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.

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.

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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. They 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.

Glaciers that end in the ocean like this are called Tidewater Glaciers. The point at which these glaciers 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).

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.

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.

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.

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

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

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

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

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 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.

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

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.

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.

Icebergs

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

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. 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’. 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.

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.

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

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

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.

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

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 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.

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].

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).

Antarctic Peninsula Ice Shelves

Prince Gustav Ice Shelf | Larsen Ice Shelf | Wordie Ice Shelf | Wilkins Ice Shelf | George VI Ice Shelf | References | Comments |

Prince Gustav Ice Shelf

Antarctic Peninsula Ice Shelves

Antarctic Peninsula Ice Shelves

Prince Gustav Ice Shelf was the most northerly ice shelf on the Antarctic Peninsula, and it was the first to collapse in 1995. It shrank progressively throughout the second half of the 20th Century, before collapsing to leave open water between James Ross Island and Trinity Peninsula1. Anecdotal evidence suggests that in the early 20th Century it was connected to an enlarged Larsen A Ice Shelf, and that it had been shrinking and thinning for many decades prior to collapse.

Following the collapse of Prince Gustav Ice Shelf, tributary glaciers were observed to accelerate, thin and recede back even 15 years after collapse2,3, directly contributing to eustatic 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.

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.

Larsen Ice Shelf

The Larsen Ice Shelf actually comprises four ice shelves; Larsen A, on the north-eastern Antarctic Peninsula, Larsen B, south of Seal Nunataks (Larsen A and B have both collapsed), Larsen D, the large currently remaining ice shelf, and Larsen D, the long, thin ice shelf fringing the south-eastern Antarctic Peninsula.

Larsen Ice Shelf in 2004

Larsen Ice Shelf in 2004

Larsen A, close to Prince Gustav Ice Shelf, used to extend from Cape Longing to Robertson Island, and merged with Larsen B at Seal Nunataks. Larsen A was relatively stable and around 4000 km2 from 1961 until the 1980s, when periodic large calving events resulted in stepwise recession1.  Larsen B underwent rapid calving and disintegration in 2002, when most of the ice shelf was lost1. Their collapse follows thinning both from basal melting and surface summer meltwater formation4. This recession began in the 1980s. Larsen C is also currently thinning5. Larsen B has been thinning throughout the Holocene6, but its recent collapse was unprecedented during the Holocene.

Following the collapse of the Larsen ice shelves, their tributary glaciers were observed to accelerate, thin and recede, resulting in a direct contribution to global sea level7.

Larsen C has been stable over the last few decades, a but a large and growing rift across the ice front is threatening to extent all the way across the ice shelf. This would calve the largest iceberg ever recorded, and could destabilise the ice shelf.

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.

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.

Wordie Ice Shelf

Wordie Ice Shelf is made up of six major glacier units that flow into it, and there are around 20 ice rises and ice rumples8. Wordie Ice Shelf disintegrated in a series of calving events during the 1970s and 1980s, and by 1992 was little more than a few disconnected glacier tongues1. This dramatic collapse could have been caused by warm summer air temperatures, which increased summer surface ablation9. Ice rises on the ice shelf, where the ice shelf is pinned to bedrock lumps on the sea floor, may have aided the ice shelf’s stability between large calving events1.

Wilkins Ice Shelf

Wilkins Ice Shelf is the largest ice shelf in West Antarctica that is currently undergoing rapid shrinkage. Recession has occurred through a series of rapid calving events, where it calved a number of large, tabular icebergs 1. Significant break-ups occurred in 1998 and March and July 2008, and finally again in April 2009. The overall area was reduced to 5434 km2, which is around two thirds of its original size.

Wilkins Ice Shelf flows only slowly, at around 30-90 metres per year (the nearby Wordie flows at 200-2000 metres per year). It has a catchment area of 16,900 km2, which is a small area of grounded ice to nourish an ice shelf, and it is sustained largely by in situ accumulation1. Melting on the Wilkins Ice Shelf is largely by basal melting and some surface melting in the summer10. The Wilkins Ice Shelf is thinning at a rate of 0.8 metres per year (1992-2008), which is driven by a basal melt rate of 1.3 ± 0.4 metre per year11. On slow-moving, thin, near-stationary ice shelves like the Wilkins Ice Shelf, basal melting rapidly melts away the original glacier ice within a few kilometres of its grounding line12. According to Rignot et al. (2013), the Wilkins Ice Sheet loses 18.4 ± 17 gigatonnes of meltwater each year as a result of basal melting, and 0.7 ± 0.4 gigatonnes of ice per year following iceberg calving.

You can explore these ice shelves using the Google Map below. Note the mountains running down the length of Alexander Island. George VI Ice Shelf lies between Alexander Island and the mainland Antarctic Peninsula (Palmer Land). On the west of Alexander Island, you have the Wilkins Ice Shelf and several smaller ice shelves. Can you see where the ice shelf starts? Look for a break in slope to see where the glacier ice starts to float.

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George VI Ice Shelf

George VI Ice Shelf is an unusual ice shelf that is trapped between the mainland (Palmer Land) and Alexander Island. Ice flows from the mainland north and south to the marine termini at either end of Prince Gustav Channel. George VI Ice Shelf is located on the -9°C annual isotherm (mean annual air temperature is -9°C), which has been proposed as the theoretical limit for ice-shelf viability13.

George VI Ice Shelf, Alexander Island

George VI Ice Shelf, Alexander Island

George VI Ice Shelf measures 24,000 km2, and is the second-largest ice shelf on the Antarctic Peninsula14. It is a slow-moving ice shelf. George VI Ice Shelf is melting rapidly at its base12, and is largely sustained by snow accumulation. George VI Ice Shelf is currently receding at a rate of 1 to 1.1 kilometres per year, and it is thinning, and the grounding line is retreating14. Frontal recession generally occurs as series of large calving events following the development of rifting.

You can read more about George VI Ice Shelf in this blog post.

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

References


1.            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).

2.            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).

3.            Davies, B.J., Carrivick, J.L., Glasser, N.F., Hambrey, M.J. & Smellie, J.L. Variable glacier response to atmospheric warming, northern Antarctic Peninsula, 1988–2009. The Cryosphere 6, 1031-1048 (2012).

4.            Shepherd, A., Wingham, D., Payne, T. & Skvarca, P. Larsen ice shelf has progressively thinned. Science 302, 856-859 (2003).

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

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

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

8.            Reynolds, J.M. The structure of Wordie Ice Shelf, Antarctic Peninsula. British Antarctic Survey Bulletin 80, 57-64 (1988).

9.            Doake, C.S.M. & Vaughan, D.G. Rapid disintegration of the Wordie Ice Shelf in response to atmospheric warming. Nature 350, 328-330 (1991).

10.          Braun, M., Humbert, A. & Moll, A. Changes of Wilkins Ice Shelf over the past 15 years and inferences on its stability. The Cryosphere 3, 41-56 (2009).

11.          Padman, L., Costa, D.P., Dinniman, M.S., Fricker, H.A., Goebel, M.E., Huckstadt, L.A., Humbert, A., Joughin, I., Lenaerts, J.T.M., Ligtenberg, S.R.M., Scambos, T. & van den Broeke, M.R. Oceanic controls on the mass balance of Wilkins Ice Shelf, Antarctica. Journal of Geophysical Research: Oceans 117, C01010 (2012).

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

13.          Morris, E.M. & Vaughan, A.P.M. 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, Vol. Volume 79 (eds. Domack, E.W., Leventer, A., Burnett, A., Bindschadler, R., Convey, P. & Kirby, M.) 61-68 (American Geophysical Union, Antarctic Research Series, Volume 79, Washington, D.C., 2003).

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

Antarctic ice shelves – the hidden villain

Sea ice and ice shelves

What is sea ice? Sea ice is frozen sea water; it perennially expands and contracts during each year’s winter and summer. Amongst the sea ice are icebergs calved from tidewater glaciers and ice shelves. Melting sea ice does not contribute directly to sea level rise (ice floats and displaces the same volume of water), but sea ice is important because it enhances climate warming. It changes the reflectivity of the sea water, reflecting lots of sunlight back (it has a high albedo), and is therefore an important component of the climate and cryospheric (icey) system.
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