Icebergs

Icebergs can be found floating freely in the ocean around the Antarctic and Greenland ice sheets, as well as in other areas with glaciers that end in the ocean. They can be important for the survival of many animals, such as polar bears in the Arctic or penguins in the Antarctic, but they can also pose danger to ships sailing in the polar regions. In 1912, an iceberg led to the sinking of the RMS Titanic in the North Atlantic Ocean.

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Figure 1. Iceberg in Antarctica. Photo credit: Bethan Davies

Iceberg A68 broke off from the Larsen C Ice Shelf in 2017. When it started drifting towards the sub-Antarctic islands of South Georgia, there was widespread concern as large icebergs can become stuck (or ‘grounded’) on the continental shelf, causing issues for local breeding wildlife like seals and penguins. Fortunately, the iceberg broke up and drifted away without causing too much harm.

What are icebergs?

An iceberg is a piece of freshwater ice that has detached from a glacier and is floating in the ocean (Figure 1). Icebergs form when pieces of ice break off the end of an ice shelf or a glacier that flows into a body of water (Figure 2). This is called “calving” and it’s a natural process that is responsible for ice loss at the edges of glaciers and ice sheets (1).

Large icebergs can also calve and break apart into multiple smaller icebergs, and ice shelves have been known to rapidly collapse and disintegrate into several or even thousands of icebergs (2, 3).

Figure 2. Simplified cartoon of iceberg formation at the edge of an ice shelf. Water can enter fractures in the ice shelf and cause the fractures to expand, eventually causing an iceberg to break off or “calve.” Ocean waves can also lead to iceberg calving. Image credit: Megan Thompson-Munson
Video of calving icebergs

In Antarctica, icebergs will calve off the marine-terminating glaciers and then float away (Figure 3). They will drift with the ocean currents. In the winter, they may become frozen into the sea ice. Eventually, they will melt away completely. The image below shows iceberg tracks, measured by satellite imagery, from 1999-2010.

Figure 3. Iceberg tracks from 1999-2010, from The Antarctic Iceberg Tracking Database

How large are icebergs?

Icebergs range in size from small “growlers,” to medium or large “bergy bits,” all the way up to true “icebergs” (Figure 4, 5). Growlers are the smallest, reaching lengths up to 2 m (4) whereas bergy bits have lengths between 2 and 5 m (5). Anything larger than a bergy bit is considered an iceberg, and they can be several thousand square kilometers in area.

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Figure 4. Sunlight shines through a grounded growler. Image credit: Bethan Davies

The largest iceberg observed by satellite was Iceberg B-15, which calved from Antarctica’s Ross Ice Shelf in 2000 and was larger than the island of Jamaica (6).

Figure 5. The three iceberg size categories: growlers (human-sized or smaller); bergy bits (car-sized to house-sized); icebergs (building-sized and larger). These cartoon icebergs are also shown in their stable orientations with their long axis approximately parallel to the ocean surface. Image credit: Megan Thompson-Munson

How do icebergs float?

Icebergs are made up of frozen freshwater, which is less dense than the liquid saltwater in the ocean. This causes icebergs to float in water. However, since the density of ice is about 90% the density of water, most of an iceberg’s mass is below the surface while only about 10% sticks out of the water. When you see an iceberg floating, you’re really only seeing a small portion of it since most of it is hidden below the surface.

As the icebergs melt, they change their position and rise up in the water column, often leading to beautiful shapes and patterns (Figures 4, 6).

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Figure 6. Icebergs weather into beautiful shapes. This one is grounded in the shallows. Figure credit: Bethan Davies

The orientation of a floating iceberg is dependent on where its center of gravity and buoyancy are relative to one another (7). Generally, icebergs will float so their long axis is parallel to the ocean surface (Figure 5).

When an iceberg begins to melt, its changing shape can cause the centers of buoyancy and gravity to shift, which might make the iceberg unstable enough to capsize or “topple” (7, 8) . Icebergs can also topple immediately after calving if they are not initially in a stable orientation to float, and this process can release energy (Figure 6). In fact, it has been suggested that energy released from thousands of capsizing icebergs may have helped to drive the rapid collapse of the Larsen B Ice Shelf in 2002 (2, 3, 9).

For this reason, it’s a good idea to always keep your distance from large icebergs!

Figure 6. Animation of an iceberg toppling and restabilizing made with Josh Tauberer’s Iceberger tool.

Draw your own iceberg and see how it floats using the Iceberger tool:

https://joshdata.me/iceberger.html

About the author

Megan Thompson-Munson is an Atmospheric and Oceanic Sciences PhD student at the University of Colorado Boulder where she studies interactions between ice sheets and the climate. She is particularly interested in better understanding firn processes in both Greenland and Antarctica.

Megan graduated with her MS degree in Geology from the University of Wyoming in 2020. Her MS work focused on Greenland Ice Sheet dynamics and involved two field seasons on the ice sheet. Field work brought her to the town of Ilulissat, Greenland where she first saw icebergs and was instantly fascinated by them. Megan earned dual BS degrees in Geology and Environmental Science in 2017 from the University of Massachusetts Amherst where she completed an honors thesis on biogeochemical reconstructions of East African paleoclimates.

Megan Thompson-Munson

Further reading

References

D. I. Benn, C. R. Warren, R. H. Mottram, Calving processes and the dynamics of calving glaciers. Earth-Sci. Rev. 82, 143–179 (2007).

2.     A. F. Banwell, D. R. MacAyeal, O. V. Sergienko, Breakup of the Larsen B Ice Shelf triggered by chain reaction drainage of supraglacial lakes. Geophys. Res. Lett. 40, 5872–5876 (2013).

3.     D. R. MacAyeal, T. A. Scambos, C. L. Hulbe, M. A. Fahnestock, Catastrophic ice-shelf break-up by an ice-shelf-fragment-capsize mechanism. J. Glaciol. 49, 22–36 (2003).

4.     Cryosphere Glossary. National Snow & Ice Data Center, (available at https://nsidc.org/cryosphere/glossary/term/growler).

5.     Cryosphere Glossary. National Snow & Ice Data Center, (available at https://nsidc.org/cryosphere/glossary/term/bergy-bit).

6.     Iceberg B-15, Ross Ice Shelf, Antarctica. NASA Earth Observatory, (available at https://earthobservatory.nasa.gov/images/552/iceberg-b-15-ross-ice-shelf-antarctica).

7.     J. F. Nye, J. R. Potter, The use of Catastrophe Theory to Analyse the Stability and Toppling of Icebergs. Ann. Glaciol. 1, 49–54 (1980).

8.     J. C. Burton, J. M. Amundson, D. S. Abbot, A. Boghosian, L. M. Cathles, S. Correa-Legisos, K. N. Darnell, N. Guttenberg, D. M. Holland, D. R. MacAyeal, Laboratory investigations of iceberg capsize dynamics, energy dissipation and tsunamigenesis. J. Geophys. Res. 117 (2012), doi:10.1029/2011jf002055.

9.     J. C. Burton, L. Mac Cathles, W. Grant Wilder, The role of cooperative iceberg capsize in ice-shelf disintegration. Ann. Glaciol. 54, 84–90 (2013).

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

Choosing the future of Antarctica

In a new article in the journal Nature, Stephen Rintoul and colleagues present two very different visions of Antarctica’s future, from the perspective of an observer looking back from 2070. In one vision, humanity continues to exploit Earth’s natural resources (such as fossils fuels) and does little to protect the environment, and in the other, there is a global movement towards conservation. The article shows how Antarctica will change over the next 50 years, should either of these two situations occur.

Post by Jacob Bendle. Continue reading

George VI Ice Shelf

George VI Ice Shelf is a floating ice shelf between Alexander Island and the Antarctic Peninsula. This webpage is based on:

Holt, T.O., Glasser, N.F., Quincey, D. and Siegfried, M.R., 2013. Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, 7: 797-816.

Holt, T., Glasser, N., Quincey, D., 2013. The structural glaciology of southwest Antarctic Peninsula Ice Shelves (ca. 2010). Journal of Maps 9, 523-531.

George VI Ice Shelf

Alexander Island and George VI Ice Shelf is an unusual area, and the ice shelf is worth investigating for several reasons. For a start, it’s unusual, being trapped between the mainland and Alexander Island (see map below), and secondly, because it’s right on the -9°C mean annual air temperature isotherm (like a contour, but of mean annual air temperatures).

George VI Ice Shelf, Alexander Island

George VI Ice Shelf, Alexander Island, showing ice flowing onto the ice shelf from both the Antarctic Peninsula and Alexander Island

Some people have argued that this mean annual air temperature is the critical threshold above which ice shelves may dramatically collapse, which has implications for accelerated flow of glaciers and ice-sheet thinning. Ice shelves are also susceptible to warming from below by warm currents penetrating onto the continental shelf. So, research into this important part of the peninsula is always welcome.

Holt and colleagues have just completed a study (open access) that investigates the response of George VI Ice Shelf (shown in the image below) to environmental change (i.e., oceanic and atmospheric temperature variations), and offer an assessment as to its future stability (Holt et al., 2013).

Alexander Island and George VI Ice Shelf, from the Landsat Image Mosaic of Antarctica

Alexander Island and George VI Ice Shelf, from the Landsat Image Mosaic of Antarctica

George VI Ice Shelf is approximately 24,000 km2 and is the second largest ice-shelf on the Antarctic Peninsula. The northern ice front calves into Marguerite Bay, and the southern ice front calves into the Ronne Entrance, where there are several ice rises (Holt et al., 2013), which are bedrock highs on which the ice front is grounded and pinned.

Along its centreline, this means that the ice shelf is around 450 km long, and is around 20 km wide and 100 m thick in the north, 600 m thick in the centre, and 75 km wide in the south. Ice flows from both Alexander Island and the Antarctic Peninsula into the ice shelf. The ice shelf primarily loses mass (ablates) as a result of calving at its termini and basal melting.

The ice shelf is unusual, because it is grounded at the margin against Alexander Island; unlike the Larsen Ice Shelf, which flows freely into the ocean, George VI Ice Shelf is laterally constrained. Here, it dams epishelf lakes in Ablation and Moutonnee valleys.

Simplified schematic figure of an ordinary ice shelf (such as the Larsen Ice Shelf) and George VI Ice Shelf, which abutts Alexander Island, forming pressure ridges and ice-shelf moraines.

Simplified schematic figure of an ordinary ice shelf (such as the Larsen Ice Shelf) and George VI Ice Shelf, which abutts Alexander Island, forming pressure ridges and ice-shelf moraines.

Ice shelf thickness and freeboard

The freeboard (height of the ice-shelf surface above sea level) of George VI Ice Shelf ranges from ~60 to 5 m asl. The ice shelf is structurally complex, with distinct flow units originating in Palmer Land flowing across to, and impinging against, Alexander Island (Davies et al. 2017; Reynolds and Hambrey, 1988; Hambrey et al., 2015).

Ice-shelf thickness and freeboard is controlled by this complex flow regime. Ice-shelf thickness varies from 100 m at the northern ice front to 600 m in the centre, before thinning again towards the southern ice front (Davies et al. 2017; Lucchitta and Rosanova, 1998; Smith et al., 2007).

The thickest ice occurs in lobes extending from the grounding-lines of the major outlet glaciers from Palmer Land. Thicknesses adjacent to Ablation Point Massif are c. 125 m (Hambrey et al., 2015), and the ice-shelf surface is 5 m asl here. In the centre of George VI Sound near Ablation Point Massif, the ice shelf reaches up to 150 m thick. The ice shelf reaches a surface elevation of ~20 m asl at Fossil Bluff, with a thickness of ~200 m.

Ice shelf thickness and freeboard, George VI Ice Shelf. From: Davies et al. 2017

Structural Glaciology

Holt et al. used Landsat images to map the surface features of ice shelves on the south-western Antarctic Peninsula. They mapped the ice front, grounding zone, longitudinal surface structures (flow stripes), pressure ridges, crevasses, fractures and rifts, surface meltwater, ice dolines, ice rises and ice rumples.

Together, these data help us to understand the structure of more ‘stable’ ice shelves, and helps us to assess their likely future response and vulnerabilities to climate change.

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.

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.

The photographs below show the northern part of George VI Ice Shelf from above.

Ice shelf collapse

Holt et al. comment that ice-shelf collapse is usually preceded by:

  • sustained ice-front retreat, with a frontal geometry that bows inland (ice shelves are more stable where they are ‘pinned’ against the land, at their lateral margins) (Doake et al., 1998);
  • continued thinning from below and above by atmospheric and oceanic warming (Fricker and Padman, 2012);
  • increasing flow speeds;
  • and finally, structural weakening along suture zones (where ice shelves or glacier ice meets) (Glasser and Scambos, 2008).
  • Meltwater ponding on the ice surface is also critical, because hydrofracture may drive rapid ice-shelf break up.

This research is aiming to investigate whether George VI Ice Shelf is at risk of collapse. Ice shelf collapse has a significant effect on the glaciers inland, causing them to accelerate and discharge ice into the ocean.

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.

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.

Changes in George VI Ice Shelf

Holt and others (2013) conducted an extensive analysis of ice shelf velocity, structural glaciology, recession, thinning and grounding-zone retreat. They found that the ice shelf is retreating at an annual rate of around 1.0 km (1936-1974) to 1.1 km per year (1974-2010).

However, recession of this particular ice shelf generally occurs during a limited number of periodic, large-scale calving events, with a two-phase recession recorded from 1974-2010 (Holt et al., 2013). Calving events followed the gradual development of large, deep rifts, with a large calving event in January 2010.

Ice rifting

Much of the ice-shelf recession is therefore controlled by long-standing rifts in the ice, which develop many years before the calving event. Rifts are structurally controlled, with areas of rifted ice occurring between different flow units of the in-flowing glaciers.

In the northern part of the ice shelf, the ice flows under an extensional regime and does not undergo longitudinal compression following rift formation. Fractures that therefore originate where glacier ice flows into the ice shelf therefore continue without impediment towards the northern end of the ice shelf, where they form large rifts, making the northern end susceptible to large, periodic, calving events.

The southern margin of George VI Ice Shelf is steadily receding (Holt et al., 2013), with recession concentrated in the centre of the ice front, where the ice is thinnest. The islands on which the ice shelf is pinned cause regular fracturing and rifting as the ice flows over and around them, as the ice shifts from compressional stresses to tensile stresses downstream. The ice rises also cause ice flow to slow around them, so they effectively act as an impediment to smooth flow.

Ice shelf acceleration

Parts of the ice shelf are now flowing faster. Recession of the most northerly margin has reduced the backstress and buttressing effect on Riley Glacier, on the Antarctic Peninsula, which is now flowing faster. In the south, the removal of large areas of ice has led to increased longitudinal extension, causing more fracturing and increased ice velocities (Holt et al., 2013).

Glacier thinning

One method of calculating elevation change is to use ICESat data. This satellite measures ice surface elevation, and repeated tracks show elevation change through time. Holt et al. analysed ICESat data across George VI Ice Shelf to show surface elevation change from 2003 to 2008.

Significant thinning was observed in the southern section of the ice shelf, which is coupled with widespread recession of the ice front terminus.

Thinning of George VI Ice Shelf. Non-significant elevation change (less than the uncertainties) was observed in the northern part of the ice shelf. Widespread and significant elevation change was recorded in the southern section, coupled with retreat of the grounding zone. From Holt et al., 2013.

Thinning of George VI Ice Shelf. Non-significant elevation change (less than the uncertainties) was observed in the northern part of the ice shelf. Widespread and significant elevation change was recorded in the southern section, coupled with retreat of the grounding zone. From Holt et al., 2013.

Future stability

The northern end of George VI Ice Shelf is currently free from large crevasses, which take several decades to form. No large-scale rifting is therefore immediately expected from this area. However, it has changed considerably over the last 40 years, and response to continued environmental change is expected (Holt et al., 2013).

Surface melt is prevalent on the ice shelf, and continued longer, warmer summers and more meltwater ponding may result in ice-shelf recession by hydrofracture, where meltwater ponds melt downwards through the ice, weakening it structurally and contributing to increased iceberg calving.

The southern ice front of the ice shelf has changed rapidly following climatic and oceanic changes, with sustained recession, mostly in the thinner central portion. This part of the ice shelf is structurally weak and prone to iceberg calving. The ice shelf is thinning following increased basal melting from the incursion of warm waters beneath the ice shelf. This part of the ice shelf is therefore very vulnerable.

Further reading

References

Davies, B.J., Hambrey, M.J., Glasser, N.F., Holt, T., Rodés, A., Smellie, J.L., Carrivick, J.L., Blockley, S.P.E., 2017. Ice-dammed lateral lake and epishelf lake insights into Holocene dynamics of Marguerite Trough Ice Stream and George VI Ice Shelf, Alexander Island, Antarctic Peninsula. Quaternary Science Reviews 177, 189-219.

Doake, C.S.M., Corr, H.F.J., Rott, H., Skvarca, P. and Young, N.W., 1998. Breakup and conditions for stability of the northern Larsen Ice Shelf, Antarctica. Nature, 391 (6669): 778-780.

Fricker, H.A. and Padman, L., 2012. Thirty years of elevation change on Antarctic Peninsula ice shelves from multimission satellite radar altimetry. Journal of Geophysical Research: Oceans, 117 (C2): C02026.

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

Hambrey, M.J., Davies, B.J., Glasser, N.F., Holt, T.O., Smellie, J.L., Carrivick, J.L., 2015. Structure and sedimentology of George VI Ice Shelf, Antarctic Peninsula: implications for ice-sheet dynamics and landform development. Journal of the Geological Society 172, 599-613.

Holt, T.O., Glasser, N.F., Quincey, D. and Siegfried, M.R., 2013. Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, 7: 797-816.

Lucchitta, B.K., Rosanova, C.E., 1998. Retreat of northern margins of George VI and Wilkins Ice Shelves, Antarctic Peninsula. Annals of Glaciology 27, 41-46.

Reynolds, J.M., Hambrey, M.J., 1988. The structural glaciology of George VI Ice Shelf, Antarctic Peninsula. British Antarctic Survey Bulletin 79, 79-95.

Smith, J.A., Bentley, M.J., Hodgson, D.A., Roberts, S.J., Leng, M.J., Lloyd, J.M., Barrett, M.S., Bryant, C.L., Sugden, D.E., 2007. Oceanic and atmospheric forcing of early Holocene ice shelf retreat, George VI Ice Shelf, Antarctic Peninsula. Quaternary Science Reviews 26, 500-516.

Glacier types

There are many different kinds of ice.

Land ice is ice grounded on land, and above sea level. If this ice melts, it contributes to sea level rise. Glaciers and ice sheets are the most common kind of land ice.

The Antarctic Ice Sheet is surrounded by many other kinds of ice. Sea ice is floating, frozen sea water. It melts away seasonally, blows around in the wind, and is not attached to the land. In the Arctic, the winter extent of sea ice is decreasing over time; in the Antarctic, increased wind strength is dispersing a thinner layer of sea ice over a wider area.

Ice shelves are the floating extensions of land ice. Where large ice streams meet the ocean in Antarctica, they start to float (the point at which they start to float is the grounding line).

Icebergs are the bits of ice that calve away from marine-terminating or lake-terminating glaciers and ice sheets. They float away into the ocean. Increased calving of land ice into the ocean contributes to sea level rise.

This video explains about sea ice.

The Larsen C Ice Shelf growing rift

Antarctic Peninsula ice shelves | Ice shelf collapse on the Antarctic Peninsula | Rifting on Larsen C | Impact of calving the large iceberg | Sea level rise following ice-shelf collapse | References | Comments |

Antarctic Peninsula ice shelves

The Antarctic Peninsula is fringed by floating ice shelves. They are floating extensions of the glaciers on land, and receive mass by snowfall and marine freeze-on. They lose mass by melting at their base and by calving icebergs. Larsen C Ice Shelf, the largest ice shelf on the Antarctic Peninsula, is currently being closely watched. Following a series of high-profile ice-shelf collapse events on the Antarctic Peninsula over the last few decades, all eyes are watching Larsen C and wondering when, and if, it will collapse.

A growing rift on Larsen C Ice Shelf

Those concerns are growing more acute as a large rift on Larsen C Ice Shelf is growing rapidly, threatening to soon calve a huge iceberg, equivalent to losing 10% of the area of the ice shelf. This could destabilise the ice shelf, making it more susceptible to a total collapse.

Larsen C rift animation uses #Sentinel1 InSAR to illustrate recent jumps in rift progression. From Prof. Adrian Luckman.

Continue reading

Grounding Lines

What is a grounding line?

Almost all of Antarctica is covered in ice. Less than 1% its land area is ice free. This means that, across Antarctica, almost all glaciers end in the ocean, whereupon they calve icebergs. These glaciers can be grounded, or can end in floating ice tongues or larger ice shelves. These floating ice shelves move with the tide. Ice shelves fringe 75% of Antarctica’s coastline, while collecting 20% of its snowfall over 11% of its area[1]. Basal melt from ice shelves is the largest melting process in Antarctica. Clearly, ice sheet – ocean interactions are extremely important for controlling ice sheet dynamics and rates of melting and recession.

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

Glaciers that end in the ocean like this are called Tidewater Glaciers. They may be grounded (the glacier is in contact with the bed entirely), or parts of the glacier terminus may be floating. Glaciers that flow into an ice shelf are tributary glaciers.

The point at which glaciers and ice shelves start to float is the Grounding Line. The location of the grounding line is important, because mass loss from Antarctica is strongly linked to changes in the ice shelves and their grounding lines[2, 3]. Change in the grounding line can result in very rapid changes in glacier and ice-shelf behaviour (for example, see Marine ice sheet instability).

Simplified grounding line image of an ice sheet. From: Huybrechts et al., 2009. Nature 458, 295-296.

The transition from grounded ice sheet to floating ice shelf plays an important role in controlling marine ice sheet dynamics, as it determines the rate at which ice flows out of the grounded part of the ice sheet[4]. This is because ice flux through the grounding line increases sharply with ice thickness at the grounding line. This means that grounding lines are unstable on reverse-bed slopes, such as those under Pine Island Glacier, because recession into deeper water increases ice flux and further encourages more glacier recession.

Summary of the impacts on Antarctica and the Southern Ocean in 2070, under a ‘high emissions’ scenario. Reprinted by permission from Nature [Nature Perspectives] [Choosing the future of Antarctica, S. Rintoul and colleagues] [Copyright 2018].

Simplified cartoon of a grounding lineMapping the grounding line

Grounding lines are actually more of a zone. The grounding zone is the region where ice transitions from grounded ice sheet to freely floating ice shelf, typically over several kilometres. The floating ice shelf changes in elevation in response to tides, atmospheric air pressure and oceanic processes. Grounding occurs when the ice shelf comes into contact with the bedrock below.

The grounding zone is the region between point F on the figure below, where there is no tidal movement, and point H, which is the seaward limit of ice flexure, where the ice is free-floating.

The grounding zone. After Fricker et al., 2009.
The grounding zone. After Fricker et al., 2009.

The grounding zone can be difficult to detect; it may take place over a wide area[5], and area can be remote and inaccessible and so difficult to monitor. Fortunately, there is a subtle feature that can be observed on satellite images. There is often an elevation minimum between points G and H (Point Im in the cartoon above). Elevation profiles across the grounding line will often show a break in slope (Point Ib).

Other methods for detecting the grounding line rely on measuring changes in surface elevation during the tidal cycle, which can be measured by GPS or satellite synthetic aperture radar (eg., InSAR) or ICESat[2, 5, 6].

Antarctic Peninsula Ice Shelves
Antarctic Peninsula Ice Shelves. The grounding line is denoted by a thick black line.

Current grounding line change

Across the Antarctic Peninsula and West Antarctica, increased upwelling of the relatively warm Circumpolar Deep Water is melting ice at the grounding line. In the Amundsen Sea, this has resulted in glacier acceleration, thinning, and grounding line recession. Circumpolar Deep Water, which is a key component of the Antarctic Circumpolar Current, is able to reach the undersides of the ice shelves and the grounding line by flowing through deep submarine troughs[7]. This has resulted in rapid grounding-line recession at Pine Island Glacier[8] – up to 31 km from 1992 to 2011.

Warm Circumpolar Deep Water is penetrating beneath the ice shelves of Pine Island Glacier and Thwaites Glacier.

Recognising grounding lines from the past

Grounding lines leave behind a distinct geomorphological and sedimentological record[9-14] on the continental shelf that scientists can use to map and date former grounding line positions. This crucial information can be used to reconstruct past ice-sheet extent; e,.g., [15, 16].

Grounding zone wedges form transverse to ice flow and can be mapped by ships equipped with swath bathymetry, which allows them to create a detailed topographical map of the sea floor[17]. These grounding zone wedges represent either past maximum ice-sheet extent, or recessional positions during deglaciation.

Grounding zone wedges (also known as ‘till deltas’ or ‘ice-contact submarine fan’[9]) build up under stable ice margins; they require the grounding line to remain in a stable position for long enough for enough sediment to accumulate to build a wedge or ridge[17]. Grounding zone wedges are sedimentary depocentres that form at the transition from grounded to floating ice. They typically consist of well-bedded foreset and bottomset deposits.

Further reading

References

  1. Rignot, E., et al., Ice Shelf Melting Around Antarctica. Science, 2013. 341(6143): p. 266-270.
  2. Brunt, K.M., et al., Mapping the grounding zone of the Ross Ice Shelf, Antarctica, using ICESat laser altimetry. Annals of Glaciology, 2010. 51(55): p. 71-79.
  3. Pritchard, H.D., et al., Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature, 2012. 484(7395): p. 502-505.
  4. Schoof, C., Ice sheet grounding line dynamics: Steady states, stability, and hysteresis. Journal of Geophysical Research: Earth Surface (2003–2012), 2007. 112(F3).
  5. Fricker, H.A., et al., Mapping the grounding zone of the Amery Ice Shelf, East Antarctica using InSAR, MODIS and ICESat. Antarctic Science, 2009. 21(5): p. 515-532.
  6. Rignot, E., J. Mouginot, and B. Scheuchl, Antarctic grounding line mapping from differential satellite radar interferometry. Geophys. Res. Lett., 2011. 38(10): p. L10504.
  7. Walker, D.P., et al., Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough. Geophysical Research Letters, 2007. 34(2): p. L02602.
  8. Rignot, E., et al., Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011. Geophysical Research Letters, 2014: p. n/a-n/a.
  9. Lønne, I., Sedimentary facies and depositional architecture of ice-contact glaciomarine systems. Sedimentary Geology, 1995. 98(1–4): p. 13-43.
  10. Powell, R.D. and B.F. Molnia, Glacimarine sedimentary processes, facies and morphology of the south-southeast Alaska shelf and fjords. Marine Geology, 1989. 85(2-4): p. 359-390.
  11. Powell, R.D., Glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on Quaternary examples. Marine Geology, 1984. 57(1-4): p. 1-52.
  12. McCabe, A.M. and N. Eyles, Sedimentology of an ice-contact glaciomarine delta, Carey Valley, Northern Ireland. Sedimentary Geology, 1988. 59(1-2): p. 1-14.
  13. Eyles, C.H., N. Eyles, and A.D. Miall, Models of glaciomarine sediments and their application to the interpretation of ancient glacial sequences. Palaeogeography, Palaeoclimatology, Palaeoecology, 1985. 15: p. 15-84.
  14. Powell, R.D. and R.B. Alley, Grounding-Line Systems: Processes, Glaciological Inferences and the Stratigraphic Record, in Geology and Seismic Stratigraphy of the Antarctic Margin, 2. 2013, American Geophysical Union. p. 169-187.
  15. Ó Cofaigh, C., et al., Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum. Quaternary Science Reviews, 2014. 100(0): p. 87-110.
  16. Ó Cofaigh, C., P. Dunlop, and S. Benetti, Marine geophysical evidence for Late Pleistocene ice sheet extent and recession off northwest Ireland. Quaternary Science Reviews, 2011. In press, corrected proof.
  17. Cofaigh, C.Ó., Ice sheets viewed from the ocean: the contribution of marine science to understanding modern and past ice sheets. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012. 370(1980): p. 5512-5539.

Shrinking ice shelves

This section is all about ice shelves. Ice shelves are floating ice, connected to the mainland. They receive ice from glaciers flowing into them from the mainland, from accumulation from snow directly onto the ice shelf, and from sea water freezing onto the bottom of the ice shelf. Most mass loss from the Antarctic continent is from ice shelves, and most of this is from just a few small ice shelves around the Antarctic Peninsula and West Antarctica.

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

Ice shelves can collapse dramatically. This can occur over just a few weeks, following progressive thinning by warm ocean waters below, and from excessive melting during a warm summer above. If an ice shelf collapses, it changes the boundary conditions for the glaciers that flow into the ice shelf. This means that ice-shelf tributary glaciers accelerate, thin and recede following ice-shelf collapse. So, although ice shelves are already floating and therefore do not contribute to sea level rise when they collapse, ice-shelf removal has significant consequences for the grounded glaciers on the mainland.

More information:

Ice shelves, icebergs and sea ice

Ice shelves | Icebergs | Sea ice | Further reading | References | Comments |

Ice shelves

An ice shelf is a floating extension of land ice. The Antarctic continent is surrounded by ice shelves. They cover >1.561 million km2 (an area the size of Greenland)[1], fringing 75% of Antarctica’s coastline, covering 11% of its total area and receiving 20% of its snow.

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

The difference between sea ice and ice shelves is that sea ice is free-floating; the sea freezes and unfreezes each year, whereas ice shelves are firmly attached to the land. Sea ice contains icebergs, thin sea ice and thicker multi-year sea ice (frozen sea water that has survived several summer melt seasons, getting thicker as more ice is added each winter).

In the photographs below, you can see the flat, floating ice shelf is almost featureless. The ice flows from the mainland into the sea, and when it becomes deep enough it floats.

Ice shelf flow

Ice shelves receive ice in several ways: flow of ice from the continent, surface accumulation (snow fall) and the freezing of marine ice to their undersides. Ice shelves lose ice by melting from below (from relatively warm ocean currents), melting above (from warm air temperatures) and from calving icebergs.  This is a normal part of their ablation.

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).

Ice shelves can be up to 2000 m thick, with a cliff edge that’s up to 100 m high. They often show flow structures on their surface – a relic of structures formed on land[2-4].

Receding ice shelves

Ice shelves around the Antarctic Peninsula are retreating[5]. These ice shelves are warmed from below by changing ocean currents, thinning them and making them vulnerable. During warm summers, ice shelves calve large icebergs – and in some cases, can catastrophically collapse.

Prince Gustav Ice Shelf in 1988. It collapsed in 1995, and the glaciers which flowed into it subsequently accelerated and thinned, transmitting lots of ice into the ocean and resulting in measureable sea level rise.
Prince Gustav Ice Shelf in 1988. It collapsed in 1995, and the glaciers which flowed into it subsequently accelerated and thinned, transmitting lots of ice into the ocean and resulting in measureable sea level rise.

Icebergs

Icebergs are floating all around Antarctica. They calve off from tidewater glaciers or ice shelves. They can range in size from small chunks you could fit into a gin and tonic to huge floating behemoths that take decades to melt and that you can land a helicopter on.

90% of the mass of an iceberg is underwater, and only a small part of the iceberg is visible above the water level. Small chunks of ice are called ‘bergy bits’, larger ones (fridge-sized) are called ‘growlers’, and chunks of ice greater than 5 m across are called ‘icebergs’.

Icebergs float in a stable position, with their long axis parallel to the water surface. Elongated icebergs will float on their side. You can draw your own icebergs here:

Iceberger, by Josh Tauberer

Ships navigating in polar waters must be careful to avoid icebergs and growlers, which can be hard to see, and will use radar to scan ahead, particularly in poor visibility or in the dark.

Bergy bits washed up at the high tide mark on James Ross Island
Bergy bits washed up at the high tide mark on James Ross Island

If you’re looking for ice to add to your drink, choose a bergy bit made from coarse clear crystal ice. See-through ice chunks are made from compressed glacier basal ice and are clean and pure enough to drink. The compressed air present in the ice bubbles away as it melts, making for the best G&T you ever had.

All shapes and sizes

Icebergs can have many colours. Blue icebergs are formed from basal ice from a glacier. The compressed crystals have a blue tint. Green and red icebergs are coloured by algae that lives on the ice.

Stripy icebergs are coloured by basal dirt and rocks, ground up by the glacier and carried away within the glacier ice. Crevasses and other glacier structures may be preserved, giving yet more texture and beauty to the iceberg.

Icebergs are studied for a number of reasons. They are tracked with satellite images as they travel around the Southern Ocean. As they drift away from the Antarctic continent, they deliver cold, fresh water, dust and minerals to the surface ocean.

The iceberg also may drag its keel on the continental shelf. Each of these processes has impacts for surface and deep-water animals[6]. The surface phytoplankton increases by up to one third in the wake of a large iceberg.

Iceberg tracks from 1999-2010, from The Antarctic Iceberg Tracking Database
Iceberg tracks from 1999-2010, from The Antarctic Iceberg Tracking Database

Tracking icebergs provides information on ocean currents. Scientists can assess whether the number of icebergs is increasing[7, 8]. The input of freshwater may affect surface water currents and even sea ice formation[9].

Sea ice

Sea ice surrounds the polar regions. On average, sea ice covers up to 25 million km2, an area 2.5 times the size of Canada. Sea ice is frozen ocean water. The sea freezes each winter around Antarctica.

This image of Antarctic sea ice is from the NASA Scientific Visualisation Studio, showing the Earth on September 21st 2005. Source: Wikimedia Commons.
This image of Antarctic sea ice is from the NASA Scientific Visualisation Studio, showing the Earth on September 21st 2005. Source: Wikimedia Commons.

Sea ice can modify climate change’s impact on terrestrial ice because it is highly reflective and because it has a strongly insulating nature. Each year, the extent of sea ice varies according to climate variability and long-term climate change.

In the Arctic, sea ice extent is steadily decreasing, with a trend of -5.3±00.6% per decade since 1985[10], as a result of long-term climate change. Year-on-year variations reflect normal variability. Because removal of sea ice changes the reflectivity of the Arctic, a diminishing sea-ice extent amplifies warming.

Frozen winter sea ice trapping calved icebergs from the margin of a tidewater glacier
Frozen winter sea ice trapping calved icebergs from the margin of a tidewater glacier

Sea ice in the Antarctic is currently increasing[9]. This is associated with cooling sea surface temperatures in the Southern Ocean, in particular near the Ross Ice Shelf.

Causes of this increasing Antarctic sea ice, which are contrasted with shrinking glaciers and ice shelves and warming deeper ocean current temperatures and atmospheric air temperatures, include changes to the Southern Annual Mode due to intensification and migration of the predominant Southern Ocean Westerlies, and cooler sea surface temperatures as a result of increased glacier and ice-shelf melting[9].

This video explains in more detail about changes in sea ice.

You can explore changes in sea ice using this ArcGIS App.

Sea Ice Aware app

Further reading

References


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

2.            Glasser, N.F., B. Kulessa, A. Luckman, D. Jansen, E.C. King, P.R. Sammonds, T.A. Scambos, and K.C. Jezek, 2009. Surface structure and stability of the Larsen C Ice Shelf, Antarctic Peninsula. Journal of Glaciology, 55(191): 400-410.

3.            Glasser, N.F. and G.H. Gudmundsson, 2012. Longitudinal surface structures (flowstripes) on Antarctic glaciers. The Cryosphere, 6: 383-391.

4.            Glasser, N.F., T.A. Scambos, J.A. Bohlander, M. Truffer, E.C. Pettit, and B.J. Davies, 2011. 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(203): 397-406.

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

6.            Schwarz, J.N. and M.P. Schodlok, 2009. Impact of drifting icebergs on surface phytoplankton biomass in the Southern Ocean: Ocean colour remote sensing and in situ iceberg tracking. Deep Sea Research Part I: Oceanographic Research Papers, 56(10): 1727-1741.

7.            Long, D.G., J. Ballantyn, and C. Bertoia, 2002. Is the number of Antarctic icebergs really increasing? Eos, Transactions American Geophysical Union, 83(42): 469-474.

8.            Ballantyne, J. and D.G. Long. A multidecadal study of the number of Antarctic icebergs using scatterometer data. in Geoscience and Remote Sensing Symposium, 2002. IGARSS ’02. 2002 IEEE International. 2002.

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

10.          Kwok, R. and D. Rothrock, 2009. Decline in Arctic sea ice thickness from submarine and ICESat records: 1958–2008. Geophysical Research Letters, 36(15).

Glaciers and Climate

This section of the website highlights how glaciers interact with climate, and how changing climate is changing glaciers around the world today.

Globally, glaciers are receding and shrinking in response to atmospheric warming. The signal is remarkably consistent across different continents and mountain ranges.

99% of Antarctica is ice-covered, and so most of the glaciers and ice streams here end in the ocean. It is in these oceanic margins that we see the most rapid changes: ice stream acceleration, thinning and grounding line migration. At the ice-ocean interface, the ice sheet is vulnerable to melting from below as well as from above, as warm ocean currents penetrate the continental shelf and melt the ice sheets at their grounding line.

This section of the website contains lots of information about how ice sheets interact with the ocean. For more information, please do see: