintegration of the West Antarctic Ice Sheet at that time, but further evidence is needed.” Above we summarize some of the more recent data on that rise above present levels, but newer data also provide insight on the rate of rise. Other than adding many better-dated and more widely distributed records to the citation list, a follower of Mercer could write the same sentence today."

"If Thwaites Glacier experiences a sustained Jakobshavn-type ice-shelf loss and retreats into the central basins beneath the West Antarctic Ice Sheet that reach >2,000 m below sea level, the resulting cliff would almost surely be highly unstable. If any m ́ lange produced were sufficiently weak, the resulting cliff failure might cause the glacier to retreat much more rapidly than simulated by models lacking this process. Pollard et al. (2014) found that a parameterization for this process increased the instability and collapse rate of the West Antarctic Ice Sheet and of marine portions of the East Antarctic Ice Sheet, with this parameterization and the forcing adopted causing West Antarctic Ice Sheet collapse to occur on decadal timescales once initiated. No fully physical model now exists that includes this process, and given the dependence on poorly known fracture mechanics of ice, a quantitatively well-constrained model appears unlikely in the near future."


"The geological record, and modeling targeting the modern setting, indicate that Thwaites Glacier will likely exhibit threshold behavior, with retreat off the sill (Figures 2 and 4) triggering much more rapid retreat that likely will be irreversible over human timescales of centuries or less. The stability threshold may already have been crossed, although the few modeling experiments to date do not provide full agreement on that. Whether the threshold has been crossed, and how rapidly the ongoing retreat may leave the stabilizing sill, may depend on processes and conditions that are not yet fully understood and measured, such that future modeling seems likely to leave substantial uncertainty for some time. Notably, retreat into the deep basins could create conditions unlike any seen on Earth today, with processes that generally are not accurately represented in the current generation of models. In particular, the possibility of cliff failure suggests that Thwaites Glacier retreat and West Antarctic Ice Sheet loss could be much faster than generally simulated."

sidd

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #367 on: May 01, 2015, 09:43:49 PM »
There is a paper out by Harig and Simons doi:10.1016/j.epsl.2015.01.029, latest GRACE data

"Ignoring GIA model uncertainty, over the period 2003–2014, West Antarctica has been losing ice mass at a rate of −121 ± 8 Gt/yr and has experienced large acceleration of ice mass losses along the Amundsen Sea coast of −18 ± 5 Gt/yr^2 , doubling the mass loss rate in the past six years. The Antarctic Peninsula shows slightly accelerating ice mass loss, with larger accelerated losses in the southern half of the Peninsula. Ice mass gains due to snowfall in Dronning Maud Land have continued to add about half the amount of West Antarctica’s loss back onto the continent over the last decade. We estimate the overall mass losses from Antarctica since January 2003 at −92 ± 10 Gt/yr."

The following is an elaboration on sidd's reference, with a link to an open access pdf and the two attachments jointly constitute Fig 3 from the paper, with the first image showing the mass loss from the WAIS and the second attachment showing the mass loss from the Antarctic Peninsula from 2003 to 2014 (ignoring GIA model uncertainty)

Christopher Harig & Frederik J. Simons (2015), "Accelerated West Antarctic ice mass loss continues to outpace East Antarctic gains", Earth Planet. Sc. Lett., 415, 134-141, doi:10.1016/j.epsl.2015.01.029
 

http://www.princeton.edu/geosciences/people/simons/pdf/EPSL-2015a.pdf


Abstract: "While multiple data sources have confirmed that Antarctica is losing ice at an accelerating rate, different measurement techniques estimate the details of its geographically highly variable mass balance with different levels of accuracy, spatio-temporal resolution, and coverage. Some scope remains for methodological improvements using a single data type. In this study we report our progress in increasing the accuracy and spatial resolution of time-variable gravimetry from the Gravity Recovery and Climate Experiment (GRACE). We determine the geographic pattern of ice mass change in Antarctica between January 2003 and June 2014, accounting for glacio-isostatic adjustment (GIA) using the IJ05_R2 model.  Expressing the unknown signal in a sparse Slepian basis constructed by optimization to prevent leakage out of the regions of interest, we use robust signal processing and statistical estimation methods.  Applying those to the latest time series of monthly GRACE solutions we map Antarctica’s mass loss in space and time as well as can be recovered from satellite gravity alone. Ignoring GIA model uncertainty, over the period 2003–2014, West Antarctica has been losing ice mass at a rate of −121 ±8 Gt/yr and has experienced large acceleration of ice mass losses along the Amundsen Sea coast of −18 ±5 Gt/yr2, doubling the mass loss rate in the past six years. The Antarctic Peninsula shows slightly accelerating ice mass loss, with larger accelerated losses in the southern half of the Peninsula. Ice mass gains due to snowfall in Dronning Maud Land have continued to add about half the amount of West Antarctica’s loss back onto the continent over the last decade. We estimate the overall mass losses from Antarctica since January 2003 at −92 ±10 Gt/yr."


Caption for image (the upper panels are for the WAIS & the lower panels are for the Antarctic Peninsula) : "Fig. 3. Time-resolved maps of ice mass change (mass corrected using the GIA model by Ivins et al., 2013) over West Antarctica and the Antarctica Peninsula for odd years from 2003 to 2013. Each panel shows the estimated mass change from January of the labeled year (e.g., 2003) to January of the following year (e.g., 2004).  Changes seen between panels are due to accelerations. The top two rows of panels correspond to the area of box a in Fig. 1 and use the top scale bar. The bottom two rows use the bottom scale bar and correspond to box b in Fig. 1. For West Antarctica, the area of the localization includes grounded ice basins with a 0.5◦ buffer along ocean borders, and is outlined with a dashed line. For the Peninsula the localization includes grounded ice and ice shelves with a 0.5◦ buffer, also shown with a dashed line. The integral values of the mass change per year are shown as “Int”, expressed in gigatons (Gt). When the color bar is saturated, as in 2011, the minimum value of the field is shown in the top right as “Min” with units of centimeters per year water equivalent."
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #368 on: May 03, 2015, 04:35:12 AM »
A hat tip to Mercer, Strong Language about Thwaites, pessimism about model improvements:

doi:10.1146/annurev-earth-060614-105344

"Oceanic Forcing of Ice-Sheet Retreat: West Antarctica and More" Alley et al.

"If Thwaites Glacier experiences a sustained Jakobshavn-type ice-shelf loss and retreats into the central basins beneath the West Antarctic Ice Sheet that reach >2,000 m below sea level, the resulting cliff would almost surely be highly unstable. If any mélange produced were sufficiently weak, the resulting cliff failure might cause the glacier to retreat much more rapidly than simulated by models lacking this process. Pollard et al. (2014) found that a parameterization for this process increased the instability and collapse rate of the West Antarctic Ice Sheet and of marine portions of the East Antarctic Ice Sheet, with this parameterization and the forcing adopted causing West Antarctic Ice Sheet collapse to occur on decadal timescales once initiated. No fully physical model now exists that includes this process, and given the dependence on poorly known fracture mechanics of ice, a quantitatively well-constrained model appears unlikely in the near future."

sidd

I just thought that I would highlight that per Alley et al 2015 cliff failures & hydrofracturing combined with weak mélange restraint (i.e. a high probability of iceberg armadas) for the Thwaites Glacier could lead to a chain reaction:  "causing West Antarctic Ice Sheet collapse to occur on decadal timescales once initiated."
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sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #369 on: May 03, 2015, 06:43:26 AM »
I find the authors (Alley 2015, doi:10.1146/annurev-earth-060614-105344)  pessimism about models telling. I see Alley, Parizek, Pollard in there. But I doubt if Rignot shares the sentiment that models cannot improve, or cannot improve quickly enuf.

For the record, i think modelling will prove more powerful than Alley et al. imagine.

sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #370 on: May 03, 2015, 05:27:23 PM »
I find the authors (Alley 2015, doi:10.1146/annurev-earth-060614-105344)  pessimism about models telling. I see Alley, Parizek, Pollard in there. But I doubt if Rignot shares the sentiment that models cannot improve, or cannot improve quickly enuf.

For the record, i think modelling will prove more powerful than Alley et al. imagine.

sidd

sidd,

First there are many different types of models, but I think that the question that Alley et al 2015 raise is whether the IPCC will accept the model findings as being directly traceable to observed/calculated physical input parameters based on deductive logic.  Certainly the cliff failure & hydrofracturing model developed by Pollard & DeConto already project the type of response that Alley et al 2015 say cannot be directly physically modeled for some (long) time to come.  This is apparently because other researchers are not willing to adopt the Bassis fracture equations for the cliff failure nor the hydrofracturing mode as apparently there are too many parameters that are poorly bounded and thus require the modeler to select and/or adjust them to match the poorly understood paleo conditions.

Second, the current WAIS conditions (particularly basal) are poorly understood/constrained, making it difficult to prescribe clear model input data without a wide range of uncertainty and/or sensitivity runs (which are normally run with low resolution).

Finally, the ACME model is committing millions of dollar to try to improve these marine ice sheet model projections; but even they understand that the projections at the end of their first 3-yr phase will be incomplete and that we will all likely need to wait until the end of their full 10-yr effort before having results that are reasonably accurate (& it is this 10-yr lag that Alley et al 2015 are lamenting).

Best,
ASLR
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #371 on: May 13, 2015, 01:49:19 AM »
With the cliff failure and hydrofracturing model of Pollard & DeConto projecting the possibility of rapid calving for marine glaciers if the mélange offers sufficiently low buttressing action, the linked modelling paper (with an open access pdf) is very topical:

Krug, J., Durand, G., Gagliardini, O., and Weiss, J.: Modelling the impact of submarine frontal melting and ice mélange on glacier dynamics, The Cryosphere, 9, 989-1003, doi:10.5194/tc-9-989-2015, 2015.

http://www.the-cryosphere.net/9/989/2015/tc-9-989-2015.html

Abstract: "Submarine melting of the calving face of tidewater glaciers and the mechanical back force applied by the ice mélange layer are two mechanisms generally proposed to explain seasonal variations at the calving front of tidewater glaciers. However, the way these processes affect the calving rate and glacier dynamics remains uncertain. In this study, we used a finite element-based model that solves the full Stokes equations to simulate the impact of these forcings on two-dimensional theoretical flow line glacier configurations. The model, which includes calving processes, suggests that frontal melting affects the position of the terminus only slightly (less than a few hundred metres) and does not affect the multiannual glacier mass balance at all. However, the ice mélange has a greater impact on the advance and retreat cycles of the glacier front (more than several kilometres) and its consequences for the mass balance are not completely negligible, stressing the need for better characterization of forcing properties. We also show that ice mélange forcing against the calving face can mechanically prevent crevasse propagation at sea level and hence prevent calving. Results also reveal different behaviours in grounded and floating glaciers: in the case of a floating extension, the strongest forcings can disrupt the glacier equilibrium by modifying its buttressing and ice flux at the grounding line."
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Re: Potential Collapse Scenario for the WAIS
« Reply #372 on: May 18, 2015, 12:47:19 PM »
In order to gain some perspective I have decided that after today, I will take a break from posting until sometime after the 4th of July.
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Re: Potential Collapse Scenario for the WAIS
« Reply #373 on: May 18, 2015, 02:01:05 PM »
ASLR, you will be much missed. Have a good break.  :)

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Re: Potential Collapse Scenario for the WAIS
« Reply #374 on: May 18, 2015, 02:30:12 PM »
Yes, have a good time, ASLR. Thanks for all the great links.
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Re: Potential Collapse Scenario for the WAIS
« Reply #375 on: May 18, 2015, 07:39:59 PM »
Thank you, ASLR, enjoy your break from posting and your effort to gain some perspective. That we may all benefit when you return :)

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Re: Potential Collapse Scenario for the WAIS
« Reply #376 on: May 19, 2015, 06:24:56 AM »
OMG.  You will be missed. 

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Re: Potential Collapse Scenario for the WAIS
« Reply #377 on: May 19, 2015, 09:44:45 AM »
Enjoy your well-earned break. 

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Re: Potential Collapse Scenario for the WAIS
« Reply #378 on: May 19, 2015, 10:42:59 AM »
Phew! at last a chance to catch up. Best  Wishes.

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Re: Potential Collapse Scenario for the WAIS
« Reply #379 on: May 20, 2015, 10:07:22 PM »
Recent lecture by Richard Alley, including remarks on Pollard et al 2015 and Applegate et al 2014 on potentially very fast ice loss from WAIS and GIS:


Thanks to Colorado Bob for posting this over at the ASIB.

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Re: Potential Collapse Scenario for the WAIS
« Reply #380 on: July 07, 2015, 02:42:13 AM »
The linked reference discusses this impact of the collapse of the WAIS on Antarctic surface climate:

Eric J. Steig, Kathleen Huybers, Hansi A. Singh, Nathan J. Steiger, Qinghua Ding, Dargan M. W. Frierson, Trevor Popp & James W. C. White (2015), "Influence of West Antarctic Ice Sheet collapse on Antarctic surface climate", Geophysical Research Letters, DOI: 10.1002/2015GL063861


http://onlinelibrary.wiley.com/doi/10.1002/2015GL063861/abstract


Abstract: "Climate model simulations are used to examine the impact of a collapse of the West Antarctic Ice Sheet (WAIS) on the surface climate of Antarctica. The lowered topography following WAIS collapse produces anomalous cyclonic circulation with increased flow of warm, maritime air toward the South Pole and cold-air advection from the East Antarctic plateau toward the Ross Sea and Marie Byrd Land, West Antarctica. Relative to the background climate, areas in East Antarctica that are adjacent to the WAIS warm, while substantial cooling (several ∘C) occurs over parts of West Antarctica. Anomalously low isotope-paleotemperature values at Mount Moulton, West Antarctica, compared with ice core records in East Antarctica, are consistent with collapse of the WAIS during the last interglacial period, Marine Isotope Stage 5e. More definitive evidence might be recoverable from an ice core record at Hercules Dome, East Antarctica, which would experience significant warming and positive oxygen isotope anomalies if the WAIS collapsed."

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Re: Potential Collapse Scenario for the WAIS
« Reply #381 on: July 29, 2015, 10:17:55 AM »
Anders Levermann of PIK on WAIS's point of no return:
http://www.project-syndicate.org/commentary/anders-levermann-explains-why-nothing-can-be-done-to-halt-the-collapse-of-the-amundsen-sea-s-ice-shelf

"Rather than reacting to global warming with gradual and predictable patterns of change, the West Antarctic ice sheet has suddenly “tipped” into a new state. A relatively small amount of melting beneath the Amundsen Sea’s ice shelf has pushed its grounding line to the top of a sub-glacial hill, from which it is now “rolling down.” Simply put, one thermal kick was enough to initiate an internal dynamic that will now continue under its own momentum, regardless of any action that humans might take to prevent it."

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Re: Potential Collapse Scenario for the WAIS
« Reply #382 on: August 01, 2015, 01:43:59 AM »
The linked reference (with an open access pdf) provides historical information indicating that continued global warming will increase ENSO activity and will telecommunicate more atmospheric energy to Antarctica (particularly West Antarctica), which will increase surface temperatures, and will increase the propensity for surface ice melting (which could fuel more hydrofracturing).

Turney, C. S. M., Fogwill, C. J., Klekociuk, A., van Ommen, T. D., Curran, M. A. J., Moy, A. D., and Palmer, J. G.  (2015), "Tropical and mid-latitude forcing of continental Antarctic temperatures", The Cryosphere Discuss., 9, 4019-4042, doi:10.5194/tcd-9-4019-2015.

http://www.the-cryosphere-discuss.net/9/4019/2015/tcd-9-4019-2015.html
http://www.the-cryosphere-discuss.net/9/4019/2015/tcd-9-4019-2015.pdf

Abstract. Future changes in atmospheric circulation and associated modes of variability are a major source of uncertainty in climate projections. Nowhere is this issue more acute than across the mid- to high-latitudes of the Southern Hemisphere (SH) which over the last few decades has experienced extreme and regional variable trends in precipitation, ocean circulation, and temperature, with major implications for Antarctic ice melt and surface mass balance. Unfortunately there is a relative dearth of observational data, limiting our understanding of the driving mechanism(s). Here we report a new 130-year annually-resolved record of δ D – a proxy for temperature – from the South Geographic Pole where we find a significant influence from extra-tropical pressure anomalies which act as "gatekeepers" to the meridional exchange of air masses. Reanalysis of global atmospheric circulation suggests these pressure anomalies play a considerably larger influence on mid- to high-latitude SH climate than hitherto believed, modulated by the tropical Pacific Ocean. Our findings suggest that future increasing tropical warmth will strengthen meridional circulation, exaggerating current trends, with potentially significant impacts on Antarctic surface mass balance.

Extract: "By extending historical observations over the South Pole and in combination with reanalysis products we find that meridional circulation changes associated with centres of pressure anomalies are part of a broader change observed over recent decades. Of particular note is the marked decrease in rainfall in southwest Australia since the 1970s. Our results demonstrate this trend is part of a hemispheric pattern of alternating northerly and southerly airflow linked to changes in the southwest Pacific and the tropical Pacific. We explore possible teleconnections via a strengthening of the Ferrel Cell and weakening Polar Cell. Comparison of 30-year running means of isotopic and climate datasets suggest the continuing trend to lower pressure anomalies in the southwest Pacific – with largely stable values in the Indian Ocean – are consistent with increased ENSO variability, implying precipitation will continue to decline in southwest.  Australia if El Niños become more frequent, and lead to greater warming over the
Antarctic, potentially impacting the future surface mass balance."
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Re: Potential Collapse Scenario for the WAIS
« Reply #383 on: August 01, 2015, 05:17:00 AM »
I don't like the sound of that "weakening polar cell" in the South. We see what that does to the north already.

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Re: Potential Collapse Scenario for the WAIS
« Reply #384 on: August 01, 2015, 11:42:00 AM »
New information about the Southern Hemisphere mid-to-upper tropospheric planetary wave, helps to explain recent surface warming over West Antarctica and the Antarctic Peninsula:

Damien Irving and Ian Simmonds (2015), "A novel approach to diagnosing Southern Hemisphere planetary wave activity and its influence on regional climate variability", Journal of Climate 2015 ; e-View doi: http://dx.doi.org/10.1175/JCLI-D-15-0287.1


http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0287.1

Abstract: "Southern Hemisphere mid-to-upper tropospheric planetary wave activity is characterized by the superposition of two zonally-oriented, quasi-stationary waveforms: zonal wavenumber one (ZW1) and zonal wavenumber three (ZW3). Previous studies have tended to consider these waveforms in isolation and with the exception of those studies relating to sea ice, little is known about their impact on regional climate variability. We take a novel approach to quantifying the combined influence of ZW1 and ZW3, using the strength of the hemispheric meridional flow as a proxy for zonal wave activity. Our methodology adapts the wave envelope construct routinely used in the identification of synoptic-scale Rossby wave packets and improves on existing approaches by allowing for variations in both wave phase and amplitude. While ZW1 and ZW3 are both prominent features of the climatological circulation, the defining feature of highly meridional hemispheric states is an enhancement of the ZW3 component. Composites of the mean surface conditions during these highly meridional, ZW3-like anomalous states (i.e. months of strong planetary wave activity) reveal large sea ice anomalies over the Amundsen and Bellingshausen Seas during autumn and along much of the East Antarctic coastline throughout the year. Large precipitation anomalies in regions of significant topography (e.g. New Zealand, Patagonia, coastal Antarctica) and anomalously warm temperatures over much of the Antarctic continent were also associated with strong planetary wave activity. The latter has potentially important implications for the interpretation of recent warming over West Antarctica and the Antarctic Peninsula."
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Re: Potential Collapse Scenario for the WAIS
« Reply #385 on: August 03, 2015, 08:35:25 PM »
sidd,

Thanks for the reference.  I believe that BISICLES is the base program to be used in the ACME program for ice sheet modeling.  This paper seems to show the fastest rate of retreat for ASE (see attached image) that I have seen BISICLES project to date, so I will be interested in seeing what projection they have by the end of the first phase of the ACME project (as well as in their final projection in about 10-years).  However, I would be even more interested if the ACME WAIS models adopt the cliff failure and hydrofracturing methodology developed by Pollard et al (2015).

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.pdf

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."

Best,
ASLR

As a follow-up to this earlier post, I provide the two attached additional images, and the two additional linked information, and I reiterate that I hope that the ACME effort learns from the methodologies currently be used by Pollard, DeConto, Applegate, and others (see the Paleo thread for August 3rd 2015), as I believe that Cornford et al 2015 are too conservative (as in transferring risk from the modeler to the general public):

See also:
http://crd.lbl.gov/departments/applied-mathematics/ANAG/about/staff/dan-martin/
http://crd.lbl.gov/assets/pubs_presos/Martin-LIWG-2015-final.pdf
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Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #386 on: August 18, 2015, 02:46:37 PM »
Cornford et al 2015 final version is now published:
http://www.the-cryosphere.net/9/1579/2015/tc-9-1579-2015.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, deploying sub-kilometer resolution around the grounding line since coarser resolution results in substantial underestimation of the response. Each of the simulations begins with a geometry and velocity close to present-day observations, and evolves according to variation in meteoric ice accumulation rates and oceanic ice shelf melt rates. 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 rate 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 and 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. Within the Amundsen Sea Embayment the largest single source of variability is the onset of sustained retreat in Thwaites Glacier, which can triple the rate of eustatic sea level rise."

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Re: Potential Collapse Scenario for the WAIS
« Reply #387 on: August 19, 2015, 06:57:15 PM »
The linked reference by Hughes et al (2015) presents state-of-the-art analysis about the Jakobshavn Effect (Hughes, 1986), focused on progressive ice-bed uncoupling due to such factors as: basal meltwater, buoyancy friction (particularly with changing surface elevation), boundary constraints of the fjord.  This work has relevance to multiple marine-terminating, and marine, glaciers in both Greenland and Antarctica (see the extract for concerns about the PIG an the Thwaites Glacier, among other Antarctic marine glaciers):

Hughes, T., Sargent, A., Fastook, J., Purdon, K., Li, J., Yan, J.-B., and Gogineni, S.: Sheet, stream, and shelf flow as progressive ice-bed uncoupling: Byrd Glacier, Antarctica, and Jakobshavn Isbrae, Greenland, The Cryosphere Discuss., 9, 4271-4354, doi:10.5194/tcd-9-4271-2015, 2015.

http://www.the-cryosphere-discuss.net/9/4271/2015/tcd-9-4271-2015.pdf

Abstract. The first-order control of ice thickness and height above sea level is linked to the decreasing strength of ice-bed coupling alone flowlines from an interior ice divide to the calving front of an ice shelf. Uncoupling progresses as a frozen bed progressively thaws for sheet flow, as a thawed bed is progressively drowned for stream flow, and as lateral and/or local grounding vanish for shelf flow. This can reduce ice thicknesses by 90 % and ice elevations by 99 % along flowlines. Original work presented here includes (1) replacing flow and sliding laws for sheet flow with upper and lower yield stresses for creep in cold overlying ice and basal ice sliding over deforming till, respectively, (2) replacing integrating the Navier–Stokes equations for stream flow with geometrical solutions to the force balance, and (3) including resistance to shelf flow caused by lateral confinement in a fjord and local grounding at ice rumples and ice rises. A comparison is made between our approach and two approaches based on continuum mechanics. Applications are made to Byrd Glacier in Antarctica and Jakobshavn Isbrae in Greenland.


Extract: "Equation (24), based only on the force balance, is especially useful here because of its robust simplicity that applies to all flowlines and flowbands (ice streams) that end at a specified ice thickness h0. It gives phi variations along x that are usually somewhat higher than when the mass balance is also included, but with the same general trend. Using Eq. (24), Pingree et al. (2011) showed how Eq. (24) produced ice elevations before and after a former surge lifecycle of Lambert Glacier in East Antarctica, and for impending surge lifecycles of Thwaites Glacier and Pine Island Glacier entering the Pine Island Bay polynya in West Antarctica that continue into East Antarctica. Using Eq. (24), Hughes (2011) has tentatively assigned inception, growth, mature, declining, and terminal lifecycle stages shown in Table 2 to all major Antarctic ice streams at the present time."

Edit: See the two attached associated images related to the Jakobshavn Effect; which elsewhere in this fold I have associated with the more dynamic "Thwaites Effect"
« Last Edit: January 24, 2016, 02:44:51 PM by AbruptSLR »
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Re: Potential Collapse Scenario for the WAIS
« Reply #388 on: August 19, 2015, 08:23:38 PM »
The 2015 paper by Hughes et al. is very reminiscent of the 2014 paper doi:10.5194/tcd-8-2043-2014
I like the simple geometric approach.

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Re: Potential Collapse Scenario for the WAIS
« Reply #389 on: November 03, 2015, 12:14:37 AM »
While I believe that the linked reference's use of the PISM ice sheet model is conservative; nevertheless, their findings support the idea the pending destabilization of the ASE marine glaciers will destabilize the remaining portions of the WAIS (however, I suspect that the estimated timeframe is scientifically conservative as PISM does not utilize either cliff failures or hydrofracturing):

Johannes Feldmann and Anders Levermann (November 2, 2015), "Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin", PNAS, doi: 10.1073/pnas.1512482112


http://www.pnas.org/content/early/2015/10/28/1512482112


Abstract: "The future evolution of the Antarctic Ice Sheet represents the largest uncertainty in sea-level projections of this and upcoming centuries. Recently, satellite observations and high-resolution simulations have suggested the initiation of an ice-sheet instability in the Amundsen Sea sector of West Antarctica, caused by the last decades’ enhanced basal ice-shelf melting. Whether this localized destabilization will yield a full discharge of marine ice from West Antarctica, associated with a global sea-level rise of more than 3 m, or whether the ice loss is limited by ice dynamics and topographic features, is unclear. Here we show that in the Parallel Ice Sheet Model, a local destabilization causes a complete disintegration of the marine ice in West Antarctica. In our simulations, at 5-km horizontal resolution, the region disequilibrates after 60 y of currently observed melt rates. Thereafter, the marine ice-sheet instability fully unfolds and is not halted by topographic features. In fact, the ice loss in Amundsen Sea sector shifts the catchment's ice divide toward the Filchner–Ronne and Ross ice shelves, which initiates grounding-line retreat there. Our simulations suggest that if a destabilization of Amundsen Sea sector has indeed been initiated, Antarctica will irrevocably contribute at least 3 m to global sea-level rise during the coming centuries to millennia."

Extract: "The result of this study is an if–then statement, saying that if the Amundsen Sea Sector is destabilized, then the entire marine part of West Antarctica will be discharged into the ocean.”
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Re: Potential Collapse Scenario for the WAIS
« Reply #390 on: November 08, 2015, 11:44:29 AM »
The linked open access reference documents a 20th century increase in snowfall in coastal West Antarctica, that evidently is associated with a deepening of the ASL, which in-turn appears to be linked to periods of coupling between the ENSO and SAM (see the attached image).  It should be remembered that such increased coastal snowfall increases the gravitational driving force on the coastal glaciers in the WAIS.  Furthermore, a deepening of the ASL can result in accelerated advection of more warm CDW to accelerate grounding line retreat of key WAIS marine glaciers.

E. R. Thomas, J. S. Hosking, R. R. Tuckwell, R. A. Warren and E. C. Ludlow (2015), "Twentieth century increase in snowfall in coastal West Antarctica", Geophysical Research Letters DOI: 10.1002/2015GL065750

http://onlinelibrary.wiley.com/doi/10.1002/2015GL065750/abstract

Abstract: "The Amundsen Sea sector of the West Antarctic ice sheet has been losing mass in recent decades; however, long records of snow accumulation are needed to place the recent changes in context. Here we present 300 year records of snow accumulation from two ice cores drilled in Ellsworth Land, West Antarctica. The records show a dramatic increase in snow accumulation during the twentieth century, linked to a deepening of the Amundsen Sea Low (ASL), tropical sea surface temperatures, and large-scale atmospheric circulation. The observed increase in snow accumulation and interannual variability during the late twentieth century is unprecedented in the context of the past 300 years and evidence that the recent deepening of the ASL is part of a longer trend."

Extract: "Ice core records from Ellsworth Land, West Antarctica reveal a twentieth century increase in regional snow accumulation and its interannual variability that is considered exceptional in the context of the past 300 years. The annual snow accumulation since 1900 increased by ~ 30%, proving that the dramatic increases observed in the Antarctic Peninsula extend into West Antarctica and that these changes occurred following a 200 year period of relatively stable conditions. Snow accumulation in this region is governed by changes in SLP in the Amundsen Sea region, resulting in enhanced meridional (onshore) winds drawing moist air to the coast of Ellsworth Land, and directly impacting global sea level rise through wind driven upwelling and subsequent thinning of West Antarctic ice shelves [Pritchard et al., 2012]. The close relationship between SLP and snow accumulation at these sites make this a unique proxy for past ASL (and onshore wind) conditions. The recent deepening trend of the ASL is predicted to continue through the 21st century in response to greenhouse gas concentration increases [Raphael et al., 2015]. The dramatic increase in snow accumulation in Ellsworth Land provides evidence that this recent deepening in the ASL region is part of a longer trend, observed as early as the 1920s, with acceleration since the 1990s when the coupling between ENSO and SAM is strongest."
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #391 on: November 11, 2015, 12:59:38 AM »
While the researchers (of the linked open access reference) are correct to include both the fingerprint effect (of ice mass loss on SLR) and corrected viscosity of the upper mantle in their Antarctic Ice-Sheet model, once the grounding line retreats to the reverse slope portion of the bed that is characteristic of most key Antarctic marine glaciers, the influence of these two considerations become less critical when cliff failures and hydrofracturing are also considered (as David Pollard is very well aware):

Natalya Gomez, David Pollard & David Holland (November 10, 2015), "Sea-level feedback lowers projections of future Antarctic Ice-Sheet mass loss", Nature Communications 6, Article number: 8798 doi:10.1038/ncomms9798

http://www.nature.com/ncomms/2015/151110/ncomms9798/full/ncomms9798.html

Abstract: "The stability of marine sectors of the Antarctic Ice Sheet (AIS) in a warming climate has been identified as the largest source of uncertainty in projections of future sea-level rise. Sea-level fall near the grounding line of a retreating marine ice sheet has a stabilizing influence on the ice sheets, and previous studies have established the importance of this feedback on ice age AIS evolution. Here we use a coupled ice sheet–sea-level model to investigate the impact of the feedback mechanism on future AIS retreat over centennial and millennial timescales for a range of emission scenarios. We show that the combination of bedrock uplift and sea-surface drop associated with ice-sheet retreat significantly reduces AIS mass loss relative to a simulation without these effects included. Sensitivity analyses show that the stabilization tends to be greatest for lower emission scenarios and Earth models characterized by a thin elastic lithosphere and low-viscosity upper mantle, as is the case for West Antarctica."

Extract: "The ice-sheet model in this paper does not include the new mechanisms of hydrofracturing by surface melt and ice-cliff failure, recently proposed to produce East Antarctic retreat as implied by (albeit uncertain) geologic evidence of high sea-level stands in past warm periods. A future paper exploring these effects with the coupled Earth–ice model is planned, but the mechanisms are somewhat speculative, and their effect is basically to accelerate WAIS retreat and amplify EAIS retreat, and the basic findings of this paper regarding negative-feedback influences of Earth-gravitational interactions are not expected to change."

Edit: I note that the forcing scenarios with names indicating various multiples of the atmospheric CO2 burden, should not be taken as literal concentrations of atmospheric CO2 for a variety of reasons including: (a) one needs to consider CO2 equivalent GHGs and aerosol affects; (b) ECS could be well above 3C; (c) Hansen et al. (2015)'s positive feedback mechanisms associated with ice mass loss was not considered by Gomez et al. (2015); (d) various Earth Systems that are normally considered slow response (permafrost degradation, etc.) could actually be fast response; which might mean that ESS could be between 4.5 and 6C by the end of this century; and (e) Gomez et al. (2015) may not be accounting correctly for vary Earth System initial states, such as the influence of the Antarctic ozone whole and ocean heat content in the Southern Ocean.

Edit2: Furthermore, I note that the amount of glacial rebound indicated by the authors would also be associated with an increase in both seismic and geothermal activity, that were almost certainly not considered within the modeled projections.
« Last Edit: November 11, 2015, 04:31:53 AM by AbruptSLR »
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Re: Potential Collapse Scenario for the WAIS
« Reply #392 on: November 18, 2015, 07:33:45 PM »
Ritz et al 2015 on potential Antarctic contribution to SLR by 2100 and 2200:
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature16147.html

Abstract
Large parts of the Antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine-ice-sheet instability (MISI), a self-sustaining retreat of the grounding line triggered by oceanic or atmospheric changes. There is growing evidence that MISI may be underway throughout the Amundsen Sea embayment (ASE), which contains ice equivalent to more than a metre of global sea-level rise. If triggered in other regions the centennial to millennial contribution could be several metres. Physically plausible projections are challenging: numerical models with sufficient spatial resolution to simulate grounding-line processes have been too computationally expensive to generate large ensembles for uncertainty assessment, and lower-resolution model projections rely on parameterizations that are only loosely constrained by present day changes. Here we project that the Antarctic ice sheet will contribute up to 30 cm sea-level equivalent by 2100 and 72 cm by 2200 (95% quantiles) where the ASE dominates. Our process-based, statistical approach gives skewed and complex probability distributions (single mode, 10 cm, at 2100; two modes, 49 cm and 6 cm, at 2200). The dependence of sliding on basal friction is a key unknown: nonlinear relationships favour higher contributions. Results are conditional on assessments of MISI risk on the basis of projected triggers under the climate scenario A1B (ref. 9), although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss. We find that contributions are restricted by a combination of these constraints, calibration with success in simulating observed ASE losses, and low assessed risk in some basins. Our assessment suggests that upper-bound estimates from low-resolution models and physical arguments (up to a metre by 2100 and around one and a half by 2200) are implausible under current understanding of physical mechanisms and potential triggers.

crandles

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Re: Potential Collapse Scenario for the WAIS
« Reply #393 on: November 19, 2015, 12:09:11 AM »
Ritz et al 2015 on potential Antarctic contribution to SLR by 2100 and 2200:
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature16147.html

BBC article on this
http://www.bbc.co.uk/news/science-environment-34859398
suggest
Quote
But the prospect of a 30cm-or-more contribution - claimed by some previous research - has just a one-in-20 chance.

and mode of just 10cm by 2100.

Seems like good news if the higher outcomes really are as unlikely as being suggested.

Quote
"We're constraining the model with the observations. Nobody has really done this sort of formal scoring before."
makes me wonder if this effectively disregards possibility of new modes of failure like 100m+ cliff failures.

Two modes at 6cm and 49cm for 2200 sounds a bit weird? Is just 6cm from Antactica by 2200 at all plausible for A1B?

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #394 on: November 19, 2015, 12:29:43 AM »
Ritz et al 2015 on potential Antarctic contribution to SLR by 2100 and 2200:
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature16147.html

BBC article on this
http://www.bbc.co.uk/news/science-environment-34859398
suggest
Quote
But the prospect of a 30cm-or-more contribution - claimed by some previous research - has just a one-in-20 chance.

and mode of just 10cm by 2100.

Seems like good news if the higher outcomes really are as unlikely as being suggested.

Quote
"We're constraining the model with the observations. Nobody has really done this sort of formal scoring before."
makes me wonder if this effectively disregards possibility of new modes of failure like 100m+ cliff failures.

Two modes at 6cm and 49cm for 2200 sounds a bit weird? Is just 6cm from Antactica by 2200 at all plausible for A1B?

Garbage in, garbage out.

Rignot stated that his lower bound for total SLR by 2100 is 1.2 m and he is still working on what he considers the upper bound to be.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #395 on: November 19, 2015, 07:35:38 AM »
makes me wonder if this effectively disregards possibility of new modes of failure like 100m+ cliff failures.

Two modes at 6cm and 49cm for 2200 sounds a bit weird? Is just 6cm from Antactica by 2200 at all plausible for A1B?

Garbage in, garbage out.

Rignot stated that his lower bound for total SLR by 2100 is 1.2 m and he is still working on what he considers the upper bound to be.

DeConto and Pollard are expected to publish a new paper on Antarctica in the coming months. So let's see what they have to say in addition to this sneak preview of their findings:
http://meetingorganizer.copernicus.org/EGU2015/EGU2015-8104.pdf

"the magnitude and rate of Antarctic ice sheet retreat are highly dependent on which future greenhouse gas scenario is followed, but even the lower emission scenarios produce an Antarctic contribution of several meters within the next several centuries. Once atmospheric CO2 concentrations exceed 2x preindustrial levels, we find that hydrofracturing by surface melt on ice shelves can trigger large-scale ice sheet retreat, regardless of circum-Antarctic ocean warming. Hence, unlike the LIG, atmospheric (not ocean) warming has the potential to become the primary mechanism driving future retreat of the Antarctic ice sheet. In simulations without atmospheric warming, we find small amounts of ocean warming can still produce large-scale retreat of the West
Antarctic Ice Sheet, although the timescale of ocean-driven retreat is slower than atmospherically driven retreat."

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #396 on: November 19, 2015, 11:12:37 AM »
DeConto and Pollard are expected to publish a new paper on Antarctica in the coming months. So let's see what they have to say ...

Agreed, and great caught on the EGU abstract.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

crandles

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Re: Potential Collapse Scenario for the WAIS
« Reply #397 on: November 19, 2015, 04:08:41 PM »

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #398 on: November 19, 2015, 04:48:08 PM »
Once atmospheric CO2 concentrations exceed 2x preindustrial levels, we find that hydrofracturing by surface melt on ice shelves can trigger large-scale ice sheet retreat, regardless of circum-Antarctic ocean warming. Hence, unlike the LIG, atmospheric (not ocean) warming has the potential to become the primary mechanism driving future retreat of the Antarctic ice sheet. In simulations without atmospheric warming, we find small amounts of ocean warming can still produce large-scale retreat of the West Antarctic Ice Sheet, although the timescale of ocean-driven retreat is slower than atmospherically driven retreat."

Per the following link the pre-industrial CO2eq atmospheric concentration was 280ppm while thru 2014 the CO2 eq atmospheric concentration was about 487ppm, and the rate of CO2 eq increase per year since 1970 is about 3ppm/yr, indicating that we are currently at about 490ppm CO2eq:

http://www.esrl.noaa.gov/gmd/aggi/aggi.html

As 280 times 2 = 560ppm, we need to increase CO2eq by another 70ppm to get to DeConto & Pollard's threshold for hydrofracturing; which could occur by 2038 (assuming 3ppm CO2eq/yr).

Furthermore, this assumes that the effective value of ECS, and the aerosol concentrations, assumed by DeConto & Pollard are correct (while I suspect that they error on the side of least drama).  Thus one mans' "sane & well crafted" projection is another man's "insane and reckless" projection.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

Timothy Astin

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Re: Potential Collapse Scenario for the WAIS
« Reply #399 on: November 19, 2015, 07:16:23 PM »
Garbage in, garbage out.

This is an unfair and unscientific comment. The Ritz et al 2015 paper is a detailed and careful piece of science. It has been peer reviewed for a journal that has excellent reviewing and editing.

This paper forms one of a range of predictions, and is at the lower end of recent estimates for contributions from the Antarctic Peninsula for sea level rise by 2100.

It is an important study to compare and contrast with other predictive approaches.

It is important to read the supplementary information (which is freely available). It includes details on the modelling assumptions, and has further discussion including comparison with other studies.

As Tamsin Edwards says in blog discussion at the link Crandles gives above, their study effectively replicates ice cliff failure in their model by another means, so simply saying that it doesn't take account of that mechanism is not fair either.

There are weaknesses in all predictive methods, especially modelling methods, the further they get away from the boundary conditions. In this case, perhaps a weakness of the predictions is in the extrapolation of statistical relationships derived from a relatively narrow time window of observations.

The other modelling approaches have weaknesses too, and other ice models which attempt to replicate complex physics can turn out to be based on poor assumptions.

That is why it is important that a variety of predictive approaches are tried.