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 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.
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?
Observations of marine ice sheet instability
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.
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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.
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5. Rignot E, Jacobs S, Mouginot J, Scheuchl B. Ice Shelf Melting Around Antarctica. Science 2013.
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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.