Box 21 Antarctic Peninsula Ice Shelves Breaking Up Is All Too Easy

Some of the most dramatic recent changes in Antarctica have taken place on the Antarctic Peninsula, the northernmost part of the continent that projects towards South America. As a direct result of temperature rise in the past 50 years or so, ice shelves on the Antarctic Peninsula have been slowly receding. Most of the ice shelves have shown two phases of recession; a climatically driven progressive recession over decades, and a more rapid ice-shelf collapse phase. Probably the most famous event was the sudden collapse over 6 weeks (February-March 2002) of the Larsen B Ice Shelf, which disintegrated with the loss of 3200 km2 of ice shelf. The recent mass loss from glaciers formerly feeding the ice shelves of the Antarctic Peninsula is estimated to be sufficient to raise eustatic sea level by between 0.1 and 0.16 ± 0.06 mm yr-1.

The 2002 Larsen B Ice Shelf collapse has been studied intensively using a number of methods. The key features of the ice-shelf collapse are:

1. Prior to its 2002 collapse, the Larsen B Ice Shelf had been in existence throughout the entire Holocene (the past 10000 years). The presence or absence of ice shelves can be detected in the sediments around the ice shelves using ocean cores (Domack et al., 2005).

2. Before it collapsed the ice shelf was fed by a number of individual tributary glaciers originating from the mountains of the Antarctic Peninsula.

3. By 2002, the ice shelf had thinned to a critical level following decades of warming, possibly to a point where structural glaciological weaknesses in the ice shelf such as crevasses and rifts caused rapid ice-shelf collapse (Glasser and Scambos, 2008).

4. Large meltwater ponds appeared on the ice-shelf surface in the summers prior to its collapse in 2002. It has been suggested that this meltwater acted as a mechanical force in the crevasses causing breaks in the ice shelf and thus accelerating ice-shelf disintegration (MacAyeal et al., 2003).

5. After 2002, the tributary glaciers responded to ice-shelf collapse by rapid thinning and acceleration (Scambos et al., 2004). This illustrates the buttressing effect of the shelf.

The images below show the collapse of the Larsen B Ice Shelf between January and March 2002 as recorded by NASA's MODIS satellite sensor. The images show the Larsen B Ice Shelf and parts of the Antarctic Peninsula (on the left). The first scene from 31 January 2002 shows the shelf in late austral summer, with dark melt ponds dotting its surface. In the next two scenes minor retreat takes place, amounting to about 800 km2, during which time several of the melt ponds well away from the ice-front drained through new cracks within the shelf. The main collapse is seen in the last two scenes, on 5 March and 7 March, with thousands of sliver icebergs and a large area of very finely divided 'bergy bits' where the shelf formerly lay. The last phases of the retreat totalled ^2600 km2. Resolution of the original images is 500 m.

2.1 The Antarctic Ice Sheet

Sources: Domack, E., Duran, D., Leventer, A., et al. (2005) Stability of the Larsen B ice shelf on the Antarctic Peninsula during the Holocene epoch. Nature, 436, 681-5. Glasser, N.F. and Scambos, T.A. (2008). A structural glaciological analysis of the 2002 Larsen B ice shelf collapse. Journal of Glaciology, 54, 3-16. MacAyeal, D.R., Scambos, T.A., Hulbe, C.L., et al. (2003). Catastrophic ice-shelf break-up by an ice-shelf-fragment-capsize mechanism. Journal of Glaciology, 49, 22-36. Scambos, T.A., Bohlander, J.A., Shuman, C.A. and Skvarca, P. (2004). Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters, 31, L18402. [Image courtesy of: NSIDC]

4. Tidewater glaciers. Where glaciers reach sea level but do not spread out to form an ice shelf they terminate as tidewater glaciers, and calve rapidly to provide another efficient means of ice loss from the ice sheet (Figure 2.4). Some glaciers, such as the neighbouring Thwaites Glacier and Pine Island Glacier, have been observed to be accelerating rapidly in recent years, at rates of up to 5% per year faster, leading to concerns that they are undergoing rapid and irreversible

Figure 2.3 The surface of the McMurdo Ice Shelf in Antarctica showing extensive debris cover and the development of surface meltponds. [Photograph: N.F. Glasser]

changes to their velocity structures. This is of concern because if this trend continues then the draw-down of ice from inland Antarctica could de-stabilise the ice sheet and potentially raise global sea levels rapidly.

Figure 2.4 Part of an ASTER satellite image acquired on 12 December 2000 showing the calving front of Pine Island Glacier in Antarctica. The image covers an area of 38 x 48 km. Note the large incipient fracture approximately 20 km behind the calving front. [Image courtesy of: NASA/GSFC/METI/ERSDAC/JAROS, and US/Japan ASTER Science Team]

Figure 2.4 Part of an ASTER satellite image acquired on 12 December 2000 showing the calving front of Pine Island Glacier in Antarctica. The image covers an area of 38 x 48 km. Note the large incipient fracture approximately 20 km behind the calving front. [Image courtesy of: NASA/GSFC/METI/ERSDAC/JAROS, and US/Japan ASTER Science Team]

5. Subglacial lakes. These are large freshwater reservoirs deep beneath the ice sheet and are found in areas of basal melting, due to high geothermal heat flux, that overlie topographic depressions. More than 70 subglacial lakes have now been identified beneath the Antarctic Ice Sheet, including the largest, Lake Vostok, which lies beneath 3 km of ice and is 230 km long, 14000 km2 in area, with a water volume of around 2000 km3. Satellite observations of the ice surface elevation in the vicinity of the subglacial lakes indicate that the lakes are connected to one another subglacially and that subglacial water moves rapidly between individual lakes by drainage beneath the ice sheet. There is landform evidence in the shape of large meltwater channel systems that some of the former Antarctic subglacial lakes may have drained catastrophically in the past (see Box 6.3).

6. Outlet glaciers and valley glaciers. Away from the fast-flowing ice streams are smaller glaciers. The most famous of these, because they feed into areas of ice-free

2.2 Greenland in the Greenhouse

land, are those of the Dry Valleys (Antartica). These cold-based glaciers have remarkably low accumulation rates of only a few centimetres per year, very low surface temperatures -30°C), very low basal temperatures -15°C) and little or no meltwater is present within the ice. As a result they have very low ice velocities and consequently documented rates of glacial erosion and landscape evolution are remarkably low in this area. In fact rates of landscape modification are so low that this environment contains a record of landscape change that can be measured in millions of years. Cold-based glaciers commonly have substantial basal debris loads, especially near their margins where stacking of debris sequences due to regelation is common and significant thicknesses of debris-rich ice can be generated, with basal debris layers of up to 5 m thickness.

0 0

Post a comment