Antarctic supraglacial lakes and ice-shelf collapse

What are supraglacial lakes?

Supraglacial lakes form on the surface of glaciers. Meltwater accumulates in depressions or hollows on the ice surface. We think of the East Antarctic Ice Sheet as being very cold and dry, with temperatures well below 0°C, but in some places, surface melting does occur in the summer melt season. In Antarctica, this occurs primarily from November to February1. Supraglacial lakes are critically important for the stability of ice sheets, glaciers and ice shelves, and can help drive ice-shelf collapse.

Settings of glacial lakes. Lakes can form in-front of glaciers and ice sheets (proglacial lakes), on top of the ice (supraglacial lakes), within the ice (englacial) and beneath glaciers and ice sheets (subglacial lakes).

Meltwater accumulates on the glacier or ice-shelf surface when firn (snow that has not yet been compressed into ice) becomes sufficiently saturated and can no longer absorb meltwater2,3.

Supraglacial lakes commonly refreeze at the end of the melt season, but some have been observed to drain into the ice6,14,16.

meltwater lakes
Supraglacial meltwater lakes on McMurdo Ice Shelf. Credit: Neil Glasser.

Why are supraglacial lakes important in Antarctica?

Supraglacial lakes are important because they can influence ice flow and mass balance in three ways. Firstly, because the ponded meltwater is darker than the surrounding white-grey ice, lakes have a lower albedo. More energy from the sun’s rays is absorbed rather than reflected,meaning the lakes can locally increase surface melting in a positive feedback effect4,5.

Secondly, supraglacial lakes can cause short-term ice flow accelerations when they drain through the ice sheet6. The meltwater can reach the bed of the ice sheet, and encourage basal sliding and thus faster ice flow.

Thirdly, supraglacial lakes have been linked to ice shelf collapse on the Antarctic Peninsula7,8. Ice shelves are the floating extensions of land ice; they receive ice from ice-flow from inland tributary glaciers and lose mass by calving icebergs. Ice shelf removal increases the flow of inland ice into the ocean and its contribution to sea-level rise9,10.

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.

It is therefore important to understand where supraglacial lakes form and how they influence the behaviour and stability of the ice sheet. We need these data so that we can identify which parts of Antarctica may be more sensitive to climate warming and how the Antarctic Ice Sheet may respond under a future warming climate11.

Landsat-8 satellite image of supraglacial lakes on the Amery Ice Shelf, East Antarctica, taken on 17th January 2019. Image size is 185 km x 180 km. Lakes (bright blue), exposed rock outcrops called nunataks (black), snow (white) and bare ice (pale blue) are visible. Image credit: U.S. Geological Survey.

Where are supraglacial lakes found in Antarctica?

Satellite remote sensing since the 1970s has allowed researchers to map supraglacial lakes over the whole Antarctic continent12,13. This has shown that thousands of supraglacial lakes are widespread around the margins of Antarctica during the summer melt season, including the periphery of the East Antarctic Ice Sheet12-16. A recent assessment found lakes up to 500 km inland, and at up to >1500 m above sea level13.

This is not a new phenomenon; satellite imagery records supraglacial lakes persistently forming annually for decades on Antarctic ice shelves. For example, lakes have been recorded on the Amery Ice Shelf in East Antarctica since at least 1974, where the largest recorded supraglacial lake (~80 km long) lake has been observed12,16-20.

Locations around Antarctic where supraglacial lakes have been observed together with examples: (a) Larsen C Ice Shelf, (b) George VI Ice Shelf, (c) Riiser-Larsen Ice Shelf, (d) Langhovde Glacier, (e) Ross Archipelago, (f) McMurdo Ice Shelf, (g) Sørsdal Glacier, (h) Mawson Glacier. Green shaded regions in the central map represent the number of published studies reporting supraglacial lakes in that location. Note that the number of studies reporting lakes in a given location does necessarily correspond to the number of lakes forming, or how long lakes have been present in this location. Lakes mapped in January 2017 in a recent East Antarctic assessment by Stokes et al. (2019) are shown in purple. Images reproduced from: Martin Truffer, University of Alaska Fairbanks (a), Frithjof C. Küpper, University of Aberdeen (b), Matti Leppäranta, University of Helsinki (c), Takehiro Fukuda, Hokkaido University (d), NASA Operation IceBridge (e), Chris Larsen, NASA Operation IceBridge (f), Sarah Thompson University of Tasmania (g), and Richard Stanaway, Australian National University (h). Credit: Arthur et al. (2020).

This interactive Google Map below allows you to explore supraglacial lakes on the floating ice tongue of Sørsdal Glacier, Princess Elizabeth Land, East Antarctica. You can clearly see meltwater filling depressions and crevasses on the ice surface and lakes being fed by meltwater from upstream.

What controls supraglacial lake distributions in Antarctica?

Most supraglacial lakes in Antarctica form on ice shelves, where the long, flat profile of the floating ice shelf and low surface elevations encourage meltwater ponding13. Supraglacial lakes typically cluster a few kilometres down-ice from the grounding line12,13,21,31.

The figure below shows how supraglacial lake distributions around the continent are controlled by interactions between local and regional wind patterns, ice surface topography and albedo21.

Distribution of supraglacial lakes in relation to wind scour zones, exposed rock, blue ice, near-surface wind speed, surface melt rate and ice shelf vulnerability to surface-melt-induced collapse.
Distribution of supraglacial lakes in relation to wind scour zones, exposed rock, blue ice, near-surface wind speed, surface melt rate and ice shelf vulnerability to surface-melt-induced collapse. Surface melt fluxes are derived from QuickScat scatterometer (2000–2009 average, Trusel et al., 2013) and include meltwater that could refreeze in the snow/firn. Ice shelf vulnerability is derived from data from the QuikSCAT satellite and represents the relative concentration of refrozen meltwater in the firn (Alley et al., 2018). The grounding line and coastline are represented by dotted and solid black lines respectively. Arrows represent near-surface wind speed vectors (Lenaerts et al., 2017; Luckman et al., 2014). Supraglacial lake distributions are reproduced from Stokes et al. (2019). On Larsen C Ice shelf (A), supraglacial lake formation is restricted to ice shelf inlets (blue stars), driven by föhn-enhanced melting. On Roi Baudouin Ice Shelf (B), lakes clustered at the grounding zone are associated with katabatic wind-enhanced melting. The co-occurrence of lakes with rock outcrops and wind-scoured blue ice is prominent on other East Antarctic ice shelves, such as Shackleton (C) and Amery (D). Credit: Arthur et al. (2020).

Antarctic Peninsula

On the Antarctic Peninsula, such as the Larsen C Ice Shelf, strong down-slope winds called föhn winds cause intense melting (>400 mm w.e. yr –1) and localised ponding, even in Antarctic winter22-26.

Fohn winds are important for warming ice shelves on the eastern Antarctic Peninsula.
Fohn winds are important for warming ice shelves on the eastern Antarctic Peninsula.
Overview map of the Antarctic Peninsula.
Overview map of the Antarctic Peninsula. Note the large floating ice shelves (labelled in blue). From Davies et al., 2012 (Quaternary Science Reviews).

East Antarctica

In coastal East Antarctica, supraglacial lakes are clustered near ice-shelf grounding lines, but are often absent further downstream towards calving fronts because higher snowfall here means surface meltwater can be absorbed into the porous snowpack before it can pond on the ice surface15.

Persistent katabatic winds induce vertical air mixing andcan scour snow surfaces to expose lower-albedo blue ice, which intensifies melting and ponding15. Lakes often form close to exposed rock, which has a low albedo and locally increases surface melting12.

Supraglacial lakes are sensitive to katabatic winds, which can cause intense short-lived snowmelt events31.

Katabatic winds (meaning “descending winds”) are ‘drainage winds’, carrying high-density cold air from a high elevation down a slope under the force of gravity. The near-surface wind field over Antarctica is dominated by katabatic winds. From: Wikipedia.

Links to ice-shelf collapse

Supraglacial lakes can flex and fracture ice shelves when they grow and drain7,8,27. A good example is the near-synchronous drainage of over 2750 lakes (up to 6.8 metres deep) on Larsen B Ice Shelf in the days before it collapsed in February-March 200227.

Lakes are thought to have drained through a process called hydrofracturing, where meltwater triggers fracturing by filling and expanding vertical crevasses7, 27. This is thought to have triggered a ‘chain reaction’ that caused widespread fracturing and lake drainage8.

Supraglacial lakes have been observed on other ice shelves before they collapsed or partially disintegrated, including Larsen A and Prince Gustav in 19957,28. They could therefore be important indicators of future ice shelf instability and collapse29,30.

Examples of supraglacial lakes on Antarctic Peninsula ice shelves prior to their disintegration or major calving
Examples of supraglacial lakes on Antarctic Peninsula ice shelves prior to their disintegration or major calving. Prince Gustav Ice Shelf (A), which disintegrated in 1995; Larsen B ice Shelf (B and C), which disintegrated in 2002; Larsen C Ice Shelf (D); George VI Ice Shelf (E); and Wilkins Ice Shelf (F), which underwent major calving in 1998 and 2008. Imagery credits: (A) Rott et al. (1996), (B) Glasser and Scambos (2008), (C) National Snow and Ice Data Center, (D) Luckman et al. (2014), (E) Labarbara and MacAyeal (2011), (F) Scambos et al. (2009). Figure credit: Arthur et al. (2020).

The likelihood of lakes triggering ice shelf collapse in other locations is lower on thicker ice shelves confined within narrow embayments, such as the George VI Ice Shelf (between the mainland Antarctic Peninsula and Alexander Island), or those that are ‘pinned’ (locally in contact with the bedrock). This is because ice-shelf flow here is stabilised and so the ice shelf can support large numbers of lakes on its surface.

Glaciological structures on George VI Ice Shelt, Antarctic Peninsula, illustrating the dominance of meltwater channels, longitudinal structures and rifts towards the south ice front. From Holt et al., 2013.

Observing Antarctic supraglacial lakes from space

The inaccessibility of lakes in Antarctica makes obtaining field measurements difficult, but satellite remote sensing can be used to map supraglacial lakes at sub-metre resolution on weekly timescales, and to measure seasonal changes in lake depth and volume (16,31).

Improvements in satellite remote sensing over time

The figure below shows how the ability to resolve individual supraglacial lakes has vastly improved since coarser sensors like AVHRR and MODIS, which were unable to distinguish these features.

Higher resolution (>10m) satellites such as Sentinel-2 and Worldview-3 can resolve lake bathymetry and surface features such as partial lake ice coverage.

Examples of the evolution of satellite image sensor resolution and detection of supraglacial lakes on Beaver Lake, adjacent to Amery Ice Shelf, East Antarctica. Panels moving from top left to bottom right represent satellite sensors in order of increasing spatial resolution. Smaller insets show the same supraglacial lake to demonstrate the improvement in detail. Imagery: Scambos et al. (1996); United States Geological Survey; Google, Maxar Technologies. Figure credit: Arthur et al. (2020).

Recent assessments have used imagery from multiple optical satellite sensors (such as Landsat-8 and Sentinel-2) to maximise the amount of cloud-free imagery for mapping supraglacial lake distributions at the continent scale12,13.

Measuring changes in supraglacial lakes

Changes in the extent, area and volume of individual supraglacial lakes can be tracked through multiple melt seasons using satellites with a short revisit time (time elapsed between observations of the same point on earth)16,31.

For example, the 16-day revisit of Landsat-8 and the 5-day revisit of the Sentinel-2A/B satellite constellation have been used to calculate seasonal fluctuations in supraglacial lake area, depth and volume through multiple melt seasons16,31. This is important for estimating the volume of meltwater stored in lakes on the surface of the Antarctic Ice Sheet and the potential for hydrofracturing (where the lake helps to break up an ice shelf).

Satellite imagery recently recorded an extreme, potentially unprecedented, melt event in January 2020 around Antarctica during unprecedented warm air temperatures, which caused widespread surface melting and lake growth.

This post draws from the review of recent progress made in understanding Antarctic supraglacial lakes using satellite remote sensing by Arthur et al. (2020) (ref. 35).

About the Author

This article was contributed by Jennifer Arthur from Durham University. Jennifer is a PhD candidate specialising in Antarctic surface hydrology and ice shelf dynamics. She’s interested in the distribution and seasonal evolution of supraglacial lakes and the potential influence of surface meltwater on outlet glacier and ice shelf stability. Jennifer’s PhD research uses satellite remote sensing to investigate this on the East Antarctic Ice Sheet. You can follow Jennifer on Twitter @AntarcticJenny and she is part of the @EGU_CR Cryosphere blog team.

Jennifer Arthur

References

1. Echelmeyer, K, Clarke, TS, Harrison, WD (1991) Surficial glaciology of Jakobshavns Isbræ, West Greenland: Part I. Surface morphology. Journal of Glaciology 37(127): 368–382.

2. Alley, KE, Scambos, TA, Anderson, RS, et al. (2018) Quantifying vulnerability of Antarctic ice shelves to hydrofracture using microwave scattering properties. Remote Sensing of Environment 210: 297–306.

3. Hubbard, B, Luckman, A, Ashmore, D, et al. (2016) Massive subsurface ice formed by refreezing of ice-shelf melt ponds. Nature Communications 7(11897): 1–6.

4. Lüthje, M, Feltham, DL, Taylor, PD, et al. (2006) Modeling the summertime evolution of sea-ice melt ponds. Journal of Geophysical Research: Oceans 111: C02001.

5. Tedesco, M, Luthje, M, Steffen, K, et al. (2012) Measurement and modeling of ablation of the bottom of supraglacial lakes in western Greenland. Geophysical Research Letters 39(2): 1–5.

6. Tuckett, PA, Ely, JC, Sole, AJ, et al. (2019) Rapid accelerations of Antarctic Peninsula outlet glaciers driven by surface melt. Nature Communications 10(4311): 1–8.

7. Scambos, T, Hulbe, CM, Fahnestock, M (2003) Climate-induced ice shelf disintegration in the Antarctic Peninsula. Paleobiology and Paleoenvironments of Eoscene Rocks Antarctic Research Series 76: 335–347.

8. Banwell, AF, MacAyeal, DR, Sergienko, OV (2013) Breakup of the Larsen B Ice Shelf triggered by chain reaction drainage of supraglacial lakes. Geophysical Research Letters 40(22): 5872–5876.

9. Scambos, TA, Bohlander, JA, Shuman, CA, et al. (2004) Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters 31(18): 2001–2004.

10. Rignot, E, Casassa, G, Gogineni, P, et al. (2004) Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf. Geophysical Research Letters 31, L18401.

11. Bell, RE, Banwell, AF, Trusel, LD, et al. (2018) Antarctic surface hydrology and impacts on ice-sheet mass balance. Nature Climate Change 8: 1044–1052.

12. Kingslake, J, Ely, JC, Das, I, et al. (2017) Widespread movement of meltwater onto and across Antarctic ice shelves. Nature 544(7650): 349–352.

13. Stokes, CR, Sanderson, JE, Miles, BWJ, et al. (2019) Widespread development of supraglacial lakes around the margin of the East Antarctic Ice Sheet. Scientific Reports 9(1): 13823.

14. Langley, ES, Leeson, AA, Stokes, CR, et al. (2016) Seasonal evolution of supraglacial lakes on an East Antarctic outlet glacier. Geophysical Research Letters 43(16): 8563–8571.

15. Lenaerts, JTM, Lhermitte, S, Drews, R, et al. (2017) Meltwater produced by wind-albedo interaction stored in an East Antarctic ice shelf. Nature Climate Change 7: 58–63.

16. Moussavi, MS, Pope, A, Halberstadt, ARW, et al. (2020) Antarctic supraglacial lake detection using Landsat 8 and Sentinel-2 imagery: Towards continental generation of lake volumes. Remote Sensing 12(1): 1–19.

17. Hambrey, MJ, Dowdeswell, JA (1994) Flow regime of the Lambert Glacier-Amery Ice Shelf system, Antarctica: Structural evidence from Landsat imagery. Annals of Glaciology 20: 401–406.

18. Mellor, M, McKinnon, G (1960) The Amery Ice Shelf and its hinterland. Polar Record 10(64): 30–34.

19. Phillips, HA (1998) Surface meltstreams on the Amery Ice Shelf, East Antarctica. Annals of Glaciology 27: 177–181.

20. Swithinbank, C (1988) Satellite image atlas of glaciers of the world: Antarctica. United States Geological Survey Professional Paper 1386-B 36(122): 122–124.

21. Arthur et al. (2020) Recent understanding of Antarctic supraglacial lakes using satellite remote sensing. Progress in Physical Geography: Earth and Environment. https://doi.org/10.1177/0309133320916114.

22. Datta, RT, Tedesco, M, Fettweis, X, et al. (2019) The effect of foehn-induced surface melt on firn evolution over the Northeast Antarctic Peninsula. Geophysical Research Letters 46: 1–10.

23. Luckman, A, Elvidge, A, Jansen, D, et al. (2014) Surface melt and ponding on Larsen C Ice Shelf and the impact of föhn winds. Antarctic Science 26(6): 625–635.

24. Kuipers Munneke, P, Luckman, AJ, Bevan, SL, et al. (2018) Intense winter surface melt on an Antarctic ice shelf. Geophysical Research Letters 45(15): 7615–7623.

25. Trusel, LD, Frey, KE, Das, SB, et al. (2013) Satellite-based estimates of Antarctic surface meltwater fluxes. Geophysical Research Letters 40(23): 6148–6153.

26. Wiesenekker, JM, Kuipers Munneke, P, van den Broeke, MR, et al. (2018) A multidecadal analysis of Föhn winds over Larsen C ice shelf from a combination of observations and modeling. Atmosphere 9(172): 1–13.

27. Glasser, NF, Scambos, TA (2008) A structural glaciological analysis of the 2002 Larsen B ice-shelf collapse. Journal of Glaciology 54(184): 3–16.

28. Rott, H, Skarvca, P, Nagler, T (1996) Rapid collapse of Northern Larsen Ice Shelf, Antarctica. Science 271(788): 788–792.

29. Bell, RE, Banwell, AF, Trusel, LD, et al. (2018) Antarctic surface hydrology and impacts on ice-sheet mass balance. Nature Climate Change 8: 1044–1052.

30. Robel, A, Banwell, AF (2019) A speed limit on ice shelf collapse through hydrofracture. Geophysical Research Letters 46(21): 12092–12100.

31. Arthur et al. (in review) Distribution and seasonal evolution of supraglacial lakes on Shackleton Ice Shelf, East Antarctica. The Cryosphere Discussions, https://doi.org/10.5194/tc-2020-101.

32. Labarbera, CH, MacAyeal, DR (2011) Traveling supraglacial lakes on George VI Ice Shelf, Antarctica. Geophysical Research Letters 38(24): L24501.

33. Scambos, T, Fricker, HA, Liu, C, et al. (2009) 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 280(1–4): 51–60.

34. Scambos, T, Bohlander, J, Raup, B (1996) Images of Antarctic Ice Shelves [Amery Ice Shelf]. Boulder, Colorado USA: National Snow and Ice Data Center. Available at: http://dx.doi.org/10.7265/N5NC5Z4N.

35. Arthur, J. F., Stokes, C., Jamieson, S. S. R., Carr, J. R., & Leeson, A. A. (2020). Recent understanding of Antarctic supraglacial lakes using satellite remote sensing. Progress in Physical Geography: Earth and Environment, 0309133320916114. https://doi.org/10.1177/0309133320916114

A note on ‘Collapse’

There is a lot in the media at the moment about the ‘collapse’ of the West Antarctic Ice Sheet. See my previous blog post for more information. But when we talk about ‘ice sheet collapse’, what do we actually mean? When we talk of people ‘collapsing’, they fall down right in front of us in the street. Buildings collapse. Bridges collapse. It’s a very bad thing. Right? Continue reading

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

Go to top or jump to Sea Level Rise.

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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Ice shelf collapse

What is an ice shelf? | Ice shelf collapse | Mechanisms of ice shelf collapse | Ice shelf buttressing | References | Comments

What is an ice shelf?

Larsen Ice Shelf in 2004

Ice shelves are floating tongues of ice that extend from grounded glaciers on land. Snow falls on glaciers, which flow downstream under gravity. Ice shelves are common around Antarctica, and the largest ones are the Ronne-Filchner, Ross and McMurdo Ice Shelves.

Ice shelves surround 75% of Antarctica’s coastline, and cover an area of over 1.561 million square kilometres (a similar size to the Greenland Ice Sheet). Ice shelves gain mass from ice flowing into them from glaciers onland, from snow accumulation, and from the freezing of marine ice (sea water) to their undersides[1]. They lose mass by calving icebergs, and basal melting towards their outer margins, along with sublimation and wind drift on their surfaces. Ice shelves are important, because they play a role in the stability of the Antarctic Ice Sheet and the ice sheet’s mass balance, and are important for ocean stratification and bottom water formation; this helps drive the world’s thermohaline circulation. Melting from beneath ice shelves is one of the key ways in which the Antarctic Ice Sheet loses mass[1].

In the satellite image of Prince Gustav Ice Shelf below, you can see that the ice shelves have a very flat appearance. In fact, you can normally tell where the ice starts to float by a sharp break in slope at the grounding line. Ice shelves are therefore composed of ice derived from snowfall on land, but they also accrete marine ice from below[2]. Ice shelves are therefore distinct from sea ice, which form solely from freezing marine water. You can see an example from northern Antarctic Peninsula below. Prince Gustav Ice Shelf was situated between Trinity Peninsula and James Ross Island. It collapsed in 1995. You can see glaciological structures on the ice shelf, indicating that it flows out from its tributary glaciers. You can also see abundant melt ponds on the ice shelf.

Ice shelves around Antarctica are up to 50,000 km2 in size, and can be up to 2000 m thick. Their front terminus is often up to 100 m high. Ice shelves intermittently calve large icebergs, which is a normal part of their ablation. Around Antarctica, ice shelves form where mean annual temperatures are less than -9°C, with sequential break up of ice shelves as temperatures increase[3-5]. The geometry of the coastline is often important for determining where ice shelves will develop. The Larsen Ice Shelf, for example, is formed in an embayment.

Ice shelf collapse

Several of the ice shelves around Antarctica have recently collapsed dramatically, rather than retreating in a slow and steady manner.  Larsen A collapsed in 1995[6], and Larsen B Ice Shelf famously collapsed in 2002. It has shrunk from 12,000 km2 in 1963 to 2400 km2 in 2010[4]. During February 2002, 3250 km2 were lost through iceberg calving and fragmentation. In the figure below, you can see the blue, mottled appearance of the ice shelf in the 2002 image, caused by the exposure of deeper blue glacier ice.

Landsat images showing the collapse of the Larsen Ice Shelf. Note the blue mottled appearance in 2002, resulting from the exposure of deep blue ice.

Several ice shelves have now collapsed around the Antarctic Peninsula (Table 1). Their collapse has made it possible to core the sub-shelf sediments to investigate whether these collapses are part of normal ice-shelf behaviour. It appears that the more northerly ice shelves, such as Prince Gustav Ice Shelf, have indeed previously collapsed, resulting in open-marine organisms living in Prince Gustav Channel for a short period 5000 years ago[7]. However, the more southerly Larsen B Ice Shelf appears to have remained a fixture throughout the Holocene[8]. This suggests that certain thresholds have been passed, with environmental changes throughout the Antarctic Peninsula now surpassing any that have occurred before.

 

 

In the video below, you can see an animation of the Larsen Ice Shelf collapse from Modis imagery:

Table 1. Dates of ice shelf collapse

Ice shelf Largest area (km2) Previous behaviour Recent behaviour
Wordie 2000 ??? 1989 collapse
Larsen Inlet 400 Frequent removal throughout Holocene 1989 collapse
Prince Gustav 2100 Removal 5000 BP 1995 collapse
Larsen A 2500 Frequent removal throughout Holocene 1995 collapse
Larsen B 11,500 Stable throughout Holocene 2002 collapse
Jones 25 ??? 2003 collapse
Wilkins 16,577 Numerous large calving events 2008 collapse
Larsen C 60,000 Stable throughout Holocene Thinning & retreating
Müller 50 Advance during the Little Ice Age Gradual  recession (50 % left)
George VI 26,000 Brief absence (9000 BP) Still present & thinning. Confined, which may increase stability.

Mechanisms of ice shelf collapse

There are several reasons why ice shelves disintegrate rapidly rather than slowly and steadily shrinking. Ice shelves collapse in response to long term environmental changes, which cause on-going thinning and shrinking. When certain thresholds are passed, catastrophic ice shelf disintegration through iceberg calving is initiated. Before collapse, ice shelves first undergo a period of long-term thinning and basal melting, which makes them vulnerable. Meltwater ponding on the surface and tidal flexure and plate bending then all contribute to rapid calving events and ice shelf disintegration.

1. Long term thinning and basal melting

Antarctic ice shelf thickness changes. Note the rapid thinning of Pine Island Glacier ice shelf in West Antarctica. From Pritchard et al., 2012, Nature. Reprinted by permission from Macmillan Publishers Ltd: Nature (Pritchard et al. 2012), copyright (2012).

Antarctic ice shelf thickness changes. Note the rapid thinning of Pine Island Glacier ice shelf in West Antarctica. From Pritchard et al., 2012, Nature. Reprinted by permission from Macmillan Publishers Ltd: Nature
(Pritchard et al. 2012), copyright (2012).

Long-term thinning from surface and basal melting preconditions the ice shelf to collapse. Negative mass balances on tributary glaciers can lead to thinning of the glaciers and ice shelves. The highest rates of thinning are where relatively warm ocean currents can access the base of ice shelves through deep troughs[9,10]. Ice-shelf structure seems to be important, with sutures between tributary glaciers resulting in weaker areas of thinner ice, which are susceptible to rifting[11].

A recent analysis of ice shelves across Antarctica has shown that basal melt rates are around 1325 ± 235 gigatonnes per year, with an additional calving flux of 1089 ± 139 gigatonnes per year. Ice shelf melting is therefore one of the largest ablation processes in Antarctica[1]. However, this massive basal melting does not occur evenly distributed across all ice shelves; the massive Ronne, Filchner and Ross ice shelves cover two thirds of the total ice shelf area but account for only 15% of net melting. Instead, the highest melt rates occur around the Antarctic Peninsula and West Antarctica, from the northern end of George VI Ice Shelf to the western end of Getz Ice Shelf[1]. These ice shelves are also rapidly thinning rapidly[9]. On slow moving ice shelves (e.g., George VI, Abbot, Wilkins), almost all of the original land ice has melted away within a few kilometres of the grounding line. So, half of the meltwater produced comes from just ten small, warm-cavity ice shelves around the SE Pacific rim of Antarctica, and these ten ice shelves occupy just 8% of total ice shelf area. All this cold water being released into the ocean has a significant impact on the formation of sea ice, resulting in higher rates of sea ice concentration around Antarctica.

Melting of ice shelves around Pine Island Glacier in West Antarctica is concerning, because the West Antarctic Ice Sheet is grounded below sea level. A collapse of this ice shelf could lead to marine ice sheet instabilty and rapid global sea level rise.

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

2. Surface melting and ponding

Increased atmospheric temperatures lead to surface melting and ponding on the ice surface. Catastrophic ice-shelf collapsed tend to occur after a relatively warm summer season, with increased surface melting[12]. Based on the seasonality of ice shelf break up, and the geographic distribution of ice shelf collapse near the southerly-progressing -9°C isotherm, it appears that surface ponding is necessary for ice-shelf collapse[12]. This meltwater melts downwards into the ice shelf, causing fractures and leading to rapid ice-berg calving[5, 12]. Increased surface meltwater also leads to snow saturation, filling crevasses with water and increasing hydrostatic pressures. Brine infiltration can also cause crack over deepening.

3. Plate bending and tidal flexure

However, meltwater ponding alone does not explain rapid ice-shelf fragmentation. We need to invole a third process. Bending at the frontal margin of the ice shelf as a result of tidal flexure may cause small cracks to form parallel to the ice front. When subject to the above conditions (thinning with abundant surface water), a threshold may be passed, causing rapid ice shelf disintegration[12].

When icebergs are formed through the above mechanisms, long, thin icebergs are formed at the ice front. These icebergs will capsize as they are thinner than they are deep. Iceberg capsize releases gravitational potential energy and increases tensile stress on the ice shelf. This may lead to a cascade of fragmentation, capsize, and iceberg break up[13].

Ice shelf buttressing

Glacier-ice shelf interactions: In a stable glacier-ice shelf system, the glacier’s downhill movement is offset by the buoyant force of the water on the front of the shelf. Warmer temperatures destabilize this system by lubricating the glacier’s base and creating melt ponds that eventually carve through the shelf. Once the ice shelf retreats to the grounding line, the buoyant force that used to offset glacier flow becomes negligible, and the glacier picks up speed on its way to the sea. Original Image by Ted Scambos and Michon Scott, National Snow and Ice Data Center.

Collapsing ice shelves do not directly contribute to global sea level rise. This is because they are floating, and so their melting does not result in sea level rise. To check this, put a few ice cubes in a glass and check the water level. Does the water rise when the “icebergs” melt?

However, ice shelves play a very important role in “buttressing” their tributary glaciers. Glaciers that feed into ice shelves are held back by the ice shelf in front of them[14, 15]. Even small ice shelves play an important role in regulating flow from ice streams that feed into them[14]. This has been observed in several cases, most notably following the Larsen Ice Shelf [16-19] and Prince Gustav Ice Shelf collapses[20, 21]. In the Landsat image of Prince Gustav Ice Shelf above, you can see the rapid glacier recession from 1988 to 2009.

With glaciers thinning, accelerating and receding in response to ice shelf collapse[20, 21], more ice is directly transported into the oceans, making a direct contribution to sea level rise. Sea level rise due to ice shelf collapse is as yet limited, but large ice shelves surrounding some of the major Antarctic glaciers could be at risk, and their collapse would result in a significant sea level rise contribution[22]. See Marine Ice Sheet Instability for more information.

Further reading

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References


1.            Rignot, E., Jacobs, S., Mouginot, J., and Scheuchl, B. (2013). Ice Shelf Melting Around Antarctica. Science.

2.            Holland, P.R., Corr, H.F.J., Vaughan, D.G., Jenkins, A., and Skvarca, P., 2009. Marine ice in Larsen Ice Shelf. Geophysical Research Letters, 2009. 36: p. L11604.

3.            Morris, E.M. and Vaughan, A.P.M., 2003. Spatial and temporal variation of surface temperature on the Antarctic Peninsula and the limit of viability of ice shelves, in Antarctic Peninsula climate variability: historical and palaeoenvironmental perspectives, E.W. Domack, et al., Editors. American Geophysical Union, Antarctic Research Series, Volume 79: Washington, D.C. p. 61-68.

4.            Cook, A.J. and Vaughan, D.G., 2010. Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years. The Cryosphere, 2010. 4(1): p. 77-98.

5.            Scambos, T., Hulbe, C., and Fahnestock, M., 2003. Climate-induced ice shelf disintegration in the Antarctic Peninsula, in Antarctic Peninsula climate variability: historical and palaeoenvironmental perspectives, E.W. Domack, et al., Editors. American Geophysical Union, Antarctic Research Series, Volume 79: Washington, D.C. p. 79-92.

6.            Rott, H., Skvarca, P., and Nagler, T., 1996. Rapid collapse of northern Larsen Ice Shelf, Antarctica. Science, 1996. 271(5250): p. 788-792.

7.            Pudsey, C.J. and Evans, J., 2001. First survey of Antarctic sub-ice shelf sediments reveals Mid-Holocene ice shelf retreat. Geology, 2001. 29: p. 787-790.

8.            Domack, E., Duran, D., Leventer, A., Ishman, S., Doane, S., McCallum, S., Amblas, D., Ring, J., Gilbert, R., and Prentice, M., 2005. Stability of the Larsen B ice shelf on the Antarctic Peninsula during the Holocene epoch. Nature, 2005. 436(4): p. 681-685.

9.            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.

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

11.          Glasser, N.F. and Scambos, T.A., 2008. A structural glaciological analysis of the 2002 Larsen B ice shelf collapse. Journal of Glaciology, 2008. 54(184): p. 3-16.

12.          Scambos, T., Fricker, H.A., Liu, C.-C., Bohlander, J., Fastook, J., Sargent, A., Massom, R., and Wu, A.-M., 2009. 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): p. 51-60.

13.          MacAyeal, D.R., Scambos, T.A., Hulbe, C.L., and Fahnestock, M.A., 2003. Catastrophic ice-shelf break-up by an ice-shelf-fragment-capsize mechanism. Journal of Glaciology, 2003. 49(164): p. 22-36.

14.          Dupont, T.K. and Alley, R.B., 2006. Role of small ice shelves in sea-level rise. Geophys. Res. Lett., 2006. 33(9): p. L09503.

15.          Dupont, T.K. and Alley, R.B., 2005. Assessment of the importance of ice-shelf buttressing to ice-sheet flow. Geophys. Res. Lett., 2005. 32(4): p. L04503.

16.          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.

17.          Rott, H., Müller, F., and Floricioiu, D., 2011. The Imbalance of glaciers after disintegration of Larsen B ice shelf, Antarctic Peninsula. The Cryosphere, 2011. 5: p. 125-134.

18.          Hulbe, C.L., Scambos, T.A., Youngberg, T., and Lamb, A.K., 2008. Patterns of glacier response to disintegration of the Larsen B ice shelf, Antarctic Peninsula. Global and Planetary Change, 2008. 63(1): p. 1-8.

19.          Rott, H., Rack, W., Nagler, T., and Ieee. 2007. Increased export of grounded ice after the collapse of northern Larsen Ice Shelf, Antarctic Peninsula, observed by Envisat ASAR, in Igarss: 2007 Ieee International Geoscience and Remote Sensing Symposium, Vols 1-12 – Sensing and Understanding Our Planet. p. 1174-1176.

20.          Glasser, N.F., Scambos, T.A., Bohlander, J., Truffer, M., Pettit, E., & Davies, B.J., 2011. From ice-shelf tributary to tidewater glacier: continued glacier recession, acceleration and thinning following the 1995 collapse of the Prince Gustav Ice Shelf on the Antarctic Peninsula. Journal of Glaciology 57, 397-406.

21.          Davies, B.J., Carrivick, J.L., Glasser, N.F., Hambrey, M.J., & Smellie, J.S., 2012. Variable glacier response to atmospheric warming, northern Antarctic Peninsula, 1988–2009. The Cryosphere 6, 1031-1048. doi:10.5194/tc-6-1031-2012

22.          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.

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