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), 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
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
Ice shelves around the Antarctic Peninsula are retreating. 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 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’.
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.
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. 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.
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, 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.
Sea ice in the Antarctic is currently increasing. 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.
This video explains in more detail about changes in sea ice.
You can explore changes in sea ice using this ArcGIS App.
- Ice shelf collapse
- Antarctic Peninsula ice shelves
- Pine Island Glacier
- Antarctic Peninsula glacier change
- Quick facts on icebergs (NSIDC), ice shelves and sea ice
- Coloured icebergs
- The Antarctic iceberg tracking database
- NASA Earth Observatory Sea Ice
- Antarctic Ice Shelves – the hidden villan
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).