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Sea level rise over the next 2000 years

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A new paper by Levermann et al. in PNAS uses the record of past rates of sea level rise from palaeo archives and numerical computer models to understand how much sea level rise we can expect per degree of warming in the future. These data suggest that we can expect a global sea level rise of 2.3 m per 1°C of warming within the next 2000 years: well within societal timeframes. A 2°C of warming would result in a global sea level rise of 4.8 m within 2000 years. This would inundate many coastal cities in Europe alone, and cause untold economic and societal damage.

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Pine Island Glacier

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Investigating Pine Island Glacier | Why is Pine Island Glacier important? | Pine Island Glacier ice shelf | Pine Island Glacier: the longer term view | Conclusions | References | Comments |

Investigating Pine Island Glacier

A fast-flowing ice stream

Pine Island Glacier is one of the largest ice streams in Antarctica. It flows, together with Thwaites Ice Stream, into the Amundsen Sea embayment in West Antarctica, and the two ice streams together drain ~5% of the Antarctic Ice Sheet1. Pine Island Glacier flows at rates of up to 4000 m per year2.

Pine Island Glacier is of interest to scientists because it is changing rapidly; it is thinning, accelerating and receding3, all of which contribute directly to sea level, and its future under a warming climate is uncertain.

Pine Island Glacier is buttressed by a large, floating ice shelf, which helps to stabilise the glacier, but this ice shelf is itself thinning and recently calved a huge iceberg.

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

Just watch how fast the ice flows in the video below, and notice especially how the ice speeds up when it reaches the floating ice shelf.

Caption: Visualisation of ice flow in the Antarctic ice sheet model PISM-PIK. The white dots show how particles move with the ice which are initially randomly distributed over the ice surface. Colours in addition show the flow speed. By Youtube user pikff1.

An inaccessible location

Despite this interest, Pine Island Glacier is difficult to access. It is remote from any research bases, so flying there means making multiple short flights, making fuel depots to allow scientists to hop to the location. Low lying cloud often makes flying hazardous. The ice stream is heavily-crevassed and dangerous, so walking on it is difficult. Sea ice keeps ships away, making it difficult to access the ice stream from the ocean.

However, scientists have several ingenious ways in which they can observe changes to this fragile, important ice stream. They can measure changes in ice extent and thinning from satellites4,5, and they have fired javelins loaded with sensors onto the ice surface, into places with too many crevasses for people to travel.

Finally, scientists on board ships have deployed ‘Autosub’ beneath the very ice shelf, to make observations where no man can go.

Autosub near the ice (from http://www.krapp.org/rupert/archiv.html/2007_08_01_archive.html)
Autosub near the ice (from http://www.krapp.org/rupert/archiv.html/2007_08_01_archive.html)

Exploring Pine Island Glacier

You can use Google Earth below to explore the ice stream. Can you identify the ice shelf? If you zoom in far enough, you’ll be able to see the huge crack in the ice shelf. You can also see how the surface of both the ice stream and ice shelf is heavily crevassed, making it difficult to walk on the surface of the ice.


View Pine Island Glacier in a larger map

Why is Pine Island Glacier important?

Pine Island Glacier drains much of the marine-based West Antarctic Ice Sheet, and it has a configuration susceptible to rapid disintegration and recession. The ice sheet in this area is grounded up to 2000 m below sea level, making it intrinsically unstable6 and susceptible to rapid melting at its base, and to rapid migration of the grounding line up the ice stream7 (see Marine Ice Sheet Instability).

The images below show how much of the West Antarctic Ice Sheet, especially around Pine Island Glacier, is grounded well below sea level.

Pine Island Glacier is one of the most dynamic features of the Antarctic Ice Sheet. It is buttressed by a large ice shelf that is currently thinning8, and the ice stream itself has a negative mass balance (the melting is not replaced by snowfall)3, it is flowing faster9,  and the grounding line is retreating further and further up into the bay.

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

The grounding line receded by more than 20 km from 1996 to 20092. The ice stream is steepening, which increases the gravitational driving stress, helping it to flow faster, and there is no indication that the glacier is approaching a steady state10.

Possible future collapse?

Pine Island Glacier could collapse – stagnate and retreat far up into the bay, resulting in rapid sea level rise – within the next few centuries, raising global sea levels by 1.5 m11,12, out of a total of 3.3 m from the entire West Antarctic Ice Sheet13.

Some studies have suggested that the entire main trunk of Pine Island Glacier could unground and become afloat within 100 years14, but more recent modelling efforts suggest that much longer timescales are needed to unground the entire trunk2.

These numerical computer models indicate that annual rates of sea level rise from Pine Island Glacier could reach 2.7 cm per 100 years2. Under the A1B “Business as Usual” emissions scenario from the IPCC (2.6°C warming by 2100), Gladstone et al. (2012) predict recession over the next 200 years with huge uncertainty over the rate of retreat, and full collapse of the trunk of Pine Island Glacier during the 22nd Century remains a possibility15.

A1B warming scenarios from the IPCC. A1B is the "Business as Usual" scenario, with emissions continuing to increase in line with present-day rates of increase.
A1B warming scenarios from the IPCC. A1B is the “Business as Usual” scenario, with emissions continuing to increase in line with present-day rates of increase. The grey bars at the right indicate the best estimate and likely range of temperatures.

It remains difficult to assess how soon a collapse of Pine Island Glacier could occur, but a new paper by Bamber and Aspinall (2013) suggest that there is a growing view that the West Antarctic Ice Sheet could become unstable over the next 100 years16.

The largest contibution to global sea level rise from the Greenland and Antarctic ice sheets combined is around 16.9 mm per year, but is more likely to be around 5.4 mm per year by 2100. This gives a total of 33 to 132 cm of global total sea level rise by 2100. Uncertainty over the future behaviour of Pine Island Glacier in West Antarctica is one of the largest constraints on accurately predicting future sea level rise16.

Current behaviour

Pine Island Glacier is currently flowing very quickly and it is accelerating, causing thinning. The velocity is well above that required to maintain mass balance – so the ice stretches longitudinally, and thins vertically3.

In the figure below, from Rignot et al. 2008, you can see that mass losses from Pine Island Glacier and Thwaites Glacier dominate Antarctic Ice Sheet ice losses. Mass loss from this basin doubled from 1996 to 2006, and it is the largest ice loss in Antarctica.

Reprinted by permission from Macmillan Publishers Ltd: Nature (Rignot et al., 2008), copyright 2008
Reprinted by permission from Macmillan Publishers Ltd: Nature
(Rignot et al., 2008), copyright 2008

Pine Island Glacier ice shelf

Pine Island Glacier has a large ice shelf, which supports the glacier. Removal of the ice shelf would likely result in rapid acceleration, thinning and recession as the glacier adjusts to new boundary conditions; these reactions have been observed following ice shelf collapse around the Antarctic Peninsula17-21.

The ice shelf around Pine Island Glacier is currently thinning, and it is warmed from below by Circumpolar Deep Water that flows onto the continental shelf22,23. This melts the ice shelf from below24, and this melting is probably the cause of the observed ice stream thinning, acceleration and grounding line recession25, which is contributing to a sea level rise of 1.2 mm per decade3.

Calving Icebergs

Pine Island Glacier ice shelf periodically calves huge icebergs. The ice shelf currently loses around 62.3 ± 5 Gigatonnes per year of ice through calving, and loses 101.2 ± 8 Gigatonnes per year through basal melting24. It calved a large iceberg in 2001, and in 2011 a huge rift developed on the ice shelf.  This iceberg was finally calved in July 2013. It’s about eight times the size of New York, or half the size of Greater London, at 720 km2.

However, this iceberg calving event is a natural process, part of how the ice shelf regularly calves – this ice shelf spawns huge icebergs every 6-10 years. Releasing a huge iceberg, by itself, is a normal process, unrelated to warming, but increased calving may occur in the future if the ice shelf continues to thin, which would make it susceptible to plate bending and hydrofracture processes21. This threshold has yet to be passed.

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

Current melting, thinning and acceleration

What is concerning is the current intense melting, thinning and glacier acceleration observed on Pine Island Glacier ice shelf22. Measurements from the British Antarctic Survey’s Autosub, the intrepid sub-ice shelf explorer, help scientists understand sub-ice conditions.

Autosub is a remotely operated vehicle, loaded with sensors that measure temperature, salinity, pressure and so on, and it can map the sea bed using downward-pointing swath bathymetry. It can dive to 1600 m and travel 400 km, and it has a clever collision avoidance system.  It’s a dangerous business; several iterations of Autosub have been lost under the ice.

However, data from Autosubs that did return indicates that more warm Circumpolar Deep Water has been in Pine Island Bay in recent summers22. Meltwater production underneath the ice shelf increased by 50% from 1994 to 2011; this increased melting results from stronger sub-ice-shelf circulation. As the ice shelf thins, more water is able to circulate beneath it22, exacerbating the problem and encouraging further melting.

Warm ocean waters are melting a cavity beneath Pine Island Glacier
Warm ocean waters are melting a cavity beneath Pine Island Glacier. After Schoof, 2010, Nature Geosci, 3, 450-451.

Pine Island Glacier ice shelf now has one of the fastest rates of ice-shelf thinning in Antarctica24,25.

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).

Pine Island Glacier: the longer term view

It is important that we take a longer-term perspective of the current changes observed on Pine Island Glacier. Are these on-going changes unprecedented, or are they part of the normal behaviour for the glacier? Marine sediment cores and swath bathymetry from ships can image the sea floor and detect and date the former behaviour of this ice stream.

These data suggest that the recession of this ice stream was largely controlled by sea level rise, with a 55 m in sea level rise during deglaciation resulting in 225 km of grounding-line recession26.

At the Last Glacial Maximum, circa 18,000 years ago, the ice stream was at the continental shelf edge27. It rapidly shrank back from around 16,400 years ago, when rising sea levels made this ice stream more buoyant, causing lift-off, decoupling from the ice sheet’s bed, and recession. 

The ice stream continued to recede from 16,400 to 12,300 years ago, controlled by global sea level rise. It reached its current position around 10,000 years ago27.

The recession of the ice stream was also controlled by the presence or absence of ice shelves. From 12300 to 10600 years ago, there was a large ice shelf throughout the Amundsen Sea Embayment. This ice shelf collapsed after 10600 years ago28, when warmer waters flowed onto the continental shelf. The grounding line of the ice stream retreated rapidly following ice-shelf collapse26.

It seems that the glacier is capable of very rapid recession within millennial timescales27, and that the dynamics between ice shelf and ice stream are intrinsically linked.  More work at a higher resolution, combined with modelling studies, is required to fine-tune and better understand the longer-term history of Pine Island Glacier.

Conclusions

Pine Island Glacier is a cause for concern, because it’s thinning rapidly, steepening, accelerating and receding. It is out of balance. Huge amounts of meltwater are generated in a large cavity beneath the ice shelf. It periodically, every 10 or so years, calves large icebergs – but on their own, they are not worrisome. The recently calved iceberg may be 720 km2, but that’s the least of this ice stream’s worries. This ice stream is unlikely to collapse in our lifetime – but the same cannot be said for future generations.

Pine Island Glacier is one of the largest ice streams in Antarctica, and drains much of the West Antarctic Ice Sheet. Because it is grounded in ever deeper sea water, it is vulnerable to melting at its base and rapid grounding line migration. A collapse of Pine Island Glacier could occur within 1000-2000 years, raising sea levels by up to 1.5 m, but it is unlikely to contribute to more than 2.7 cm of sea level rise over the next 100 years.

Wider Reading

References


1.            Vaughan, D.G., Smith, A.M., Corr, H.F.J., Jenkins, A., Bentley, C.R., Stenoien, M.D., Jacobs, S.S., Kellogg, T.B., Rignot, E. & Lucchitta, B.K. A Review of Pine Island Glacier, West Antarctica: Hypotheses of Instability Vs. Observations of Change. in The West Antarctic Ice Sheet: Behavior and Environment 237-256 (American Geophysical Union, 2001).

2.            Joughin, I., Smith, B.E. & Holland, D.M. Sensitivity of 21st century sea level to ocean-induced thinning of Pine Island Glacier, Antarctica. Geophysical Research Letters 37, L20502 (2010).

3.            Rignot, E., Bamber, J.L., van den Broeke, M.R., Davis, C., Li, Y., van de Berg, W.J. & van Meijgaard, E. Recent Antarctic ice mass loss from radar interferometry and regional climate modelling. Nature Geosci 1, 106-110 (2008).

4.            Rignot, E., Mouginot, J. & Scheuchl, B. Ice Flow of the Antarctic Ice Sheet. Science (2011).

5.            Shepherd, A., Wingham, D.J., Mansley, J.A.D. & Corr, H.F.J. Inland thinning of Pine Island Glacier, West Antarctica. Science 291, 862-864 (2001).

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

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

8.            Rignot, E. Ice-shelf changes in Pine Island Bay, Antarctica, 1947-2000. Journal of Glaciology 48, 247-256 (2002).

9.            Rignot, E. Changes in ice dynamics and mass balance of the Antarctic ice sheet. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 364, 1637-1655 (2006).

10.          Scott, J.B.T., Gudmundsson, G.H., Smith, A.M., Bingham, R.G., Pritchard, H.D. & Vaughan, D.G. Increased rate of acceleration on Pine Island Glacier strongly coupled to changes in gravitational driving stress. Cryosphere 3, 125-131 (2009).

11.          Hughes, T. A simple holistic hypothesis for the self-destruction of ice sheets. Quaternary Science Reviews 30, 1829-1845 (2011).

12.          Vaughan, D.G. West Antarctic Ice Sheet collapse – the fall and rise of a paradigm. Climatic Change 91, 65-79 (2008).

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

14.          Wingham, D.J., Wallis, D.W. & Shepherd, A. Spatial and temporal evolution of Pine Island Glacier thinning, 1995-2006. Geophysical Research Letters 36, L17501 (2009).

15.        Gladstone, R.M., Lee, V., Rougier, J., Payne, A.J., Hellmer, H., Le Brocq, A., Shepherd, A., Edwards, T.L., Gregory, J. & Cornford, S.L. Calibrated prediction of Pine Island Glacier retreat during the 21st and 22nd centuries with a coupled flowline model. Earth and Planetary Science Letters 333–334, 191-199 (2012).

16.          Bamber, J. L., and Aspinall, W. P. (2013). An expert judgement assessment of future sea level rise from the ice sheets. Nature Clim. Change 3, 424-427.

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

18.          De Angelis, H. & Skvarca, P. Glacier surge after ice shelf collapse. Science 299, 1560-1562 (2003).

19.          Rott, H., Rack, W., Skvarca, P. & De Angelis, H. Northern Larsen Ice Shelf, Antarctica: further retreat after collapse. Annals of Glaciology 34, 277-282 (2002).

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

21.          Scambos, T., Fricker, H.A., Liu, C.-C., Bohlander, J., Fastook, J., Sargent, A., Massom, R. & Wu, A.-M. 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, 51-60 (2009).

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

23.          Jenkins, A., Dutrieux, P., Jacobs, S.S., McPhail, S.D., Perrett, J.R., Webb, A.T. & White, D. Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat. Nature Geoscience 3, 468-472 (2010).

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

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

26.          Kirshner, A.E., Anderson, J.B., Jakobsson, M., O’Regan, M., Majewski, W. & Nitsche, F.O. Post-LGM deglaciation in Pine Island Bay, West Antarctica. Quaternary Science Reviews 38, 11-26 (2012).

27.          Lowe, A.L. & Anderson, J.B. Reconstruction of the West Antarctic ice sheet in Pine Island Bay during the Last Glacial Maximum and its subsequent retreat history. Quaternary Science Reviews 21, 1879-1897 (2002).

28.          Jakobsson, M., Anderson, J.B., Nitsche, F.O., Dowdeswell, J.A., Gyllencreutz, R., Kirchner, N., Mohammed, R., O’Regan, M., Alley, R.B., Andandakrishnan, S., Eriksson, B., Kirshner, A., Fernandez, R., Stolldorf, T., Minzoni, R. & Majewski, W. Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica. Geology 39, 691-694 (2011).

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Antarctica

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The Antarctic continent | The Antarctic Ice Sheets | Ice streams, subglacial lakes and ice shelves in Antarctica | Wildlife of Antarctica | Exploration of Antarctica | References | Comments |

The Antarctic continent

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

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

Antarctica: the enigmatic, romantic, remote white continent. Antarctica lies at the bottom of the world and all waters south of 60°S latitude are designated Antarctic, where no country owns the land and where only scientific and peaceful operations may take place. Military activity is banned in Antarctica, and it is a haven for wildlife.

Unlike the Arctic, where floating sea ice annual melts and refreezes, Antarctica is a solid ice sheet lying on a solid continent1. The Antarctic summer is during the northern Hemisphere winter. Antarctica may be remote and isolated, but the dynamics of Antarctic glaciers affect us all.

Antarctica is huge. The Earth’s southernmost continent is twice the size of Australia, and 98% of it is covered by ice. Antarctica is cold (the coldest recorded temperature is -89°C, from Vostok), but the peripheral islands and Antarctic Peninsula may have positive air temperatures in summer.

There is no permanent human population in Antarctica, but around 1000 people, mostly scientists and support staff, overwinter each year. Summer populations can be as high as 5000 (excluding the many hundreds of visitors who briefly visit on tourist ships). The British Antarctic Survey maintains eight research stations and operates many summer field camps each year.

Research stations and summer camps in Antarctica.

Research stations and summer camps in Antarctica.

You can use Google Earth to explore Antarctica for yourself. You can see how the great continent is surrounded by cold ocean waters. Note the Antarctic Peninsula, the thin spine of mountains pointing towards South America, the huge flat and floating ice shelves, and the large, high, East Antarctic Ice Sheet.

View Larger Map

Antarctic mountains

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 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 Antarctic continent lies on a large landmass. Underneath that smooth ice sheet there are mountains and valleys.

The surface of the Antarctic Ice Sheet is up to 4000 m high, and in places the ice is 4000 m deep, but the Gamburtsev Mountain range is up to 2,700 m high and lies underneath the East Antarctic Ice Sheet.

The Transantarctic Mountains divide East and West Antarctica. This mountain range is 3500 km long and 100-300 km wide. The summits of these mountains poke through the ice to form some of the only ice-free areas of Antarctica; these ‘nunataks’ are up to 4,500 m high.

The Transantarctic Mountains contain some of the oldest glacial sediments in Antarctica, and the Sirius Group, from Mount Sirius,  indicates that there has been ice here for at least 15 million years. This webpage has beautiful photographs of the Transantarctic Mountains.

You can use the Google Map below to easily explore the Transantarctic Mountains. You can see how they go through the ice sheet. In this map, the Byrd and Shackleton glaciers are in the centre, and they flow into the giant, floating, flat Ross Ice Shelf. How does this compare with the BEDMAP2 figures above and below?

View Larger Map

The first ice in Antarctica grew on the Transantarctic Mountains and Gamburtsev Mountains around 34 million years ago2, when global air temperatures were around 4°C warmer than today. Since then, with on-going cooling, the ice sheets have fluctuated, growing and shrinking at different timescales.

During the Quaternary Period, the ice sheets fluctuated first at 41,000 year timescales, and after around 1 million years ago, they fluctuated at 100,000 year timescales. These huge ice sheets came to dominate and influence the Earth’s climate and global sea levels. The last glacial cycle ended around 11,000 years ago and the Last Glacial Maximum was around 18,000 years ago.

Antarctica's subglacial topography, with main mountain ranges shown.

Antarctica’s subglacial topography, with main mountain ranges shown, using the BEDMAP2 dataset (Fretwell et al., 2012).

The Antarctic ocean

Simplified schematic map of ocean currents of the Southern Ocean.

Simplified schematic map of ocean currents of the Southern Ocean.

There is a strong circumpolar circulation around Antarctica. This results in a cooler continent, as heat exchange from the tropics is limited. The circulation in the Weddell Sea brings ice bergs and cold water north, up the Antarctic Peninsula, and is one of the reasons why the eastern Antarctic Peninsula is much warmer than the western Antarctic Peninsula.

Antarctica is globally important, and not just because melting Antarctic glaciers have the potential to raise global sea levels. Cold, salty water forms around Antarctica, which sinks to the sea floor and drives global ocean currents. The Global Thermohaline Circulation drives large currents around the world, and brings the warm Gulf Stream to Britain, moderating its climate.

Global thermohaline circulation. From: Wikimedia Commons

Global thermohaline circulation. From: Wikimedia Commons

The Antarctic Ice Sheets

East Antarctic Ice Sheet

Satellite image of the Dry Valleys

Satellite image of the Dry Valleys

There are three ice sheets in Antarctica; the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet and the Antarctic Peninsula Ice Sheet. Each of these ice sheets has its own unique characteristics and behaviour. East Antarctica is grounded mostly above sea level and forms the bulk of the Antarctic Ice Sheet; if it melted, the East Antarctic Ice Sheet would raise global sea levels by 53 m3.

The EAIS holds the bulk of frozen fresh water on planet Earth, and it’s the highest, driest, coldest and windiest ice sheet in Antarctica by far.

In fact, the East Antarctic Ice Sheet is so cold and dry, it is the world’s most southerly desert. The Dry Valleys of East Antarctica receive around 10 mm of precipitation per year, and the mean annual air temperature is -19.8°C, making this one of the harshest places in the world.

West Antarctic Ice Sheet

Antarctic Ice Sheet without ice

Isostatically corrected Antarctic continent with the ice removed. From the Global Warming Art Project

The West Antarctic Ice Sheet is grounded largely below sea level. If it melted, it would raise global sea levels by a mere 3.3 m4, but unlike the East Antarctic Ice Sheet, rapid ice-sheet melt is a threat and a possibility.

The West Antarctic Ice Sheet is grounded well below sea level and the base of the ice sheet deepens landwards; it is therefore known as a “Marine Ice Sheet“.

The West Antarctic Ice Sheet is located in a region of rapid warming, and warm ocean waters threaten to melt the ice sheet at its base5.

During past interglacials, it is likely that the West Antarctic Ice Sheet almost entirely disappeared, and was left as a series of islands – as shown in the figure opposite. A future collapse of the West Antarctic Ice Sheet could rapidly raise global sea levels6. The likely hood of this happening, when it would happen and how long it would take is currently a topic of hot debate7.

Antarctic Peninsula Ice Sheet

Antarctic Peninsula Ice Shelves

Antarctic Peninsula Ice Shelves

The Antarctic Peninsula Ice Sheet is the smallest, holding only 0.24 m of sea level equivalent. However, this small ice sheet, situated on a mountain range, is perhaps the most vulnerable to climate change.

The glaciers of the Antarctic Peninsula are small and located in a region of rapid warming8. This has already resulted in numerous observable changes: collapsing ice shelves9, thinning and accelerating glaciers10-12, and widespread glacier recession13.

Ice streams, subglacial lakes and ice shelves in Antarctica

Ice Streams

Ice streams of Antarctica. From Rignot et al. (2011).

Ice streams of Antarctica. From Rignot et al. (2011).

The Antarctic Ice Sheets are not just domes of ice spreading slowly out to their margins. The Antarctic Ice Sheets are drained by fast-flowing ice streams14. The Twaites Ice Stream and Pine Island Glacier, for example, together drain 30% of the West Antarctic Ice Sheet.

Pine Island Glacier moves at about 4000 metres per year, and the stability and dynamics of this ice stream is essential for the stability of the larger Antarctic Ice Sheet. Ice streams send dendritic fingers deep into the Antarctic continent, and you can see on the figure of ice velocities the slow-moving ice divides at the centre of the different ice sheets.

Recent data published by Rignot et al. 2011 shows the ice flow across the Antarctic continent. This image, made from data downloaded from the NSIDC[5] is shown on alogarithmic scale. This emphasises the ice divides clearly. You can see, by comparing with the BEDMAP figure above, that these tend to follow the mountain ranges. Large ice streams drain into fast-flowing, floating ice shelves.

Subglacial Lakes

379 subglacial lakes have now been identified beneath the Antarctic continent. This map, using data from Wright and Siegert 2012 [1] shows that many are located in ice-stream onset zones as well as underneath slow-moving ice domes.

379 subglacial lakes have now been identified beneath the Antarctic continent. This map, using data from Wright and Siegert 2012 [1] shows that many are located in ice-stream onset zones as well as underneath slow-moving ice domes.

Despite being so cold, there is water at the base of the Antarctic Ice Sheet. The huge weight of the ice above melts ice at the base of the ice sheet, aided by geothermal heating. This water lubricates the base of the ice sheet, and helps the ice streams achieve their great speeds.

The water ponds in lows and hollows beneath the ice sheet, and it may exist at huge hydrostatic pressure, enabling water to flow uphill.

379 subglacial lakes have now been mapped across Antarctica15, and more are being found all the time. These subglacial lakes influence the behaviour of the ice streams of Antarctica, and drainage of lakes may add more water to the base of an ice stream – helping it to flow faster16.

Ice Shelves

Larsen Ice Shelf in 2004

Larsen Ice Shelf in 2004

Antarctica is fringed with ice shelves; in fact, 75% of the Antarctic continent is buttressed with ice shelves. Ice shelves are floating extensions of Antarctic glaciers, supplemented by snow fall directly onto the ice shelves and freezing of marine waters below5.

Ice shelves cover ~1.561 million km2, which is similar in area to the Greenland Ice Sheet. Ice shelves collect 20% of Antarctica’s snowfall and cover 11% of its area.

Ice shelves lose mass by melting from below and by calving ice bergs. In fact, basal melting from ice shelves accounts for most of the ice loss from Antarctica, and most of this ice loss comes from a few small ice shelves in West Antarctica and along the western Antarctic Peninsula5.

Sea Ice

Breaking sea ice with HMS Protector, northern Antarctic Peninsula, March 2012.

Breaking sea ice with HMS Protector, northern Antarctic Peninsula, March 2012.

Sea ice is seasonal and consists of frozen sea water, together with icebergs calved from Antarctic glaciers and ice shelves. Winter sea ice around Antarctica is increasing, in contrast with winter sea ice in the Arctic, which is decreasing.

This seasonal increase in sea ice may be due to colder, fresher water, released from the melting ice shelves, which accumulates in a cool, fresh surface layer and shields surface waters from the warmer, deeper waters that are melting the ice shelves17.

Wildlife of Antarctica

Adelie penguin

Adelie penguin

Antarctica is a wild continent. It is also largely deserted; all the wildlife lives in the ocean. It may come ashore briefly, but all the food is in the ocean. Small mites and springtails are the only animals that actually live on the small land oases around Antarctica. Birdlife, however, is prevalent in Antarctica.

Flying birds include Albatross, terns, cormorants, gulls, skuas, petrels and fulmar.

Penguins are Antarctica’s poster child, and Antarctica has seven species: Adélie penguins, chinstrap penguins, emperor penguins, Gentoo penguins, king penguins, rockhopper penguins and royal penguins.

The warm upwelling ocean currents around Antarctica make it a haven for sea animals, and this accounts for the high numbers of Antarctic whales and seals. There are true (earless) seals and fur seals, which have ear flaps.

Whales are common in Antarctica and for decades were hunted, in some cases nearly to extinction. The Antarctic Treaty has allowed some species to recover, although some are still vulnerable.

Exploration of Antarctica

Antarctica was first explored in the 19th Century. Captain James Cook, in the ships HMS Resolution and HMS Adventure, crossed the Antarctic Circle for the first time on 17th January 1773, and was repeatedly beaten back by sea ice.  Land was first sighted, probably around 1820. James Clark Ross sailed through the Ross Sea in 1841, and sailed near the Ross Ice Shelf.

Ernest Shackleton lead the Nimrod expedition in 1907 and reached the magnetic South Pole. An expedition led by Roald Amundsen reached the geographic South Pole on 14th December 1911.

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References

1.        Siegert, M.J. Antarctic subglacial topography and ice-sheet evolution. Earth Surface Processes and Landforms 33, 646-660 (2008).

2.        Siegert, M.J., Barrett, P., Decont, R., Dunbar, R., Cofaigh, C.O., Passchier, S. & Naish, T. Recent advances in understanding Antarctic climate evolution. Antarctic Science 20, 313-325 (2008).

3.        Fretwell, L.O., H. D. Pritchard, D. G. Vaughan, J. L. Bamber, N. E. Barrand, R. Bell, C. Bianchi, R. G. Bingham, D. D. Blankenship, G. Casassa, G. Catania, D. Callens, H. Conway, A. J. Cook, H. F. J. Corr, D. Damaske, V. Damm, F. Ferraccioli, R. Forsberg, S. Fujita, P. Gogineni, J. A. Griggs, R. C. A. Hindmarsh, P. Holmlund, J. W. Holt, R. W. Jacobel, A. Jenkins, W. Jokat, T. Jordan, E. C. King, J. Kohler, W. Krabill, M. Riger-Kusk, K. A. Langley, G. Leitchenkov, C. Leuschen, B. P. Luyendyk, K. Matsuoka, Y. Nogi, O. A. Nost, S. V. Popov, E. Rignot, D. M. Rippin, A. Riviera, J. Roberts, N. Ross, M. J. Siegert, A. M. Smith, D. Steinhage, M. Studinger, B. Sun, B. K. Tinto, B. C. Welch, D. A. Young, C. Xiangbin & Zirizzotti, A. Bedmap2: improved ice bed, surface and thickness datasets for Antarctica. The Cryosphere 7, 375-393 (2013).

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

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

6.        Vaughan, D.G. West Antarctic Ice Sheet collapse – the fall and rise of a paradigm. Climatic Change 91, 65-79 (2008).

7.        Bamber, J.L. & Aspinall, W.P. An expert judgement assessment of future sea level rise from the ice sheets. Nature Clim. Change 3, 424-427 (2013).

8.        Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carelton, A.M., Jones, P.D., Lagun, V., Reid, P.A. & Iagovkina, S. Antarctic climate change during the last 50 years. International Journal of Climatology 25, 279-294 (2005).

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

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

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

12.      Pritchard, H.D. & Vaughan, D.G. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula. Journal of Geophysical Research-Earth Surface 112, F03S29, 1-10 (2007).

13.      Cook, A.J., Fox, A.J., Vaughan, D.G. & Ferrigno, J.G. Retreating glacier fronts on the Antarctic Peninsula over the past half-century. Science 308, 541-544 (2005).

14.      Rignot, E., Mouginot, J. & Scheuchl, B. Ice Flow of the Antarctic Ice Sheet. Science (2011).

15.      Wright, A. & Siegert, M. A fourth inventory of Antarctic subglacial lakes. Antarctic Science 24, 659-664 (2012).

16.      Smith, B.E., Fricker, H.A., Joughin, I.R. & Tulaczyk, S. An inventory of active subglacial lakes in Antarctica detected by ICESat (2003-2008). Journal of Glaciology 55, 573-595 (2009).

17.      Bintanja, R., van Oldenborgh, G.J., Drijfhout, S.S., Wouters, B. & Katsman, C.A. Important role for ocean warming and increased ice-shelf melt in Antarctic sea-ice expansion. Nature Geosci advance online publication(2013).

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West Antarctic Ice Sheet

13

Introduction | Topography | Oceanography | Ice streams and ice shelves | References | Comments |

Introduction

Landsat Image Mosaic of Antarctica, showing the different ice sheets of Antarctica

The West Antarctic Ice Sheet (the WAIS) is capable of rapid change as it is a marine ice sheet and therefore could be unstable. It has the potential to raise global sea level by 3.3 m[1] over a matter of centuries. The Transantarctic Mountains divide the West Antarctic Ice Sheet from the East Antarctic Ice Sheet[2]. West Antarctica is approximately 97% ice-covered, and is 1.97 x 106 km2 in area. The West Antarctic Ice Sheet flows into the Bellingshausen, Weddell, Amundsen and Ross seas.

There are principally three sectors of the ice sheet, which flow northeast-ward into the Weddell Sea, westward into the Ross Ice Shelf and northward into the Amundsen/Bellingshausen seas. The highest elevations reached are 3000 m above sea level[2], occurring at the divides between these sectors. The size of the West Antarctic Ice Sheet is limited, despite its high average snow falls, by the faster speeds of its ice streams.

Topography

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)

The West Antarctic Ice Sheet is, in places, over 2000 m thick, with the geological floor well below sea level. The marine basins are variable, with both rough mountainous terrain and flat, deep oceanic basins[2], with a maximum depth of 2555 m below present sea level.

During past interglacials, the West Antarctic Ice Sheet has been completely removed[3], which is one of the arguments supporting a Marine Ice Sheet Instability hypothesis. During past glacials, the West Antarctic Ice Sheet extended to the continental shelf edge[4-6], drained by numerous ice streams[7, 8], such as the Pine Island and Thwaites ice streams, which flow out into the Amundsen Sea. In the four-panel figure opposite, you can see these two ice streams clearly. They are grounded below sea level and drain a large proportion of the West Antarctic Ice Sheet.

In the map below, showing ice thicknesses across the Antarctic continent, you can see that the West Antarctic Ice Sheet has ice thicknesses of up to 2000 m, but that it is largely grounded below sea level. The maximum altitude of the ice surface is less than 2000 m above sea level. The West Antarctic Ice Sheet is divided from the East Antarctic Ice Sheet by the large Transantarctic Mountains.

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.

Oceanography

Simplified schematic map of ocean currents of the Southern Ocean.

West Antarctica is surrounded by a strong clockwise circumpolar circulation. These currents play a significant role in the global thermohaline circulation, and are one of the reasons why Antarctica is so cold.

At shallower depths, Circumpolar Deep Water can move across the continental shelf and reach the underside of ice shelves[2], which it can rapidly melt due to its relatively warm temperatures.

Ice streams and ice shelves

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

The West Antarctic Ice Sheet is drained by several large ice streams. The basal sediments of West Antarctica comprise soft marine sediments. Combined with geothermal heating at the base, this is sufficient to allow glaciers to slide rapidly: see Glacial Processes. This ice flow is partly constrained by buttressing ice shelves. The ice streams flow from an inland reservoir of ice towards the ocean, passing over a grounding line and, in places, into an ice shelf. Nearly all the precipitation received in West Antarctica eventually passes through these ice streams[2].

Further reading

To learn more about the West Antarctic Ice Sheet, you can read:

Go to top or jump to Antarctic Peninsula Ice Sheet.

References

1.            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.

2.            Bindschadler, R., 2006. The environment and evolution of the West Antarctic ice sheet: setting the stage. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2006. 364(1844): p. 1583-1605.

3.            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.

4.            Bentley, M.J. and Anderson, J.B., 1998. Glacial and marine geological evidence for the ice sheet configuration in the Weddell Sea-Antarctic Peninsula region during the Last Glacial Maximum. Antarctic Science, 1998. 10(3): p. 309-325.

5.            Lowe, A.L. and Anderson, J.B., 2002. Reconstruction of the West Antarctic ice sheet in Pine Island Bay during the Last Glacial Maximum and its subsequent retreat history. Quaternary Science Reviews, 2002. 21(16-17): p. 1879-1897.

6.            Anderson, J.B., Shipp, S.S., Lowe, A.L., Wellner, J.S., and Mosola, A.B., 2002. The Antarctic Ice Sheet during the Last Glacial Maximum and its subsequent retreat history: a review. Quaternary Science Reviews, 2002. 21(1-3): p. 49-70.

7.            Graham, A.G.C., Larter, R.D., Gohl, K., Hillenbrand, C.-D., Smith, J.A., and Kuhn, G., 2009. Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control. Quaternary Science Reviews, 2009. 28(25-26): p. 2774-2793.

8.            Livingstone, S.J., O Cofaigh, C., Stokes, C.R., Hillenbrand, C.-D., Vieli, A., and Jamieson, S.S.R., 2012. Antarctic palaeo-ice streams. Earth-Science Reviews, 2012. 111(1-2): p. 90-128.

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Marine ice sheet instability

15

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

15

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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