Is the West Antarctic Ice Sheet collapsing?

Marine ice sheet instability

The West Antarctic Ice Sheet (WAIS) is the world’s most vulnerable ice sheet. This is because it is grounded below sea level, and marine ice sheets such as these are susceptible to rapid melting at their base. Fast-flowing ice streams draining the WAIS (Pine Island Glacier and Thwaites Glacier in particular) into the Amundsen Sea have a grounding line on a reverse bed slope, becoming deeper inland. Recession of the grounding line means that the ice stream is grounded in deeper water, with a greater ice thickness. Stable grounding lines cannot be established on these reverse bed slopes1, because ice thickness is a key factor in controlling ice flux across the grounding line. Thicker ice in deeper water drives increased calving, increased ice discharge, and further grounding-line recession in a positive feedback loop2, 3.  This process is called Marine Ice Sheet Instability4.

Ice streams of Antarctica with Pine Island Glacier and Thwaites glacier highlighted.

Ice streams of Antarctica with Pine Island Glacier and Thwaites glacier highlighted.

Ice shelf collapse

The situation is exacerbated by ice shelf collapse. Floating ice shelves5 provide back stress, supporting the ice streams6, 7. If they thin and melt away, this support is removed, leading to increase ice discharge, accelerated flow, grounding line recession8, 9, 10, and marine ice sheet instability. These ice shelves are being thinned from below as they are warmed by Circumpolar Deep Water11, which is being increasingly transported onto the continental shelf12, 13, 14. Glacial troughs, excavated throughout maximum glaciations over the last 2 million years, provide a major route for Circumpolar Deep Water to access the ice shelves11. Pine Island Glacier’s ice shelf in particular is currently accelerating, thinning and receding13, 15.

Warm Circumpolar Deep Water is penetrating beneath Pine Island Glacier's ice shelf

Warm Circumpolar Deep Water is penetrating beneath Pine Island Glacier’s ice shelf

Scientists fear that the large ice streams draining the WAIS into the Amundsen Sea, particularly Pine Island Glacier and Thwaites Glacier, could be becoming unstable. But what is the risk of this actually happening, and what does it mean in terms of sea level rise?

NASA’s DC-8 flies across the crack forming across the Pine Island Glacier ice shelf on Oct. 26, 2011. The ice shelf is in the midst of a natural process of calving a large iceberg, which it hasn’t done since 2001. Credit: Jefferson Beck/NASA

NASA’s DC-8 flies across the crack forming across the Pine Island Glacier ice shelf on Oct. 26, 2011. The ice shelf is in the midst of a natural process of calving a large iceberg, which it hasn’t done since 2001. Credit: Jefferson Beck/NASA

Observations of marine ice sheet instability

groundingline

Schematic cartoon of a glacier flowing into an ice shelf, showing the grounding line and calving at the ice cliff at the edge of the ice shelf.

A recent study by Mouginot and colleagues detected acceleration and increased ice discharge. Flow speeds have increased dramatically since 1973, and much of this increase occurred between 2009-201316. The large acceleration in ice velocity and ice discharge in Pine Island Glacier from 2002-2008 coincided with rapid recession of its grounding line. This speed-up is spreading far inland, and it indicates a strong link between glacier recession and the observed ice-shelf melting. The authors herald this as the start of marine ice sheet instability. They state that these changes are a possible sign of a progressive collapse of these ice streams, in response to the high melting of their buttressing ice shelves.

Predicting future ice stream behaviour

A second study published this week by Ian Joughin and colleagues looks at the future of the WAIS. They use computer models to investigate the sensitivity of Thwaites Glacier to ocean melt, and to determine whether the current recession is unstable17. They found that early-stage collapse had already begun, and that the ice stream exhibited threshold behaviour. Once melt passed 1 mm per year, rapid collapse (within decades) occurred as the grounding line reached the deepest parts of the marine basin (for reference, total global sea level rise today is ~3 mm per year, so this is a significant contribution!). The highest-melt simulations pass the threshold of 1 mm per year within 250 years. For all but the very lowest melt simulations, this critical threshold was passed within 1000 years.

Observed ice losses from 1995-2013 fall within the range predicted under the highest melt scenarios for this modelling experiment. The early stages of the highest-melt simulations therefore reasonably approximate present conditions. These simulations provide strong evidence that the process of marine-ice sheet instability is already underway on Thwaites Glacier, largely due to the observed high sub-ice shelf melt rates. Rapid losses of 1 mm per year will ensue once the grounding line reaches the basin’s deepest parts, which could occur within two centuries. Once this threshold is passed, rapid ice sheet collapse could occur, which would spill over into other basins and perhaps spell an end for the West Antarctic Ice Sheet. This process is difficult to accurately model, but rapid ice sheet collapse would certainly result in dramatically higher rates of sea level rise once this critical threshold is passed.

This numerical model suggests that a full-scale collapse of this sector of the West Antarctic Ice Sheet may be inevitable. But it leaves a large uncertainty in the timing; more complex coupled ice-sheet and climate models are needed to model this more thoroughly in the future.

Two studies, one result

Thus two completely independent studies, using completely different methods, both find that the early stages of marine ice sheet instability may be underway in West Antarctica. Unless the Circumpolar Deep Water recedes enough to reduce melt well below today’s levels, we may be seeing an irreversible process that could result in dramatic ice stream recession. Given that the West Antarctic Ice Sheet has a total sea level equivalent of 3.3 m1,  with 1.5 m from Pine Island Glacier alone4, marine ice sheet collapse could be a significant challenge for future generations, with major changes in rates of sea level rise being possible within just the next couple of hundred years.

References

1.            Bamber JL, Riva REM, Vermeersen BLA, Le Brocq AM. Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet. Science 2009, 324(5929): 901-903.

 

2.            Schoof C. Ice sheet grounding line dynamics: Steady states, stability, and hysteresis. Journal of Geophysical Research-Earth Surface 2007, 112(F3).

 

3.            Joughin I, Alley RB. Stability of the West Antarctic ice sheet in a warming world. Nature Geosci 2011, 4(8): 506-513.

 

4.            Vaughan DG. West Antarctic Ice Sheet collapse – the fall and rise of a paradigm. Climatic Change 2008, 91(1-2): 65-79.

 

5.            Rignot E, Jacobs S, Mouginot J, Scheuchl B. Ice Shelf Melting Around Antarctica. Science 2013.

 

6.            Dupont TK, Alley RB. Role of small ice shelves in sea-level rise. Geophys Res Lett 2006, 33(9): L09503.

 

7.            Dupont T, Alley R. Assessment of the importance of ice‐shelf buttressing to ice-sheet flow. Geophysical Research Letters 2005, 32(4).

 

8.            Berthier E, Scambos T, Schuman CA. Mass loss of Larsen B tributary glaciers (Antarctic Peninsula) unabated since 2002. Geophysical Research Letters 2012, 39: L13501.

 

9.            Scambos T, Fricker HA, Liu C-C, Bohlander J, Fastook J, Sargent A, et al. Ice shelf disintegration by plate bending and hydro-fracture: Satellite observations and model results of the 2008 Wilkins ice shelf break-ups. Earth and Planetary Science Letters 2009, 280(1-4): 51-60.

 

10.          Scambos TA, Bohlander JA, Shuman CA, Skvarca P. Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters 2004, 31: L18402.

 

11.          Walker DP, Brandon MA, Jenkins A, Allen JT, Dowdeswell JA, Evans J. Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough. Geophysical Research Letters 2007, 34(2).

 

12.          Pritchard HD, Ligtenberg SRM, Fricker HA, Vaughan DG, van den Broeke MR, Padman L. Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature 2012, 484(7395): 502-505.

 

13.          Jacobs SS, Jenkins A, Giulivi CF, Dutrieux P. Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nature Geoscience 2011, 4(8): 519-523.

 

14.          Thoma M, Jenkins A, Holland D, Jacobs S. Modelling circumpolar deep water intrusions on the Amundsen Sea continental shelf, Antarctica. Geophysical Research Letters 2008, 35(18).

 

15.          Jenkins A, Dutrieux P, Jacobs SS, McPhail SD, Perrett JR, Webb AT, et al. Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat. Nature Geoscience 2010, 3(7): 468-472.

 

16.          Mouginot J, Rignot E, Scheuchl B. Sustained increase in ice discharge from the Amundsen Sea Embayment, West Antarctica, from 1973 to 2013. Geophysical Research Letters 2014: 2013GL059069.

 

17.          Joughin I, Smith BE, Medley B. Marine Ice Sheet Collapse Potentially Underway for the Thwaites Glacier Basin, West Antarctica. Science 2014.

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14 thoughts on “Is the West Antarctic Ice Sheet collapsing?

  1. Your reference list does not do justice to the early work that identified this issue of potential marine ice instability that could lead to collapse. These studies pointed the way to investigating the Pine Island Glacier 30 years ago. Hughes (1973 and 1975) and Thomas (1979) for example. This also shows we have understood the general glacier dynamics for quite sometime and were in the right place to observe the beginning of the collapse.
    Hughes, T.]. 1973. Is the West Antarctic ice sheet disintegrating? Journal of Geophysical Research, Vol. 78, No. 33, p. 7884-910
    Hughes, T. ]. 1975. The West Antarctic ice sheet: instability, disintegration, and initiation of ice ages. Reviews of Geophysics and Space Physics, Vol. 13, No. 4, p. 502-526
    http://www.igsoc.org/journal.old/24/90/igs_journal_vol24_issue090_pg167-177.pdf

  2. I’ve been trying to find an explanation for why a glacier that sits in a bowl should become *more* unstable as it melts. To the rank amateur (me) the mental image makes no sense. Presumably, even if it’s all water, it’ll just sit in the bowl.

    This is the first site I’ve come across that actually explains how the process works. And that process is truly terrifying. But thanks for the clarity anyway!

  3. Mouginot? That was slightly confusing at first glance. Rignot is the lead and corresponding author.

    Nice post overall, but your use of conditional language seems hard to justify given the findings of the two papers. Neither lead author has been so hesitant about the irreversibility.

    OTOH without the conditionals Revkin might not have linked to you. :/

    “Thicker ice in deeper water drives increased calving, increased ice discharge, and further grounding-line recession in a positive feedback loop.” As a general matter, yes, but AIUI the increasing height (depth) of the ice face is the key factor for accelerating retreat of these glaciers since it creates more surface area for the warm water to work on.

    • Mouginot is the lead author so it is appropriate to cite him in the post.

      I think I will need to write a longer post defending my ‘reticent’ ‘conditional’ language!

      • Thanks, Bethan, I look forward to that.

        Re the paper, Rignot was lead on the one “published this week” that’s been getting all the attention. Mouginot was lead on a related paper published (also in GRL) a couple months ago. I haven’t read either yet, but the potential for conflating them is obvious.

        • And of course it’s the latter that’s in your references, which I should have looked at before. But didn’t the new paper add something worth mentioning?

  4. If the grounding line retreat is irreversible, how did the grounding line advance as far as it did in the first place?

    • Marine ice sheet instability is a classic example of hysteresis. A positive feedback cycle is initiated, which leads to a number of changes that reinforce the intial perturbation. In this case, incursions of circumpolar deep water onto the continental shelf are melting the ice stream at its base and encouraging grounding line recession, which results in a positive feedback loop and further grounding line recession. However, a change in ocean circulation could result in these currents not flowing so far onto the continental shelf. This melt may then reduce and the grounding line recession may halt. This is noted in the published paper.

  5. I suppose that as the under side of the glacier melts, the resulting fresh water mixes with the adjacent salt water, making it lighter and causing it to flow upwards along the under side of the ice. This will pull new warm denser water in under the ice to melt even more.

  6. The melt rate is in a sense analogous to an air lift. In an Air lift, air is introduced into a pipe, making the column of water in the pipe less dense than the water outside the pipe. This density difference powers the water up the pipe and the deeper the mouth of the pipe, the greater suction such a pipe can exert. Below the West Antarctic ice sheet, as the ice melts it produces fresh water which mixes with the adjacent sea water. This mix is lighter than the surrounding water and flows up the slope. This sucks in more water from the ocean which in turn is freshened and flows up the slope. Since there is a reverse slope as you go landward, the grounding line gets deeper and deeper. The deeper it is, the stronger the current and the faster the melt. The collapse rate should accelerate. Incidentally, this may explain part of the increase of Antarctic floating ice. The fresher water on the surface freezes more easily, increasing ice extent. This very increase may signal the acceleration of the collapse of the West Antarctic ice shelf.

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