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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #400 on: November 19, 2015, 07:39:42 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.

"All models are wrong but some models are useful", but the key issue is useful for what?  If one is happy to let science plod along very slowly then perhaps the comment is "unscientific"; but as scientists take no responsibility for the catastrophic impact on society if the probabilities from such studies of abrupt ice sheet collapse are wrong, then the comment is not in my opinion "unfair".  The Ritz et al. 2015 paper does not consider hydrofracturing, which DeConto & Pollard show may be the greatest risk of abrupt marine ice sheet collapse, possibly starting as soon as circa 2038.  I think that if a researcher ignores the most important input for abrupt ice sheet mass loss, then it is fair for me to say GIGO.

Edit: The attached image [for Pliocene conditions] from Pollard et al. (2015) shows the importance of including both cliff failures and hydrofracturing within a model whose results are to be relied upon for policy making.
« Last Edit: November 19, 2015, 07:54:56 PM by AbruptSLR »
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oren

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Re: Potential Collapse Scenario for the WAIS
« Reply #401 on: November 21, 2015, 12:17:06 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.

I have long wondered on the negative feedback associated with gravitational effects and marine terminating glaciers. The effects of gravity changes following loss of ice mass are immediate and relatively easy to model, as opposed to mantle rebound which is a slower process with many more unknowns.
However, I have some issues with this paper's assumptions and model results.
First, it mixes gravitation and rebound together and I find it hard to understand each effect separately. An error in the rebound model could undermine the results which arise from gravitation effects. In Figure 3 where these effects are separated, it seems that when sea level rate of change is stable adding gravitational effects lowers sea level in the first 50-100 years. I find this result implausible (should be no mass change ==> no gravitational effects compared to present) and therefore the whole analysis of the separate contributions of these factors problematic.
In addition, some of the scenarios (2x, 4x and 8x pre-industrial CO2, immediate or over 1000 years) are unrealistic. 2x of pre-industrial CO2 is expected in about 60 years (or less when counting all GHGs). The most probable scenarios are 2x "immediate", and 4x to 8x over 1000 years.
In addition, a 2 deg C rise in ocean temperatures is assumed (immediate or over 1000 years). There is no sensitivity analysis of this important number, and no correlation between it and the CO2 level. So running worst-case 8x CO2 but limiting ocean warming to 2 deg limits the worst-case in an unknown way.
Finally, in the 1000 year scenarios with fully coupled model (blue lines in Fig.1), for the first 500 years Antarctica will cause 0.3m of sea level drop. I believe this result is wrong and this undermines the whole paper. Even if Zwally is right about present AIS negative contribution to SLR, this is not expected to continue for 500 years or even 100 years at this rate under warming scenarios.

Bottom line, I find it hard to take this paper's summary as is. Indeed it seems gravitational effects form a significant negative feedback, and so do rebound effects under slower scenarios. But how strong are these feedbacks? I'm still not sure.

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #402 on: November 21, 2015, 12:44:12 AM »
Bottom line, I find it hard to take this paper's summary as is. Indeed it seems gravitational effects form a significant negative feedback, and so do rebound effects under slower scenarios. But how strong are these feedbacks? I'm still not sure.

It is even harder to understand the references finding of a significant negative feedback when one considers that even with all of the ice mass loss to date in Antarctica the local sea level has not dropped at all (see attached image of satellite measurements of local sea level changes), due to such factors contributing an increase in local sea level of: (a) increasing ocean heat content; (b) decreasing average seawater salinity; and (c) the fingerprint effect of ice mass loss from North Hemisphere ice sheets (Greenland) and other glaciers.

Caption for attached image: "Sea Level Anomalies on 2012/01/01 exploiting 4 altimeters: Jason-2, Jason-1, Envisat and Cryosat-2. Credits Cnes-Ssalto/Duacs-Esa"

Edit: I do not expect the local Antarctic sea levels to drop until well after cliff failures and hydrofracturing have been well initiated (circa 2035 to 2040).
« Last Edit: November 21, 2015, 09:20:31 AM by AbruptSLR »
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Csnavywx

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Re: Potential Collapse Scenario for the WAIS
« Reply #403 on: November 21, 2015, 02:40:31 AM »
Wait, what? A1B assumes a mean CO2 concentration of 700 ppm by 2100. And the statistical mode for their low end scenario is 6cm of Antarctic contribution by 2200?

What?

Okay, let's start with the most generous assumptions. What's the current rate of estimated Antarctic contribution to sea level rise?

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #404 on: November 21, 2015, 03:34:07 AM »
What's the current rate of estimated Antarctic contribution to sea level rise?

Different sources give slightly different values, but the attached image provides the official AR5 values:

Caption for the image: "Caption for Figure 2: Contribution of global glaciers (red), Greendland (green) and Antarctica (blue) to sea level rise between 1992-2012. Positive correlation between cumulative ice mass loss and sea level equivalent, shaded areas indicate uncertainty"
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Richard Rathbone

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Re: Potential Collapse Scenario for the WAIS
« Reply #405 on: November 21, 2015, 03:43:57 AM »
Wait, what? A1B assumes a mean CO2 concentration of 700 ppm by 2100. And the statistical mode for their low end scenario is 6cm of Antarctic contribution by 2200?

What?

Okay, let's start with the most generous assumptions. What's the current rate of estimated Antarctic contribution to sea level rise?

They haven't got failure modes in their model. They assume the ice just flows like it is at the moment. And as long as ice flows like it is at the moment, their analysis looks good to me. However thats only reasonably certain for another couple of decades. The moment cliff failure might set in, their analysis is irrelevant as an upper bound, but would still serve as a lower bound. So beyond 2040, treat everything they say as being the likely bound, as being a lower bound, not an upper one.


Csnavywx

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Re: Potential Collapse Scenario for the WAIS
« Reply #406 on: November 21, 2015, 04:30:09 AM »
Thanks ASLR and Richard.

That really bothered me. Even a generously low extrap puts 7cm in the ocean by 2200 using 1998-2012 average melt rates.

With no failure modes, the entire paper makes more sense as a lower bound.

sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #407 on: November 21, 2015, 08:18:50 AM »
The Pollard(2015) paper is apt as a comparison from the dark side.  A first reading of Ritz(2015) led me to it also after meandering through some calculation, Schoof, Bassis, the Jacobshawn thread and eventually Pollard. In the following, i refer to

Ritz(2015) doi:10.1038/nature16147
Mouginot(2014) doi:10.1002/2013GL059069
Shepherd(2012) doi:10.1126/science.1228102
Schoof(2007) doi:10.1029/2006JF000664
Bassis(2012) doi:10.1098/rspa.2011.0422
Bassis(2013) doi:10.1038/NGEO1887
Pollard(2015) doi:10.1016/j.epsl.2014.12.035


My thoughts on Ritz were:

1)Ritz only looks at MISI instability, but marine fronted ice on flatland topo will also retreat under forcing

2)Ritz Suppl. 1.7 mass waste calibration from Shepherd(2012) uses only laser and i/o method, but not the larger mass waste from GRACE which is also in Shepherd. Ritz reference Mouginot(2014) but do not use the (much larger) mass waste estimate in Mouginot for calibrationI have asked one of the authors about this on Dr. Connolley's blog.

3)Ritz have no calving law but claim a horizontal mass waste term which is similar to Bassis mechanism for cliff failure. But Bassis treats granular flow which to me is uncompliant with Ritz "no suction" check on max tensile stress (which Ritz set equal to the free water opposing buttress) to get a max strain rate. (Bassis(2012) explicitly does the case of no cohesive strength.) If Ritz turn the "no suction" check off, they get much larger result, see Ritz extended data fig 10

4)Ritz has no explicit ice shelf collapse or basal melt, these are also parametrised with the low end calibration in 2)

5)Ritz treats heat balance such as melt thru parametrisation. This one gets me. The Schoof treatment assumes that water is at pressure melting point, but in reality is hotter. So the Schoof condition they apply is optimistic. Pollard does a quadratic melt dependence in temperature in excess
of pressure melting point, which may not be exact, but at least begins to address this issue.

6)Pollard does a step Pliocene level forcing,  and a step change of 2C in ocean temperature but i am sure they can do A1B or other scenario as well. I do like the incorporation scheme for Bassis cliff failure in Pollard. So I prefer the Pollard treatment and look forward to a treatment of A1B, perhaps they will say something in the coming paper.

7)There is no simulation in Ritz or Pollard about binge purge cycles.

If you look at Schoof, say Fig 5a), 8, or 9a),b) you see the hysteresis loop. Schoof refers to this:

"A possible interpretation of Heinrich events is that the discharge of sediment-laden icebergs occurred as a result of an abrupt and irreversible retreat of the grounding across this overdeepening ..."

i attach fig 5 and 9 at the end.

What i have in mind is a an ice sheet with two stable states on each side of an overdeepened topo as in Schoof. The thing collapses back across the hole thru cliff failure, and the hole is emptied thru iceberg transport.

As Bassis(2013) puts it:
"Our simulations indicate that calving is a two-part process that requires both ice fracture and the subsequent transport of detached icebergs away  from the calving front."

Then if there is enuf ice left, the thing jumps back across the hole to the seaward stable point and jumps back for another cycle. The longer time constant in this sawtooth is, i think, the berg transport away, but in the large like a relaxation oscillator. In the Thwaites basin the hole goes to the Transantarctic mountains, and one cycle would drain a great deal of WAIS.

Which reminded me of the recent Jacobshawn calving and that Pollard has this to say:

"Huge calving events observed at the fronts of Jakobshavn Isbrae and  Helheim glaciers ... in water depths of ∼700 m to 1000 m with no contiguous  ice shelves, may be the closest example, but may still not be a good analog  as discussed in Appendix A."

In Appendix A.

"The closest analog today might be the separation and overturning of km-scale bergs as observed currently at Jakobshavn and Helheim glaciers  ...  For subaerial cliffs with no ice shelves, it might involve fracturing and violent seaward expelling of ice near the water line (the rough location of maximum unbalanced hydrostatic stress for tidewater cliffs, Bassis and Walker (2012) in a process not seen today."

Eggzackly. We are seeing the beginning at Jacobshawn. Thwaites is at least ten times wider than Jacobshawn. I dont want to see what will happen when it lets go like Jacobshawn is doing, but I fear I shall.

On the whole I am quite thankful to Ritz et al. for making me reread all these nice papers and think. Now I am still confused, but at a higher level.

sidd

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #408 on: November 21, 2015, 09:49:55 AM »
On the whole I am quite thankful to Ritz et al. for making me reread all these nice papers and think. Now I am still confused, but at a higher level.

Nice analysis.  I only wish that Ritz et al (2015) had also taken the time to review all these nice papers, so that policy makers would be less confused (because Ritz et al would have reached the same conclusion that you have).
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S.Pansa

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Re: Potential Collapse Scenario for the WAIS
« Reply #409 on: November 23, 2015, 07:33:11 AM »
Richard Alley, one of the co-authors of the Ritz paper, has now added some comments about the strength of the paper - and about what is missing in the model. Found at Gred Laden's blog here:

Quote
Alley lauds the Ritz, Edwards, et al paper as representing “a great amount of careful work, and provid[ing] a particularly broad exploration of some of the poorly known parameters that control the ice sheet.” However, he finds that the study did not address some important mechanisms.

   " …the model does not allow loss of any ice shelves, does not allow grounding-line retreat from calving of icebergs following ice-shelf loss, and does not allow faster retreat from breakage of cliffs higher than those observed today, especially if aided by meltwater wedging in crevasses. The model restricts grounding-line retreat to the rate given by thinning of ice during viscous flow of an unbuttressed but still-present ice shelf, with a specified upper limit enforced on the rate of that retreat. The model also does not allow retreat up a sloping bed under forcing, something that is widely observed. The Supplementary Information includes discussion of checks that the authors did to assess the importance of these assumptions, which the authors argue justify omitting the mechanisms. However, it remains that with the model not allowing very rapid retreat, not allowing ice-cliff crumbling after ice-shelf loss, and not allowing retreat up sloping beds, the model cannot exhibit some possible behaviors that could cause rapid ice-sheet shrinkage.

    So, I view this as an important step forward for the scientific community, but the qualification in the last sentence of the paper leads to additional information showing that we cannot yet confidently place quantitatively reliable limits on the possible sea-level rise from the Antarctic ice sheet. I personally hope that the new paper is right, but I will continue research on this topic in the hope of providing improved estimates. Until such work is successful, I do not believe we can exclude the possibility of faster sea-level rise than suggested in the new paper."

Seems to confirm what others have said here.

Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #410 on: November 23, 2015, 09:38:33 AM »
Thanks, S.Pansa, for pointing to those comments by Alley (who btw is co-author of a review-paper, but not the Ritz-paper).

Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #411 on: December 03, 2015, 11:35:11 PM »
Watch out for the coming AGU Fall Meeting, starting December 13th.

Big lecture by Rignot:
https://agu.confex.com/agu/fm15/meetingapp.cgi/Paper/60094

And presentation by Pollard:
https://agu.confex.com/agu/fm15/meetingapp.cgi/Paper/60833

And by Hansen on scientific reticence and sea level rise:
https://agu.confex.com/agu/fm15/meetingapp.cgi/Paper/86614

It seems the Pollard presenation will not be shown on AGU on Demand, unfortunately.

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #412 on: December 03, 2015, 11:58:23 PM »
Lennart,

Thank you very much for the links.

Further, just because I like it so much, I provide the abstract for the Pollard, DeConto, Chang, Applegate and Haran (2015) AGU paper that Lennart linked.  While I cannot attend the presentation, it should be a good one, and I note that it would be nice if the authors applied their same approach on Jakobshavn Glacier, for which there are excellent records for the past 30,000 years to compare any model runs that they might make:

David Pollard, Robert DeConto, Won Chang, Patrick Applegate and Murali Haran (Dec 18, 2015), "Modeling of past and future variations of the Antarctic Ice Sheet with Large Ensembles" AGU Fall Meeting, Paper 60833.

https://agu.confex.com/agu/fm15/meetingapp.cgi/Paper/60833

Abstract: "Recent observations of thinning and retreat of the Pine Island and Thwaites Glaciers identify the Amundsen Sea Embayment (ASE) sector of West Antarctica as particularly vulnerable to future climate change. To date, most future modeling of these glaciers has been calibrated using recent and modern observations. As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the last deglacial retreat of Antarctica, making use of geologic data from ~20,000 years BP to present, focusing on the ASE but including other sectors of Antarctica.
Following several recent ice-sheet studies, we use Large Ensemble statistical methods, performing sets of ~600 runs over the last 30,000 years with systematically varying model parameters. Objective scores for each run are calculated using modern data and past reconstructed grounding lines, relative sea level records, cosmogenic elevation-age data and uplift rates. Two types of statistical methods are used to analyze the Large-Ensemble
results: simple averaging weighted by the aggregate score, and more advanced Bayesian emulation and calibration methods that rigorously account for some of the uncertainties in the model and observations.

Results for best-fit parameter ranges and envelopes of equivalent sea-level rise with the simple averaging method agree quite well with the more advanced techniques, but only for a Large Ensemble with dense (Full Factorial) parameter sampling. Runs are extended into the future using RCP scenarios, with drastic retreat mechanisms of hydrofracturing and structural ice-cliff failure. In most runs this produces grounding-line retreat into the West Antarctic interior, and into East Antarctic basins for RCP8.5, and the Large Ensemble analysis provides sea-level-rise envelopes with well defined parametric uncertainty bounds."

Best,
ASLR
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #413 on: December 04, 2015, 08:51:04 PM »
The linked draft reference provides some insight on what we can expect from the Pollard et al (2015) AGU presentation:

Won Chang, Murali Haran, Patrick Applegate, David Pollard (October 7, 2015), "Improving Ice Sheet Model Calibration Using Paleoclimate and Modern Data"

http://arxiv.org/pdf/1510.01676.pdf

Abstract: "Human-induced climate change may cause significant ice volume loss from the West Antarctic Ice Sheet (WAIS). Projections of ice volume change from ice-sheet models and corresponding future sea-level rise have large uncertainties due to poorly constrained input parameters. In most future applications to date, model calibration has utilized only modern or recent (decadal) observations, leaving input parameters that control the long-term behavior of WAIS largely unconstrained. Many paleo-observations are in the form of localized time series, while modern observations are non-Gaussian spatial data; combining information across these types poses nontrivial statistical challenges. Here we introduce a computationally efficient calibration approach that utilizes both modern and paleo-observations to generate better-constrained ice volume projections.

Using fast emulators built upon principal component analysis and a reduced dimension calibration model, we can efficiently handle high-dimensional and non-Gaussian data. We apply our calibration approach to the PSU3D-ICE model which can realistically simulate long-term behavior of WAIS. Our results show that using paleo observations in calibration significantly reduces parametric uncertainty, resulting in sharper projections about the future state of WAIS. One benefit of using paleo observations is found to be that unrealistic simulations with overshoots in past ice retreat and projected future regrowth are eliminated."

Captions: " Figure 7: Predictive distribution for the ice volume change projection based on the real observations described in Section 4.2. The solid red line shows the predictive distribution based on our approach using both the past grounding line positions the modern binary patterns and the dashed and dotted blue line represents the result based on only the modern binary patterns. The dashed green line shows the projection without calibration. The results show that using the past grounding line leads to a significantly sharper projection by removing the unrealistic ice volume increase in the results solely based on the modern observations."
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #414 on: December 05, 2015, 06:37:50 PM »
As a short follow-up to my last post about Won Chang, Murali Haran, Patrick Applegate, David Pollard (2015), while I greatly respect their approach and findings, I note that the dynamic ice mass loss for Pine Island Bay in the last 30,000 years (which likely includes ice mass loss input from Pine Island Bay to Meltwater Pulse 1A) may not be as dynamic as what may occur in the Byrd Subglacial Basin area once the grounding line for the Thwaites Glacier reaches the negative bottom slope (possibly as soon as in the next couple of decade with the mean global surface temperature anom. possibly near 2C).  Therefore, the calibration of the cliff failures and hydrofracturing parameters may currently err somewhat on the side of least drama (while still performing much better than most other comparable marine ice sheet models).

Therefore, I reiterate that if Chang et al. or other researchers have the budget that it would be helpful if they were to create a cliff failure and hydrofacturing model of the Jakobshavn Glacier and to calibrate it against the observed behavior during the past 11,000 years (including the Holocene Optimum) as discussed in Reply #171 of the "Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe" thread; because the grounding line for that glacier appears to have retreated down a bottom slope with a markedly negative slope in that timeframe, and because the surface temperatures during the Holocene Optimum are closer to today's conditions (which might help hydrofracturing).
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #415 on: December 22, 2015, 10:47:39 PM »
The projected future behavior of the ASL per the linked reference will likely force more warm CDW into the ASE thus promoting more than previously projected ice mass loss from marine glaciers in this key area:

J. Scott Hosking, Andrew Orr, Thomas J. Bracegirdle and John Turner (2015), "Future circulation changes off West Antarctica: Sensitivity of the Amundsen Sea Low to projected anthropogenic forcing", Geophysical Research Letters, DOI: 10.1002/2015GL067143

http://onlinelibrary.wiley.com/doi/10.1002/2015GL067143/abstract

Abstract: "The Amundsen Sea Low (ASL) is a major driver of West Antarctic climate variability, with the potential to accelerate the loss of glacial ice. Using the eleven global climate models which most reliably simulate the seasonality in ASL location, we assess the ASL sensitivity to projected future changes using the CMIP5 historical (1951-2000) and representative concentration pathway experiment RCP8.5 (2051-2100). For the first time, we show that the future ASL will likely migrate polewards in summer (DJF) and autumn (MAM), and eastward in autumn and winter (JJA). The autumn-winter changes drive colder southerly winds over the Ross Sea and warmer northerly winds towards the Antarctic Peninsula. This is consistent with recent trends in ERA-Interim reanalysis meridional winds (1979-2014) and reconstructed temperature (1957-2006), suggesting that the impact of anthropogenic forcing on the ASL is likely to play an important role on both past and future patterns of West Antarctic climate variability."
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #416 on: January 01, 2016, 01:08:14 AM »
The linked (open access) reference (and attached associated image) uses both proxy, and climate model, evidence to show that as the Eastern Tropical Pacific continues to warm (both due to climate change and also due to a possible multi-decadal positive phase(s) of the PDO/IPO) the coastal areas of the WAIS and in particular the ASE will sustain anomalously high increases in surface temperature.  This increases the risk of possible hydrofracture and cliff failures in the affected areas, possibly as soon as the 2035 to 2040 timeframe:


Turney, C. S. M., Fogwill, C. J., Klekociuk, A. R., 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, 9, 2405-2415, doi:10.5194/tc-9-2405-2015.

http://www.the-cryosphere.net/9/2405/2015/tc-9-2405-2015.html
http://www.the-cryosphere.net/9/2405/2015/tc-9-2405-2015.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-latitudes to high latitudes of the Southern Hemisphere (SH), which over the last few decades have experienced extreme and regionally 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 δD – a proxy for temperature – from the geographic South Pole where we find a significant influence from extratropical pressure anomalies which act as "gatekeepers" to the meridional exchange of air masses. Reanalysis of global atmospheric circulation suggests these pressure anomalies play a significant influence on mid- to high-latitude SH climate, modulated by the tropical Pacific Ocean. This work adds to a growing body of literature confirming the important roles of tropical and mid-latitude atmospheric circulation variability on Antarctic temperatures. 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: "With projected weakening of the trade winds (England et al., 2014), the observed links to Antarctic temperatures suggest the tropics may in fact play an increasingly significant role in driving high-latitude warming (Figs. 4 and 10), with potentially important implications for understanding past climate states (McGlone et al., 2010; Pedro et al., 2011; Visser et al., 2003) and future Antarctic surface mass balance."
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Re: Potential Collapse Scenario for the WAIS
« Reply #417 on: January 01, 2016, 01:59:55 AM »
The linked (open access) reference finds that there are already large committed ice mass losses for the Smith, Pope and Kohler Glaciers in the ASE over the next 30-years, even without any new oceanic or atmospheric forcings:


Goldberg, D. N., Heimbach, P., Joughin, I., and Smith, B.  (2015), "Committed retreat of Smith, Pope, and Kohler Glaciers over the next 30 years inferred by transient model calibration", The Cryosphere, 9, 2429-2446, doi:10.5194/tc-9-2429-2015.


http://www.the-cryosphere.net/9/2429/2015/tc-9-2429-2015.html


Abstract. A glacial flow model of Smith, Pope and Kohler Glaciers is calibrated by means of control methods against time varying, annually resolved observations of ice height and velocities, covering the period 2002 to 2011. The inversion – termed "transient calibration" – produces an optimal set of time-mean, spatially varying parameters together with a time-evolving state that accounts for the transient nature of observations and the model dynamics. Serving as an optimal initial condition, the estimated state for 2011 is used, with no additional forcing, for predicting grounded ice volume loss and grounding line retreat over the ensuing 30 years. The transiently calibrated model predicts a near-steady loss of grounded ice volume of approximately 21 km3 a−1 over this period, as well as loss of 33 km2 a−1 grounded area. We contrast this prediction with one obtained following a commonly used "snapshot" or steady-state inversion, which does not consider time dependence and assumes all observations to be contemporaneous. Transient calibration is shown to achieve a better fit with observations of thinning and grounding line retreat histories, and yields a quantitatively different projection with respect to ice volume loss and ungrounding. Sensitivity studies suggest large near-future levels of unforced, i.e., committed sea level contribution from these ice streams under reasonable assumptions regarding uncertainties of the unknown parameters.
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Re: Potential Collapse Scenario for the WAIS
« Reply #418 on: January 11, 2016, 09:31:49 PM »
I believe that the behavior of marine glaciers adjoining the Filchner-Ronne ice shelf in the following linked reference should be considered as a lower bound, as it ignores/discounts numerous considerations, including:
- Cliff failures and hydrofracturing.
- Possible interconnection of ocean currents between the Weddell and Amundsen Seas, through submarine channels currently filled with marine ice.
- Possible impacts on ice loss associated with possible/probable changes in the adjoining sea ice.
- Positive feedbacks not considered by the authors.

M. Mengel, J. Feldmann & A. Levermann (2016), "Linear sea-level response to abrupt ocean warming of major West Antarctic ice basin", Nature Climate Change, Volume: 6, Pages: 71–74, doi:10.1038/nclimate2808


http://www.nature.com/nclimate/journal/v6/n1/full/nclimate2808.html

Abstract: "Antarctica’s contribution to global sea-level rise has recently been increasing. Whether its ice discharge will become unstable and decouple from anthropogenic forcing or increase linearly with the warming of the surrounding ocean is of fundamental importance. Under unabated greenhouse-gas emissions, ocean models indicate an abrupt intrusion of warm circumpolar deep water into the cavity below West Antarctica’s Filchner–Ronne ice shelf within the next two centuries. The ice basin’s retrograde bed slope would allow for an unstable ice-sheet retreat, but the buttressing of the large ice shelf and the narrow glacier troughs tend to inhibit such instability. It is unclear whether future ice loss will be dominated by ice instability or anthropogenic forcing. Here we show in regional and continental-scale ice-sheet simulations, which are capable of resolving unstable grounding-line retreat, that the sea-level response of the Filchner–Ronne ice basin is not dominated by ice instability and follows the strength of the forcing quasi-linearly. We find that the ice loss reduces after each pulse of projected warm water intrusion. The long-term sea-level contribution is approximately proportional to the total shelf-ice melt. Although the local instabilities might dominate the ice loss for weak oceanic warming, we find that the upper limit of ice discharge from the region is determined by the forcing and not by the marine ice-sheet instability."
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Re: Potential Collapse Scenario for the WAIS
« Reply #419 on: January 15, 2016, 05:01:55 PM »
The linked April 2015 video of Richard Alley's talk about the risk that mainstream science may not have yet fully recognized the risk of abrupt sea level rise contribution from the WAIS (say due to cliff failures & hydrofracturing), has been posted before.  Nevertheless, I repost it here because Alley concludes his talk by calling for more oceanic-ice interaction investigation; which is precisely what the following linked research to be conducted at Scripps from Dec 2016 to Dec 2017 as part of the ACME program:



The linked website indicates that from Dec 2016 until Dec 2017, Scripps (lead by Julie McClean) will investigate Ocean and Sea-Ice Processes for the ACME program.  Therefore, I believe that it is reasonable to assume that preliminary findings from Phase 1 of the ACME program will not be publically available before 2018.

https://scripps.ucsd.edu/research/proposals/accelerated-climate-modeling-energy-acme-ocean-and-sea-ice-processes

Extract: "For cryosphere, the team will examine the near-term risks of initiating the dynamic instability and onset of the collapse of the Antarctic Ice Sheet due to rapid melting by warming waters adjacent to the ice sheet grounding lines.
 
The experiment would be the first fully coupled global simulation to include dynamic ice shelf–ocean interactions for addressing the potential instability associated with grounding line dynamics in marine ice sheets around Antarctica."
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Re: Potential Collapse Scenario for the WAIS
« Reply #420 on: January 22, 2016, 03:52:44 PM »
The linked reference discusses 300-years of ice core data demonstrating that the Amundsen Sea Low, ASL, is deepening as part of a long-term anthropogenically driven trend.  This trend is accelerating the timeline for the collapse of the WAIS by:

(a) Increasing the advection of warm CDW to key marine glaciers,
(b) Working together with the SAM & El Ninos to advect warm Tropical Pacific atmospheric energy to Western Antarctica, and
(c) Increasing the recent snowfall that has been, and still is, increasing the driving gravitational head on key marine glaciers in this area.

E. R. Thomas, J. S. Hosking, R. R. Tuckwell, R. A. Warren & 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."

See also:
Strelich, L. (2016), Climate change drives increasing snowfall in western Antarctica, Eos, 97, doi:10.1029/2016EO044021. Published on 21 January 2016

Extract: "Scientists predict that one symptom of a changing climate will be a deepening of the Amundsen Sea Low, a climatological low-pressure center that plays a big role in the regional climate. Here Thomas et al. provide new evidence that this change—and, ultimately, the melting of West Antarctic ice sheets—is already under way.

These phenomena influence regional sea surface temperature, atmospheric circulation, and sea level pressure. Changes in sea level pressure generate changes in snowfall and drive meridional winds that bring moist air up onto the Ellsworth coast, causing ocean upwelling and melting of the West Antarctic ice shelves."
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Re: Potential Collapse Scenario for the WAIS
« Reply #421 on: February 01, 2016, 05:03:01 PM »
The linked reference indicates that current atmospheric & ocean conditions are not sufficient to cause a collapse of the WAIS comparable to that which occurred in the LIG (Eemian); however, it cites a Southern Ocean temperature increase threshold of 2 to 3C (based on paleo-data) for a likely collapse of the WAIS (which most current CMIP models indicate will not happen before 2100).  However, I note that scientists do not put their work into perspective for the reader, as that assume that the reader will provide their own context.  In this case, I note that the authors ignore the risk of cliff failures and hydrofracturing and thus can only be considered a lower-bound approximation.  Furthermore, I note that as the Southern Ocean freshens (with continued warming and ice melt) there should be more local evaporation (as freshwater evaporates more easily than saltwater), which in the short-term will result in more snowfall (particularly in West Antarctica), but in a few decades time could result in more rainfall which would greatly accelerated both surface ice mass loss & hydrofracturing (which would need to be included into any upper bound forecast of the future risk of WAIS collapse before 2100):

Paul Gierz, Klaus Grosfeld, Malte Thoma & Gerrit Lohmann (2016), "Ocean temperature thresholds for Last Interglacial West Antarctic Ice Sheet collapse", Geophysical Research Letters, DOI: 10.1002/2016GL067818


http://onlinelibrary.wiley.com/doi/10.1002/2016GL067818/abstract

Abstract: "The West Antarctic Ice Sheet (WAIS) is considered the major contributor to global sea level rise in the Last Interglacial (LIG) and potentially in the future. Exposed fossil reef terraces suggest sea levels in excess of 7 meters in the last warm era, of which probably not much more than 2 meters are considered to originate from melting of the Greenland Ice Sheet. We simulate the evolution of the Antarctic Ice Sheet during the LIG with a 3D thermomechanical ice sheet model forced by an atmosphere ocean general circulation model (AOGCM). Our results show that high LIG sea levels, cannot be reproduced with the atmosphere-ocean forcing delivered by current AOGCMs. However, when taking reconstructed Southern Ocean temperature anomalies of several degrees, sensitivity studies indicate a Southern Ocean temperature anomaly threshold for total WAIS collapse of 2-3∘C, accounting for a sea level rise of 3-4 meters during the LIG. Potential future Antarctic Ice Sheet dynamics range from a moderate retreat to a complete collapse, depending on rate and amplitude of warming."
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Re: Potential Collapse Scenario for the WAIS
« Reply #422 on: February 09, 2016, 02:08:32 AM »
The linked reference quantifies the zone of "passive" ice shelf that can be lost without accelerating the contribution of the buttressed grounded marine glacial to sea level rise.  The Amundsen and Bellingshausen ice shelves were found to have the least area of passive ice shelf in Antarctica (see image):

Johannes Jakob Fürst, Gaël Durand, Fabien Gillet-Chaulet, Laure Tavard, Melanie Rankl, Matthias Braun & Olivier Gagliardini  (2016), "The safety band of Antarctic ice shelves", Nature Climate Change, doi:10.1038/nclimate2912

http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2912.html

Abstract: "The floating ice shelves along the seaboard of the Antarctic ice sheet restrain the outflow of upstream grounded ice. Removal of these ice shelves, as shown by past ice-shelf recession and break-up, accelerates the outflow, which adds to sea-level rise. A key question in predicting future outflow is to quantify the extent of calving that might precondition other dynamic consequences and lead to loss of ice-shelf restraint. Here we delineate frontal areas that we label as ‘passive shelf ice’ and that can be removed without major dynamic implications, with contrasting results across the continent. The ice shelves in the Amundsen and Bellingshausen seas have limited or almost no ‘passive’ portion, which implies that further retreat of current ice-shelf fronts will yield important dynamic consequences. This region is particularly vulnerable as ice shelves have been thinning at high rates for two decades and as upstream grounded ice rests on a backward sloping bed, a precondition to marine ice-sheet instability. In contrast to these ice shelves, Larsen C Ice Shelf, in the Weddell Sea, exhibits a large ‘passive’ frontal area, suggesting that the imminent calving of a vast tabular iceberg will be unlikely to instantly produce much dynamic change."

Caption: "Map showing the percentage area of "passive" ice in all of Antarctica's ice shelves. Passive ice can be lost without dramatically changing glacier and ice sheet dynamics"

See also:
http://www.climatecentral.org/news/weakening-ice-shelves-sea-level-rise-20003

Extract: "The Amundsen and Bellingshausen sea sectors in West Antarctica are home to the most at-risk ice on the continent. Both regions’ ice shelves average less than 10 percent of their ice being “safe” to lose (in a relative sense anyways) before major ice sheet dynamics start to change."
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Re: Potential Collapse Scenario for the WAIS
« Reply #423 on: February 09, 2016, 07:47:26 PM »
The linked reference indicates that the current generation of AOGCM projections are incapable of hindcasting the sea level contribution from the WAIS during the Last Interglacial (LIG or Eemian).  Hopefully, future generations of climate models will include cliff failure and hydrofractuing mechanisms (ala Pollard et al 2015).  Also, while the reference cites a "… Southern Ocean temperature anomaly threshold for total WAIS collapse of 2-3∘C …", it does not acknowledge that the current ozone hole over Antarctica has no parallel during the LIG, and that the current ozone how can/is creating westerly wind that is advecting warm CDW to the grounding lines of key WAIS marine glaciers much more quickly than likely occurred during the LIG/Eemian:

Johannes Sutter, Paul Gierz, Klaus Grosfeld, Malte Thoma & Gerrit Lohmann (2016), "Ocean temperature thresholds for Last Interglacial West Antarctic Ice Sheet collapse", Geophysical Research Letters, DOI: 10.1002/2016GL067818


http://onlinelibrary.wiley.com/doi/10.1002/2016GL067818/abstract?systemMessage=Wiley+Online+Library+will+be+unavailable+on+Saturday+27th+February+from+09:00-14:00+GMT+/+04:00-09:00+EST+/+17:00-22:00+SGT+for+essential+maintenance.++Apologies+for+the+inconvenience.

Abstract: "The West Antarctic Ice Sheet (WAIS) is considered the major contributor to global sea level rise in the Last Interglacial (LIG) and potentially in the future. Exposed fossil reef terraces suggest sea levels in excess of 7 meters in the last warm era, of which probably not much more than 2 meters are considered to originate from melting of the Greenland Ice Sheet. We simulate the evolution of the Antarctic Ice Sheet during the LIG with a 3D thermomechanical ice sheet model forced by an atmosphere ocean general circulation model (AOGCM). Our results show that high LIG sea levels, cannot be reproduced with the atmosphere-ocean forcing delivered by current AOGCMs. However, when taking reconstructed Southern Ocean temperature anomalies of several degrees, sensitivity studies indicate a Southern Ocean temperature anomaly threshold for total WAIS collapse of 2-3∘C, accounting for a sea level rise of 3-4 meters during the LIG. Potential future Antarctic Ice Sheet dynamics range from a moderate retreat to a complete collapse, depending on rate and amplitude of warming."
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Re: Potential Collapse Scenario for the WAIS
« Reply #424 on: February 18, 2016, 10:00:18 PM »
The linked reference discusses field research documenting the manner in which the Ross Ice Sheet/Shelf retreated during the Holocene in a series of relatively abrupt stair-steps that periodically sent armadas of icebergs across the continental shelf, gouging furrows as they moved into the open ocean.  While this information has some relevance to projecting the potential future break-up of the existing Ross Ice Shelf; it also has a good degree of relevance to projecting the armadas of icebergs that are likely to exit out of the Amundsen Sea Embayment if/when the adjoining marine glaciers break-up (see associated image)

Yusuke Yokoyama, John B. Anderson, Masako Yamane, Lauren M. Simkins, Yosuke Miyairi, Takahiro Yamazaki, Mamito Koizumi, Hisami Suga, Kazuya Kusahara, Lindsay Prothro, Hiroyasu Hasumi, John R. Southon, and Naohiko Ohkouchi (2016), "Widespread collapse of the Ross Ice Shelf during the late Holocene", Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1516908113


http://www.pnas.org/content/early/2016/02/09/1516908113.abstract

Abstract: "The stability of modern ice shelves is threatened by atmospheric and oceanic warming. The geologic record of formerly glaciated continental shelves provides a window into the past of how ice shelves responded to a warming climate. Fields of deep (−560 m), linear iceberg furrows on the outer, western Ross Sea continental shelf record an early post-Last Glacial Maximum episode of ice-shelf collapse that was followed by continuous retreat of the grounding line for ∼200 km. Runaway grounding line conditions culminated once the ice became pinned on shallow banks in the western Ross Sea. This early episode of ice-shelf collapse is not observed in the eastern Ross Sea, where more episodic grounding line retreat took place. More widespread (∼280,000 km2) retreat of the ancestral Ross Ice Shelf occurred during the late Holocene. This event is recorded in sediment cores by a shift from terrigenous glacimarine mud to diatomaceous open-marine sediment as well as an increase in radiogenic beryllium (10Be) concentrations. The timing of ice-shelf breakup is constrained by compound specific radiocarbon ages, the first application of this technique systematically applied to Antarctic marine sediments. Breakup initiated around 5 ka, with the ice shelf reaching its current configuration ∼1.5 ka. In the eastern Ross Sea, the ice shelf retreated up to 100 km in about a thousand years. Three-dimensional thermodynamic ice-shelf/ocean modeling results and comparison with ice-core records indicate that ice-shelf breakup resulted from combined atmospheric warming and warm ocean currents impinging onto the continental shelf."

See also:
http://phys.org/news/2016-02-colossal-antarctic-ice-shelf-collapse-ice.html

Extract: ""We found that about 10,000 years ago, this thick, grounded ice sheet broke apart in dramatic fashion," Anderson said. "The evidence shows that an armada of icebergs—each at least twice as tall as the Empire State Building—was pushed out en masse. We know this because this part of the Ross Sea is about 550 meters (1,804 feet) deep, and the icebergs were so large and so tightly packed that they gouged huge furrows into the seafloor as they moved north."


The really big breakup began around 3000 B.C.," Anderson said. "We believe it was similar, in many respects, to the breakup of the Larsen B Ice Shelf in 2002. The Larsen is far smaller than the Ross Ice Shelf, but satellite imagery that year showed the Larsen dramatically breaking apart in just a few weeks. We believe the large breakup of the Ross Ice Shelf occurred at roughly this same pace, but the area involved was so much larger—about the size of the state of Colorado—that it took several centuries to complete."

By 1500 B.C. the breakup had exposed about 100,000 square miles of the Ross Sea that had been either wholly or largely ice-covered for many millennia, Anderson said."
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Re: Potential Collapse Scenario for the WAIS
« Reply #425 on: February 21, 2016, 07:43:52 PM »
The linked (open access) Fogwill et. al. (2015) reference first notes that in order to achieve the observed sea levels during recent past interglacial periods (particularly the Eemian) as yet unidentified mechanisms must have contributed to the accelerated collapse of major portions of the WAIS.  The reference then assumes various collapse scenarios for the WAIS and uses a regional climate model to study the sensitivity of the Southern Ocean to the assumed scenarios in order to study one of several likely feedback mechanisms that could contribute to accelerated WAIS collapse.  The attached image shows the projected decrease in AABW formation and the increase in CDW (400 to 700m deep) temperature associated with these scenarios.  Of particular, concern is that the projections indicate that the scenarios result in the advection of the warmer CDW to key marine glaciers (notable those in the ASE), which results in a positive feedback for sustained WAIS collapse.  The authors acknowledge that their findings are likely conservative (ESLD) as they do not consider continued GHG emissions nor possible high values for ECS/ESS, nor the positive feedbacks cited by Hansen et. al. (2015).  The reference concludes by calling for the development of more advanced climate models (such as ACME [both initial and final]) to further investigate the many and complex issues associated with this matter:

C. J. Fogwill, S. J. Phipps, C. S. M. Turney & N. R. Golledge (2015), "Sensitivity of the Southern Ocean to enhanced regional Antarctic ice sheet meltwater input", Earth's Future, Volume 3, Issue 10, Pages 317–329, DOI: 10.1002/2015EF000306

http://onlinelibrary.wiley.com/doi/10.1002/2015EF000306/full

Abstract: "Despite advances in our understanding of the processes driving contemporary sea level rise, the stability of the Antarctic ice sheets and their contribution to sea level under projected future warming remains uncertain due to the influence of strong ice-climate feedbacks. Disentangling these feedbacks is key to reducing uncertainty. Here we present a series of climate system model simulations that explore the potential effects of increased West Antarctic Ice Sheet (WAIS) meltwater flux on Southern Ocean dynamics. We project future changes driven by sectors of the WAIS, delivering spatially and temporally variable meltwater flux into the Amundsen, Ross, and Weddell embayments over future centuries. Focusing on the Amundsen Sea sector of the WAIS over the next 200 years, we demonstrate that the enhanced meltwater flux rapidly stratifies surface waters, resulting in a significant decrease in the rate of Antarctic Bottom Water (AABW) formation. This triggers rapid pervasive ocean warming (>1°C) at depth due to advection from the original site(s) of meltwater input. The greatest warming is predicted along sectors of the ice sheet that are highly sensitized to ocean forcing, creating a feedback loop that could enhance basal ice shelf melting and grounding line retreat. Given that we do not include the effects of rising CO2—predicted to further reduce AABW formation—our experiments highlight the urgent need to develop a new generation of fully coupled ice sheet climate models, which include feedback mechanisms such as this, to reduce uncertainty in climate and sea level projections."

Extract: "One major uncertainty, however, is how the marine-based West Antarctic Ice Sheet (WAIS) will respond to future climate change, and particularly how it may contribute to future global mean sea level (GMSL) [Lenton et al., 2008; Pritchard et al., 2012; Vaughan et al., 2013; Golledge et al., 2015]. In part, this question arises from analogy with past interglacial periods when, despite only small apparent increases in mean atmospheric and ocean temperatures, GMSL is predicted to have been far higher than present [Dutton et al., 2015; Dutton and Lambeck, 2012; Kopp et al., 2009]. To achieve these levels, undefined mechanisms must have been at work that substantially increased the net contribution of the Earth's ice sheets to global sea level [Fogwill et al., 2014].
One such mechanism could have been through ice-ocean feedbacks that arose as a consequence of enhanced meltwater discharge to the Southern Ocean. This has been highlighted in recent studies investigating the apparent coupling between Antarctic ice sheet change and atmospheric temperatures during past interglacials [Holden et al., 2010]. In conclusion, this detailed study of the Last Interglacial demonstrated that feedbacks from WAIS retreat were required to simulate the magnitude of the observed warming within Antarctic ice core records.



To summarize, the changes in the properties of AABW triggered by increasing freshwater input in the Southern Ocean surrounding Antarctica have critical implications for the dynamics of the Antarctic ice sheet. Intriguingly, several recent studies provide growing evidence of rapid contemporary changes in the properties of AABW [Jacobs et al., 2002; Rhein et al., 2013; van Wijk and Rintoul, 2014]. Observations suggest that the AABW layer is warming, freshening, and contracting in volume [Jacobs et al., 2002], although the drivers of these changes are not yet clear. Our simulations and the mechanism described above suggests that contemporary Southern Ocean freshening may already be occurring as a result of increasing delivery of meltwater from Antarctic ice, with the possibility that a marked reduction in the rate of AABW production may be imminent [Purkey and Johnson, 2013; Rhein et al., 2013], triggering further warming at depth in the Southern Ocean. When combined with uncertainties regarding potential increases in ocean temperatures due to shifting winds and/or changing ocean circulation patterns, the potential for marked changes in ocean ice sheet dynamics over the next century is high [Fogwill et al., 2014; Hellmer et al., 2012; Miles et al., 2013; Spence et al., 2014]. Our experiments provide a unique insight into potential future changes in the Southern Ocean that have important implications for the stability of the Antarctic ice sheets. This study examines just one of a number of strong feedback mechanisms operating at the ocean ice sheet interface that question current sea level rise projections; clearly, modeling studies will need to integrate these feedbacks to gain a more realistic picture of future change."


See also:
Sullivan, C. (2016), Antarctic meltwater makes the ocean warmer and fresher, Eos, 97, doi:10.1029/2016EO044811. Published on 1 February 2016.

https://eos.org/research-spotlights/antarctic-meltwater-makes-the-ocean-warmer-and-fresher
Extract: "It’s well known that anthropogenic warming will affect global climate and sea levels far into the future. At the edges of ice sheets and glaciers, water, air, and ice all come together in a complex union that carries implications for predicting climate in the future. As global temperatures rise, ice sheets and glaciers melt, dumping their meltwater into the ocean. The future of some areas, such as the West Antarctic Ice Sheet (WAIS), remains a wildcard when it comes to predicting how the oceans might change.
The WAIS spans much of West Antarctica and holds 2.2 million square kilometers of ice. To try and piece together how the ice sheet might change in the future, Fogwill et al. used climate models to examine the meltwater’s potential effects, focusing on three specific Antarctic regions: the Amundsen, Ross, and Weddell embayments. The scientists’ experiments considered numerous scenarios, ranging from only portions of the ice sheet disappearing to complete melting.
Fresh water flowing off the ice sheet reduces the salinity and density of the ocean’s surface water. With lower-density water sitting on top, the ocean’s vertical layers become more distinct and don’t mix as much. The researchers also found that the formation rate of Antarctic Bottom Water—a layer of dense cool water that pools near the ocean floor—decreased by 25%–50% within decades when compared to a preindustrial control. In all scenarios, the results show that waters ranging from 400 to 700 meters in depth will warm rapidly within the first 200 years, with warming at depth fluctuating between 0.5°C and >1°C, depending on the scenario.
The scientists demonstrated that an increase in fresh meltwater from the WAIS decreases ocean salinity and increases water temperatures in critical locations around the edge of the Antarctic ice sheet, including areas of the East Antarctic Ice Sheet (EAIS). In addition, by investigating specific sites along the ice sheet’s edges, the researchers showed that the location of melting is just as critical to ocean dynamics as the volume of meltwater flowing into the ocean, with sites such as the Amundsen Sea, an area of rapid contemporary WAIS change, being critical.
These effects extend beyond the Southern Ocean. Although the impact is strongest there, all across the Southern Hemisphere, salinity and water temperature are changing. In these models, however, the scientists didn’t include rising atmospheric carbon dioxide levels, which could accelerate the rate of ice sheet melting. Given that, the estimations provided could be conservative."
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Re: Potential Collapse Scenario for the WAIS
« Reply #426 on: February 21, 2016, 10:32:34 PM »
There is another paper wil Fogwill in 2014 DOI:10.1002/jqs.2683   which was also disturbing. I believe i have commented before on Fig 7 therein and the following discussion of concave basin profiles and fast drawdown.

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Re: Potential Collapse Scenario for the WAIS
« Reply #427 on: February 22, 2016, 07:00:51 PM »
The linked reference compares paleo ice core data with a coupled climate model projections to that Southern Ocean deep convection can drive Antarctic multidecadal warming events.  The projections indicate that such convection driven warming events must be preconditioned by: (1) heat accumulation in the depth Southern Ocean (which is occurring now); (2) changes in wind and/or sea ice patterns (which have been projected in the near future); and (3) fast sea-ice-albedo feedback.
While Hansen et al (2015) projects this type of deep convective ocean behavior most other AR5 model projections do not; thus it is possible that in the future that: (a) initially the forecast increase in snow will fall more on sea ice than on land, where it will contribute to SLR; and (b) after a few decades of the warming event the sea ice extent will be reduced and the increased precipitation will fall more frequently as rain (rather than snow) that contributing the hydrofracturing and abrupt SLR.

J.B. Pedro, T. Martin, E. J. Steig, M. Jochum, W. Park & S.O. Rasmussen (20 February 2016), "Southern Ocean deep convection as a driver of Antarctic warming events", Geophysical Research Letters, DOI: 10.1002/2016GL067861

http://onlinelibrary.wiley.com/doi/10.1002/2016GL067861/abstract

Abstract: "Simulations with a free-running coupled climate model show that heat release associated with Southern Ocean deep convection variability can drive centennial-scale Antarctic temperature variations of up to 2.0 °C. The mechanism involves three steps: Preconditioning: heat accumulates at depth in the Southern Ocean; Convection onset: wind and/or sea-ice changes tip the buoyantly unstable system into the convective state; Antarctic warming: fast sea-ice–albedo feedbacks (on annual–decadal timescales) and slow Southern Ocean frontal and sea-surface temperature adjustments to convective heat release (on multidecadal–century timescales) drive an increase in atmospheric heat and moisture transport toward Antarctica. We discuss the potential of this mechanism to help drive and amplify climate variability as observed in Antarctic ice-core records."

Caption for second image: "Figure S2. Map showing the surface-air-temperature (SAT) anomaly during stage 2 (cf. Figure 3d). Circles mark locations of ice-core records. Color-coding of the circles depicts the maximum lagged correlation coefficient of modeled local SAT with SAT over the convection area (black cross in Weddell Sea). Lower panels show time series of modeled SAT anomalies at selected ice-core sites (red) together with the SAT anomaly over the convection region (black). Note different y-axis scaling for red lines."
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sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #428 on: February 23, 2016, 07:09:58 AM »
Here are the long awaited Pollard/DeConto/... papers from ANDRILL 2 in PNAS

unfortunately the dois printed on the pdf for papers are the same ?! but they are available from pnas early content, altho the abstract for one leads to the other ? oddities abound

doi:10.1073/pnas.1516030113
doi:10.1073/pnas.1516130113

nyhoo i enclose a heartstopping graf from the Gasson paper, with many meters of sea level rise in much less than a millennium after including cliff failure and hydrofracture.



Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #429 on: February 23, 2016, 08:58:56 AM »
Tnx for catching these, sidd.
So they concentrated on the Miocene first and left the RCP-scenarios for later.
In their scenario A they seem to find about 6m of SLR in the first century after collapse, and another 4-5m in the second century, as their fig3 that sidd provides, shows. Also see their fig2 attached below for the different ice sheeets under scenarios A (modern bedrock topography) and B (assumed Miocene bedrock topography).

In their methods section they say:
"an instantaneous warming experiment is performed. This experiment uses the equilibrated ice sheets from the colder climate simulations as boundary and initial conditions. Atmospheric CO2 is increased to either 500 or 840 ppm, and a warm austral summer astronomical configuration is specified (January insolation at 70° S, 539 W m−2). Because of the significant computational expense of the asynchronous GCM–RCM climate forcing, it is not currently feasible to perform simulations with a transient astronomical forcing."

In this situation the extra 340 ppm apparently does not cause faster SLR in the first centuries.

Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #430 on: February 23, 2016, 11:34:26 AM »
Stefan Rahmstorf connects the dots at RealClimate between these papers and two other new SLR-papers in PNAS (and discussed in the Consequences folder under the SLR and social cost of carbon thread:
http://www.realclimate.org/index.php/archives/2016/02/millennia-of-sea-level-change/

He points to:
"two further new papers, also appearing in PNAS this week, by Gasson et al. and by Levy et al.. These papers look at the stability of the Antarctic Ice Sheet during the early to mid Miocene, between 23 and 14 million years ago. What is most relevant here is the advances in modelling the Antarctic ice sheet by including new mechanisms describing the fracturing of ice shelves and the breakup of large ice cliffs. The improved ice sheet model is able to capture the highly variable Antarctic ice volume during the Miocene; the bad news is that it suggests the Antarctic Ice Sheet can decay more rapidly than previously thought."

As noted above, Gasson et al seem to find that about 10m of SLR in two centuries may be likely once WAIS collapse gets going, so maybe from around 2050 or so?

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Re: Potential Collapse Scenario for the WAIS
« Reply #431 on: February 23, 2016, 12:11:34 PM »
Correction, I didn't read carefully enough: Gasson et al don't find 10m in two centuries, but in four, with about 6m in the first two and about 4m in the next two.

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Re: Potential Collapse Scenario for the WAIS
« Reply #432 on: February 23, 2016, 12:45:17 PM »
Also see:
https://www.sciencedaily.com/releases/2016/02/160222155615.htm

Gasson says:
"The ice sheets will take hundreds of years to respond, so although CO2 may be at the same level as during the Miocene in the next 30 years, it doesn't mean that they will melt in 30 years,"

But how many centuries are we talking about? Two, three, more? And how certain are we of that? We're at about two centuries now. According to fig2b (attached below) in Rohling et al 2013 we may not need more time for the ice sheets to start fully responding:
http://www.nature.com/articles/srep03461

Hopefully it will take a few more centuries, but we're in risky territory.

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Re: Potential Collapse Scenario for the WAIS
« Reply #433 on: February 23, 2016, 01:16:28 PM »

Lennart van der Linde

Quote
Correction, I didn't read carefully enough: Gasson et al don't find 10m in two centuries, but in four, with about 6m in the first two and about 4m in the next two.

Is that from WAIS alone? Or does it include other sources?

And thanks.
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Re: Potential Collapse Scenario for the WAIS
« Reply #434 on: February 23, 2016, 05:12:48 PM »

Lennart van der Linde

Quote
Correction, I didn't read carefully enough: Gasson et al don't find 10m in two centuries, but in four, with about 6m in the first two and about 4m in the next two.

Is that from WAIS alone? Or does it include other sources?

And thanks.

Geoff,

As the linked Robert Scribbler article indicates that during the Middle Miocene Climate Optimum sea level was 36.5 to 58m meters higher than today, it is safe to say that the Gasson et al (2016) values only include contribution from Antarctica and not from other sources.

Best
ASLR

http://robertscribbler.com/2015/03/11/entering-the-middle-miocene-co2-likely-to-hit-404-parts-per-million-by-may/

Extract: "The Middle Miocene Climate Optimum occurred between about 15 and 17 million years ago. It hosted an atmosphere in which carbon dioxide levels varied wildly from 300 parts per million to 500 parts per million. Temperatures were between 3 to 5 degrees Celsius hotter than the 19th Century. And sea levels were about 120 to 190 feet higher."

See also:
https://www.washingtonpost.com/news/energy-environment/wp/2016/02/22/antarctica-could-be-more-vulnerable-to-major-melting-than-we-thought/

Also I provide the following reference information about the Levy et al (2016) paper:

R. Levy, D. Harwood, F. Florindo, F. Sangiorgi, R. Tripati, H. von Eynatten, E. Gasson, G. Kuhn, A. Tripati, R. DeConto, C. Fielding, B. Field, N. Golledge, R. McKay, T. Naish, M. Olney, D. Pollard, S. Schouten, F. Talarico, S. Warny, V. Willmott, G. Acton, K. Panter, T. Paulsen, and M. Taviani (2016), "Antarctic ice sheet sensitivity to atmospheric CO2variations in the early to mid-Miocene", Proceedings of the National Academy of Sciences, pp. 201516030, doi: 10.1073/pnas.1516030113


http://www.pnas.org/content/early/2016/02/17/1516030113.abstract?sid=054ec02a-cd04-4970-9a0b-91d9ea9d0fb7

Significance: "New information from the ANDRILL-2A drill core and a complementary ice sheet modeling study show that polar climate and Antarctic ice sheet (AIS) margins were highly dynamic during the early to mid-Miocene. Changes in extent of the AIS inferred by these studies suggest that high southern latitudes were sensitive to relatively small changes in atmospheric CO2 (between 280 and 500 ppm). Importantly, reconstructions through intervals of peak warmth indicate that the AIS retreated beyond its terrestrial margin under atmospheric CO2 conditions that were similar to those projected for the coming centuries."

Abstract: " Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23–14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene."
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Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #435 on: February 23, 2016, 05:55:27 PM »
Hi Geoff,

In addition to ASLR: yes, Gasson et al are talking only about Antarctica, so this implies that we have to add SLR from thermal expansion and from melting GIS and smaller ice caps and glaciers to the SLR from WAIS and parts of EAIS in the coming centuries.

This could then add maybe another 2-3m until say 2250-2300 to the potential 6m from WAIS and EAIS.

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Re: Potential Collapse Scenario for the WAIS
« Reply #436 on: February 23, 2016, 10:06:14 PM »
Fig S4 in Gasson shows the dramatic effect of hydrofracture and cliff failure

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Re: Potential Collapse Scenario for the WAIS
« Reply #437 on: February 23, 2016, 10:33:35 PM »
I thought that for ease of reference, I might as well provide the following for the Gasson et al (2016) paper (and the attached image with two figures).  Further, I thought that I would note that (a) our current CO₂-e is already near 490 ppm (less aerosol negative forcing) so if we clean-up our aerosols quickly we could be at mid-Miocene atmospheric conditions soon than many people are thinking; and (b) the attached images shows that our current WAIS bathymetry makes our current West Antarctic marine glaciers more sensitive to abrupt collapse than was the case during the mid-Miocene period:

E. Gasson, R.M. DeConto, D. Pollard, and R.H. Levy (2016), "Dynamic Antarctic ice sheet during the early to mid-Miocene", Proceedings of the National Academy of Sciences, pp. 201516130, doi: 10.1073/pnas.1516130113

http://www.pnas.org/content/early/2016/02/17/1516130113

Significance: "Atmospheric concentrations of carbon dioxide are projected to exceed 500 ppm in the coming decades. It is likely that the last time such levels of atmospheric CO2 were reached was during the Miocene, for which there is geologic data for large-scale advance and retreat of the Antarctic ice sheet. Simulating Antarctic ice sheet retreat is something that ice sheet models have struggled to achieve because of a strong hysteresis effect. Here, a number of developments in our modeling approach mean that we are able to simulate large-scale variability of the Antarctic ice sheet for the first time. Our results are also consistent with a recently recovered sedimentological record from the Ross Sea presented in a companion article."

Abstract: "Geological data indicate that there were major variations in Antarctic ice sheet volume and extent during the early to mid-Miocene. Simulating such large-scale changes is problematic because of a strong hysteresis effect, which results in stability once the ice sheets have reached continental size. A relatively narrow range of atmospheric CO2 concentrations indicated by proxy records exacerbates this problem. Here, we are able to simulate large-scale variability of the early to mid-Miocene Antarctic ice sheet because of three developments in our modeling approach. (i) We use a climate–ice sheet coupling method utilizing a high-resolution atmospheric component to account for ice sheet–climate feedbacks. (ii) The ice sheet model includes recently proposed mechanisms for retreat into deep subglacial basins caused by ice-cliff failure and ice-shelf hydrofracture. (iii) We account for changes in the oxygen isotopic composition of the ice sheet by using isotope-enabled climate and ice sheet models. We compare our modeling results with ice-proximal records emerging from a sedimentological drill core from the Ross Sea (Andrill-2A) that is presented in a companion article. The variability in Antarctic ice volume that we simulate is equivalent to a seawater oxygen isotope signal of 0.52–0.66‰, or a sea level equivalent change of 30–36 m, for a range of atmospheric CO2 between 280 and 500 ppm and a changing astronomical configuration. This result represents a substantial advance in resolving the long-standing model data conflict of Miocene Antarctic ice sheet and sea level variability."

Caption for second part of the attached image: "Fig. S3. Sensitivity of model to bedrock topography for instantaneous warming experiments (warmer climate forcing, 500-ppm CO2, no climate feedbacks).  Note the increased initial ice volume for the older topographies, which are more similar to the Eocene/Oligocene topography of ref. 40 and have a greater area of land above sea level, particularly in the West Antarctic. Despite the large difference in response, the change in sea level is similar for all topographies. “max” and “min” are scaled between either the Eocene–Oligocene maximum or minimum reconstructions of ref. 40."
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AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #438 on: March 30, 2016, 12:32:47 AM »
While all of the linked IGSOC 2015 abstracts are of interest, I picked four that I thought had particular relevance to the discussion of the stability/timing of the potential partial collapse of the WAIS, this century:

http://www.igsoc.org/symposia/2015/cambridge/proceedings/procsfiles/procabstracts_68.html

Extract: "73A1780
Large-ensemble modeling of last deglacial and future variations of the Antarctic ice sheet
David Pollard, Robert DeConto, Won Chang, Patrick Applegate, Murali Haran
Corresponding author: David Pollard
Corresponding author e-mail: pollard@essc.psu.edu
Recent observations of thinning and retreat of Pine Island and Thwaites Glaciers identify the Amundsen Sea Embayment (ASE) sector of West Antarctica as particularly vulnerable to future climate change. To date, most future modeling of these glaciers has been calibrated using recent and modern observations. As an alternate approach, we apply a hybrid 3-D ice-sheet–shelf model to the last deglacial retreat of Antarctica, making use of geologic data from ~20 000 years BP to present, focusing on the ASE but including other sectors of Antarctica. Following several recent ice-sheet studies, we use large-ensemble statistical techniques, performing sets of ~600 runs with varying model parameters. The model is run for the last 40 ka, both on continental domains and on nested domains over West Antarctica. Various types of objective RMS scores for each run are calculated using reconstructed past grounding lines, relative sea-level records, measured uplift rates, cosmogenic elevation-age data, and modern ice distribution. Runs are extended into the future few millennia using simple warming scenarios. The goal is to produce calibrated probabilistic ranges of model parameter values and quantified envelopes of future ice retreat. Two types of results are described and compared, using (1) statistical techniques with emulation, likelihood functions and MCMC, and (2) a much simpler technique of taking ensemble-mean averages weighted by aggregate RMS scores. One robust conclusion is that for future warming scenarios, most reasonable parameter combinations produce retreat deep into the West Antarctic interior.

&
73A1817
Strong effects of thermodynamic interactions of the Antarctic ice shelves with the ocean circulation on the Southern Ocean and sea-ice formation in a global coupled ocean circulation model
Olga Sergienko, Mathew Harrison
Corresponding author: Olga Sergienko
Corresponding author e-mail: osergien@princeton.edu
Melting/refreezing of ice shelves have strong impacts both on ice shelves (through modification of their shape) and on the ocean circulation (through modification of their water masses). Representation of ice-shelf/ocean interaction in the global ocean circulation models continues to be challenging. Using a high-resolution (1/8 deg) global isopycnal ocean model, MOM6, and a sea-ice model, SIS, we investigate the effects of thermodynamic coupling of the Antarctic ice shelves on the various aspects of ocean circulation. Such high (3–8 km) horizontal spatial resolution allows for detailed resolution of the sub-ice-shelf cavity circulations. The computed ice-shelf melt rates are in very good agreement with observationally derived melt rate estimates. The spatial distributions of simulated melting/freezing rates indicate enhanced melting in the vicinity of the grounding line and very strong melting at the ice-shelf front. Results of our simulations show strong effects of sub-ice-shelf meltwater on circulation of the Southern Ocean. We also find that simulations accounting for the thermodynamic coupling of the Antarctic ice shelves produce consistently thicker sea ice compared with the uncoupled simulations.

&
73A1887
1000 year adaptive mesh simulations of Antarctic ice dynamics
Stephen Cornford, Daniel Martin, Antony Payne, Esmond Ng
Corresponding author: Stephen Cornford
Corresponding author e-mail: ggslc@bristol.ac.uk
Numerical modelling of Antarctic ice dynamics becomes more demanding as the simulation time increases, partly because drainage basins evolve and even merge over long timescales, and to a great extent because fine-scale features – such as the grounding line – can migrate over continental length scales. Century long calculations – for example, the simulations of Pine Island Glacier described by Joughin (2010), Favier (2014) and Seroussi (2014) – need only consider single ice streams, and can take advantage of the relatively little grounding line migration likely to occur to limit fine resolution to a region close to the present-day grounding line. As integration times grow the grounding line tends to sweep out a larger area – meaning that the region of fine resolution must either cover that growing area or evolve with it. At the same time, neighbouring ice streams may merge, so that they can no longer be treated separately. Ultimately, it becomes necessary to carry out simulations of the whole of Antarctica, potentially applying fine resolution everywhere. We present 1000 year simulations of the whole Antarctic response to simplified ocean forcing using the BISICLES ice-sheet model. Some of the simulations feature the complete collapse of the West Antarctic ice sheet, and we are able to use time-evolving adaptive mesh refinement to track the grounding line and reduce the computational complexity by orders of magnitude. We compare the size of pure numerical errors, caused by spatial and temporal under-resolution, with the differences due to approximation made in the model physics, and estimate an upper bound on the speed of West Antarctic collapse.

&
73A1935
Committed near-future retreat of Smith, Pope and Kohler Glaciers inferred from transient model calibration
Daniel Goldberg, Patrick Heimbach
Corresponding author: Daniel Goldberg
Corresponding author e-mail: dngoldberg@gmail.com
A glacial flow model is used to investigate near-future thinning and and grounding line retreat of Pope, Smith, and Kohler Glaciers, West Antarctica. The model is calibrated against observations to infer unknown parameters. We investigate two methods of calibration: the more commonly used ‘snapshot calibration’, which does not consider time dependence and assumes all observations are contemporaneous; and ‘transient calibration’, which accounts for the transient nature of observations. The transiently calibrated model achieves good agreement with time-dependent observations of surface elevation and velocity from 2001 to 2011, while snapshot calibration is unable to reproduce transient observed behaviour – although the poor fit of the snapshot-calibrated model is not apparent when examining an areally integrated metric such as total 2001–2011 sea-level contribution from the region. The models are then run from 2011 to 2041 with no additional forcings. The transiently calibrated model predicts near-steady grounded ice loss of 22.5 km3 a–1 over this period, while the snapshot-calibrated figure, while still large, is nearly 50% less. Moreover the transiently calibrated model reproduces past grounding line retreat and predicts further retreat, while the snapshot-calibrated model does neither. This demonstrates the need for ice models to be able to reproduce time-dependent observational histories in order to make more accurate predictions of near-future ice-stream behavior. Still, these results – along with additional sensitivity studies – suggest that this region will continue to have significant sea-level contributions over the next several decades, regardless of external forcings or uncertainties in unknown parameters.
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sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #439 on: March 30, 2016, 05:32:52 AM »
yes, i was waiting for someone to come up with the approach in the Cornford Igsoc abstract.

" ... we are able to use time-evolving adaptive mesh refinement to track the grounding line and reduce the computational complexity by orders of magnitude ... "

that may be good enough without comoving mesh.

And i note the sentence in the Sergienko abstract
 
"We also find that simulations accounting for the thermodynamic coupling of the Antarctic ice shelves produce consistently thicker sea ice compared with the uncoupled simulations"

Another piece falls into place.

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Re: Potential Collapse Scenario for the WAIS
« Reply #440 on: March 30, 2016, 08:50:43 PM »
Per the linked (open access) reference Antarctica could contribute 1.05 +/-0.3 m of SLR by 2100 (see attached image):

Robert M. DeConto & David Pollard (31 March 2016), "Contribution of Antarctica to past and future sea-level rise", Nature, Volume: 531, Pages: 591–597, doi:10.1038/nature17145

http://www.nature.com/articles/nature17145.epdf?referrer_access_token=px-zRubs4M6aBBPl42_1GdRgN0jAjWel9jnR3ZoTv0M-pvJMg7VLINRa2mnTNsvXfjbAFNU4M9sSVFBNmnefzinIWT5DIW6fVmmjzqPkWPG0EWAexculA_Dh1H0gVAzIYAUjdsj8uznmBvFk8_blNOM5-opyiSaKMyaJis4af48A0kgec2kZ8QcJLEQ0CKHzo1BxzQZ7aHlC6ggm5qLKPX8C4yz0OZ4SKpsmFZlbgUA%3D&tracking_referrer=www.nature.com

Abstract: "Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6–9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics—including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs—that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years."


See also:
Jeff Tollefson (31 March 2016), "Antarctic model raises prospect of unstoppable ice collapse", Nature, Volume: 531, Pages: 562, doi:10.1038/531562a


http://www.nature.com/news/antarctic-model-raises-prospect-of-unstoppable-ice-collapse-1.19638


Extract: "Sea levels could rise by more than 15 metres by 2500 if greenhouse-gas emissions continue to grow."
« Last Edit: March 30, 2016, 11:16:03 PM by AbruptSLR »
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sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #441 on: March 30, 2016, 10:33:58 PM »
Nice, thorough analysis by DeConto and Pollard, among other things, vindicating Mercer's insistence on the importance of the 0C midsummer isotherm. Unfortunately they did not cover the effect of the melt and berg effux freshening the upper waters of the southern ocean, which would have been a nice contact point to the latest Hansen.

But they sound as though they might do that soon.

"In particular, the model lacks two-way coupling between the ice sheet and the ocean. This is especially relevant for RCP8.5, in which >1 Sv of freshwater and icebergs would be supplied to the Southern Ocean during peak retreat (Extended Data Fig. 8 ). Rapid calving and ice-margin collapse also implies ice mélange in restricted embayments that could provide buttressing and a negative feedback on retreat. The loss of ice mass would also have a strong effect on relative sea level at the margin owing to gravitational and solid-earth deformation effects [48], which could affect MISI and MICI dynamics because of their strong dependency on bathymetry. Future simulations should include coupling with Earth models that account for these processes."
« Last Edit: March 30, 2016, 10:45:00 PM by sidd »

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #442 on: March 30, 2016, 11:10:27 PM »
Nice, thorough analysis by DeConto and Pollard, among other things, vindicating Mercer's insistence on the importance of the 0C midsummer isotherm. Unfortunately they did not cover the effect of the melt and berg effux freshening the upper waters of the southern ocean, which would have been a nice contact point to the latest Hansen.

But they sound as though they might do that soon.

"In particular, the model lacks two-way coupling between the ice sheet and the ocean. This is especially relevant for RCP8.5, in which >1 Sv of freshwater and icebergs would be supplied to the Southern Ocean during peak retreat (Extended Data Fig. 8 ). Rapid calving and ice-margin collapse also implies ice mélange in restricted embayments that could provide buttressing and a negative feedback on retreat. The loss of ice mass would also have a strong effect on relative sea level at the margin owing to gravitational and solid-earth deformation effects [48], which could affect MISI and MICI dynamics because of their strong dependency on bathymetry. Future simulations should include coupling with Earth models that account for these processes."

I do not know whether the doted line in the attached Figure 8 from the extended data gives a hint about the impact of including ice-climate feedback.
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sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #443 on: March 31, 2016, 08:06:52 AM »
DeConto has 1Sverdrup peak in 2100 from your attachment, 60mm/yr SLR for a hundred years from fig 4c. Hansen has 1 Sverdrup in 2040 (fig S14) in the north atlantic, and the caption sez " Freshwater injection onto the Southern Ocean is double the North Atlantic rate" with a 10 year double time. So he probably has 1Sverdrup from Antarctica right now,  and an ocean model exhibiting melt layer feedback, which deConto lacks.

So say doubling time in Hansen ought to be 40 yr to give a rough match in magnitude of 1Sv in 2100, then deConto and hansen are not so far apart for the 2050-2100 period. Or say, when deConto puts in the melt layer feedback, he will get the same doomsday projections as Hansen.

They are both working on the next iteration, we shall see.

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Re: Potential Collapse Scenario for the WAIS
« Reply #444 on: March 31, 2016, 04:03:22 PM »
DeConto has 1Sverdrup peak in 2100 from your attachment, 60mm/yr SLR for a hundred years from fig 4c. Hansen has 1 Sverdrup in 2040 (fig S14) in the north atlantic, and the caption sez " Freshwater injection onto the Southern Ocean is double the North Atlantic rate" with a 10 year double time. So he probably has 1Sverdrup from Antarctica right now,  and an ocean model exhibiting melt layer feedback, which deConto lacks.

So say doubling time in Hansen ought to be 40 yr to give a rough match in magnitude of 1Sv in 2100, then deConto and hansen are not so far apart for the 2050-2100 period. Or say, when deConto puts in the melt layer feedback, he will get the same doomsday projections as Hansen.

They are both working on the next iteration, we shall see.

sidd,

Thanks for your comments & observations, and I concur that all of this good work is still just a work in progress and as you correctly point-out that DeConto & Pollard (2016) appear to be converging towards Hansen et al (2016), I will make more pointed comments in the Hansen et al thread.  Nevertheless, I point-out here that very few people make any effort to understand the numerous complexities of abrupt sea level rise, as they assume that leading scientists on the IPCC AR5 (like Richard Alley) had their backs to protect them from unexpected surprises.  Unfortunately, as the attached image from the AR5 Ch 13 (sea level) Supplement shows that the new preliminary findings of DeConto & Pollard (2016) much more than double the RCP 8.5 GMSL projections for 2100, without accounting for the risk that the GIS contributions will likely exceed the AR5 projections.

It is my opinion that AR5 WG1 authors (like Richard Alley) should be held accountable for their "happy talk" and should be replaced by people like Eric Rignot for AR6.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

sidd

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Re: Potential Collapse Scenario for the WAIS
« Reply #445 on: April 01, 2016, 01:27:05 AM »
I should read more carefully. Hansen has 720 GT/yr (2mm/yr SLR equivalent) meltwater injection from Antarctica in 2011,

"We take freshwater injection to be 720 Gt/year from Antarctica and 360 Gt/year in the North Atlantic in 2011, with injection rates at earlier and later times defined by assumption of a 10-year doubling time."

Not so unreasonable when one recalls this is total melt, not mass waste. 1Sv is about 3e4 GT/yr, which a doubling time of 10 yr attains shortly after 2060. Hansen quits injection in 2060.

"Cumulative North Atlantic freshwater forcing in sverdrup years (Sv years) is 0.2 Sv years in 2014, 2.4 Sv years in 2050, and 3.4 Sv years (its maximum) prior to 2060 (Fig. S14)."

Looking at the fearsome fig 4c and extended data fig 8 in DeConto gives 1Sv/yr for fifty years past 2100, accompanied by 50mm/yr SLR for a hundred. So even worse than Hansen, just delayed 50 yr later. So I feel that when they put the meltwater feedback in, their estimates will approach Hansen.

I keep coming back to Blanchon (2009) doi:10.1038/nature07933 supplementary fig 3. If you take the the integral of the SLR curve from 4c in DeConto and integrate it to find the cumulative sea level change, you will get a sharp rise in the beginning in the cumulative graph, which comes from the inital peak in fig 4c, and then a slowing  rate _exactly as in one of the fast jumps in Blanchon record_

I fear more and more that we are in the exponential blowout phase at Thwaites, that we have decades at best to retreat from the ocean. It is much easier to relocate when SLR is 3 mm/yr than when it is 10, 30 or 60.

Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #446 on: April 03, 2016, 08:33:02 AM »
Ted Scambos interviewed by Peter Sinclair on DeConto & Pollard:
http://climatecrocks.com/2016/04/02/the-weekend-wonk-what-this-weeks-antarctic-study-means/

AbruptSLR

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Re: Potential Collapse Scenario for the WAIS
« Reply #447 on: April 04, 2016, 02:01:41 AM »
The linked Phys Org website has a nice video of the man of the moment, David Pollard:

http://phys.org/news/2016-03-sea-level-earlier-years.html

Extract: ""So, at a time in the past when global average temperatures were only slightly warmer than today," says DeConto, "sea levels were much higher. Melting of the smaller Greenland Ice Sheet can only explain a fraction of this sea-level rise, most which must have been caused by retreat on Antarctica."

To investigate this, DeConto and Pollard developed a new ice sheet-climate model that includes "previously under-appreciated processes" that emphasize the importance of future atmospheric warming around Antarctica."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #448 on: April 04, 2016, 07:24:33 PM »
Nice to hear/see Pollard talking, as he and DeConto aren't exactly all over youtube...

Lennart van der Linde

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Re: Potential Collapse Scenario for the WAIS
« Reply #449 on: April 04, 2016, 07:27:49 PM »
Here's one with DeConto, from Jan 13th this year: