Marine ice sheet instability

Introduction | Past evidence of ice sheet collapse | Hypothesis of marine ice sheet instability | References | Comments |

Introduction

Images of the Amundsen Sea Embayment, showing: Landsat image (LIMA); BEDMAP bed elevation (from Lythe et al., 2001); and ice velocity (from Rignot et al. 2011)

In 1978, Mercer was one of the first to identify that rising temperatures could have catastrophic consequences in West Antarctica, triggering a collapse of the West Antarctic Ice Sheet[1]. This is because much of the West Antarctic Ice Sheet lies below sea level[2], making it a Marine Ice Sheet. West Antarctica is currently the world’s largest marine ice sheet, although they may have been common during the Last Glaciation, circa 18,000 years ago. Portions of the Greenland Ice Sheet and East Antarctic Ice Sheet are also marine, but have shallower bathymetries than West Antarctica. The ice sheet is currently stable due to its buttressing ice shelves and local regions where the bathymetry opposes the general trend[3].

The figure panel opposite shows the Pine Island Glacier and Twaites ice streams, which are grounded well below sea level and drain a large proportion of West Antarctica. Their accumulation areas flow from the Transantarctic Mountains and out into the Amundsen Sea. The map below, from the BEDMAP2 database, shows ice sheet thicknesses and a cross section across the entire Antarctic continent. Here, you can clearly see the difference between the West and East Antarctic ice sheets. They are separated by the 2000 m high Transantarctic Mountains. The East Antarctic Ice Sheet is grounded largely above sea level, whereas the West Antarctic Ice Sheet is mostly grounded well below sea level.

The BEDMAP 2 dataset shows how ice thickness across the Antarctic continent is variable, with thin ice over the mountains and thick ice over East Antarctica. The cross section shows how the West Antarctic Ice Sheet is grounded below sea level.

The BEDMAP 2 dataset (Fretwell et al. 2013) shows how ice thickness across the Antarctic continent is variable, with thin ice over the mountains and thick ice over East Antarctica. The cross section shows how the West Antarctic Ice Sheet is grounded below sea level.

The figures below show how, firstly, the West Antarctic Ice Sheet is grounded below sea level, and that both the West and East Antarctic Ice sheet have water (lakes and channels) at their base; secondly, bedrock topography of Antarctica; thirdly, ice streams of Antarctica, and fourthly, what the Antarctic continent would look like if all the ice were to be removed. Note how West Antarctica becomes a series of islands.

Past evidence of ice sheet collapse

Profile through the Antarctic ice sheet (A) Bellingshausen Sea – West Antarctic ice sheet – Ross ice shelf – Ross Sea (B). The profile shows that most of the West Antarctic ice sheet is grounded below sea level which makes it sensitive to sea level rise. If the contact of the ice to the bottom rocks is lost seaward of the grounding line, the ice sheet becomes significantly thinner (some 100 m), forming a shelf ice.
By Hannes Grobe 21:51, 12 August 2006 (UTC), Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (Own work) [CC-BY-SA-2.5 (www.creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons.

There is some evidence to suggest that, in previous interglacials, the West Antarctic Ice Sheet completely disappeared, leading to sea levels about 5m higher than at present[1].  For example, marine micro-organisms have been found in in glacial sediments at the base of ice cores beneath Ice-Stream B[4]. This occurred during a period of anomalous warmth during MIS 5e in East Antarctica. Evidence from bryozoan and other marine micro-organisms indicates open seaways across West Antarctica at various periods during the last few million years, and even during the past one or more interglacials[3].

 

Hypothesis of marine ice sheet instability

Much of West Antarctica drains through the Pine Island Glacier and Thwaites ice streams into Pine Island Bay. These ice shelves are warmed from below by Circumpolar Deep Water[5], which has resulted in system imbalances, more intense melting, glacier acceleration and drainage basin drawdown[6-8]. This is the “Weak Underbelly” of the West Antarctic Ice Sheet[9], which may be prone to collapse. Pine Island Glacier is currently thinning[10], and, combined with rapid basal melting of the Amundsen Sea ice shelves[11], means that there is concern for the future viability of its fringing ice shelves.

Marine Ice Sheet instability hypothesis flow chart

The Marine Ice Sheet Instability hypothesis is that atmospheric and oceanic warming could result in increased melting and recession at the grounding line on a reverse slope gradient[12]. This would result in the glacier becoming grounded in deeper water and a greater ice thickness. This is because the grounding line in this region has a reverse-bed gradient, becoming deeper inland.  Stable grounding lines cannot be located on upward-sloping portions of seafloor[13]. Ice thickness at the grounding line is a key factor in controlling flux across the grounding line[3], so thicker ice grounded in deeper water would result in floatation, basal melting, increased iceberg production, and further retreat within a positive feedback loop. This would result in a rapid melting of the West Antarctic Ice Sheet, triggering rapid sea level rise.

 

Simplified cartoon of a tributary glacier feeding into an ice shelf, showing the grounding line (where the glacier begins to float).

This could be exacerbated by the removal of fringing ice shelves around the Amundsen Sea sector of the West Antarctic Ice Sheet. Removal of buttressing ice shelves around ice streams tends to result in glacier acceleration, thinning, and grounding line migration[14, 15].

This is a low-probability, high-magnitude event, with a 5% probability of the West Antarctic Ice Sheet contributing 10 mm sea level rise per year within 200 years[16]. The most recent numerical models predict a sea level rise of 3.3 m if this event was to occur[12].

This hypothesis has recently featured prominently in the science news, for example, on the Discovery News.

Further reading

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References


1.            Mercer, J.H., 1978. West Antarctic Ice Sheet and CO2 Greenhouse effect – threat of disaster. Nature, 1978. 271(5643): p. 321-325.

2.            Lythe, M.B., Vaughan, D.G., and the BEDMAP Consortium. 2001. BEDMAP: a new ice thickness and subglacial topographical model of Antarctica. Journal of Geophysical Research, 2001. 106(B6): p. 11335-11351.

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

4.            Scherer, R.P., Aldahan, A., Tulaczyk, S., Possnert, G., Engelhardt, H., and Kamb, B., 1998. Pleistocene Collapse of the West Antarctic Ice Sheet. Science, 1998. 281(5373): p. 82-85.

5.            Jacobs, S.S., Jenkins, A., Giulivi, C.F., and Dutrieux, P., 2011. Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nature Geoscience, 2011. 4(8): p. 519-523.

6.            Shepherd, A., Wingham, D., and Rignot, E., 2004. Warm ocean is eroding West Antarctic Ice Sheet. Geophysical Research Letters, 2004. 31(23): p. L23402.

7.            Shepherd, A., Wingham, D.J., Mansley, J.A.D., and Corr, H.F.J., 2001. Inland thinning of Pine Island Glacier, West Antarctica. Science, 2001. 291: p. 862-864.

8.            Wingham, D.J., Wallis, D.W., and Shepherd, A., 2009. Spatial and temporal evolution of Pine Island Glacier thinning, 1995-2006. Geophysical Research Letters, 2009. 35: p. L17501.

9.            Hughes, T.J., 1981. The weak underbelly of the West Antarctic Ice Sheet. Journal of Glaciology, 1981. 27: p. 518-525.

10.          Pritchard, H.D., Arthern, R.J., Vaughan, D.G., and Edwards, L.A., 2009. Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature, 2009. 461(7266): p. 971-975.

11.          Pritchard, H.D., Ligtenberg, S.R.M., Fricker, H.A., Vaughan, D.G., van den Broeke, M.R., and Padman, L., 2012. Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature, 2012. 484(7395): p. 502-505.

12.          Bamber, J.L., Riva, R.E.M., Vermeersen, B.L.A., and Le Brocq, A.M., 2009. Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet. Science, 2009. 324(5929): p. 901-903.

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

14.          Scambos, T.A., Bohlander, J.A., Shuman, C.A., and Skvarca, P., 2004. Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters, 2004. 31: p. L18402.

15.          De Angelis, H. and Skvarca, P., 2003. Glacier surge after ice shelf collapse. Science, 2003. 299: p. 1560-1562.

16.          Vaughan, D.G. and Spouge, J.R., 2002. Risk estimation of collapse of the West Antarctic Ice Sheet. Climatic Change, 2002. 52: p. 65-91.

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9 thoughts on “Marine ice sheet instability

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  4. Hi, In your chart, Marine Ice Sheet instability hypothesis flow chart, you show a positive feedback loop which you have left as continuous without any output (of calving). Why is that? Surely the chart needs to show an output from the “increased calving” box. Without it, it seems incomplete.
    Chris.

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