Ice shelf

An ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are found in Antarctica, Greenland and Canada only. The boundary between the floating ice shelf and the grounded (resting on bedrock) ice that feeds it is called the grounding line. When the grounding line retreats inland, water is added to the ocean and sea level rises.

In contrast, sea ice is formed on water, is much thinner, and forms throughout the Arctic Ocean. It also is found in the Southern Ocean around the continent of Antarctica.

Ice shelves flow by gravity-driven horizontal spreading on the ocean surface. That flow continually moves ice from the grounding line to the seaward front of the shelf. The primary mechanism of mass loss from ice shelves is iceberg "calving", in which a chunk of ice breaks off from the seaward front of the shelf. Typically, a shelf front will extend forward for years or decades between major calving events. Snow accumulation on the upper surface and melting from the lower surface are also important to the "mass balance" of an ice shelf.

The thickness of modern-day ice shelves ranges from about 100 to 1000 meters. The density contrast between glacial ice, which is denser than normal ice, and liquid water means that only about 1/9 of the floating ice is above the ocean surface. The world's largest ice shelves are the Ross Ice Shelf and the Filchner-Ronne Ice Shelf in Antarctica.

The term captured ice shelf has been used for the ice over a subglacial lake, such as Lake Vostok.

Canadian ice shelves

All Canadian ice shelves are attached to Ellesmere Island and lie north of 82°N:
*Alfred Ernest Ice Shelf
*Ayles Ice Shelf (broke up in 2005)
*M'Clintock Ice Shelf (broke up 1963-1966)
*Markham Ice Shelf (broke up in 2008)
*Milne Ice Shelf
*Ward Hunt Ice Shelf
*Smith Ice Shelf

Antarctic ice shelves

44 percent of the Antarctic coastline has ice shelves attached. Their aggregate area is 1,541,700 km² [http://www.stat.fi/tup/maanum/01_maantieteellisia_tietoja_aihepiireittain.xls] . The individual ice shelf areas are listed below, in a clockwise manner, starting in the west of Eastern Antarctica:
*Abbot Ice Shelf
*Amery Ice Shelf
*Baka Ice Shelf
*Brunt Ice Shelf
*Cook Ice Shelf
*Cosgrove Ice Shelf
*Crosson Ice Shelf
*Dotson Ice Shelf
*Edward VIII Ice Shelf
*Ekstrom Ice Shelf
*Filchner Ice Shelf
*Fimbul Ice Shelf
*George VI Ice Shelf
*Getz Ice Shelf
*Gillett Ice Shelf
*Hannan Ice Shelf
*Jelbart Ice Shelf
*Jones Ice Shelf
*Larsen Ice Shelf
*Lazarev Ice Shelf
*McMurdo Ice Shelf
*Moscow University Ice Shelf
*Müller Ice Shelf
*Nansen Ice Sheet
*Nickerson Ice Shelf
*Prince Gustav Ice Shelf
*Publications Ice Shelf
*Quar Ice Shelf
*Riiser-Larsen Ice Shelf
*Ronne Ice Shelf
*Ross Ice Shelf
*Shackleton Ice Shelf
*Slava Ice Shelf
*Stange Ice Shelf
*Sulzberger Ice Shelf
*Swinburne Ice Shelf
*Venable Ice Shelf
*Voyeykov Ice Shelf
*West Ice Shelf
*Wilkins Ice Shelf
*Wordie Ice Shelf
*Wyers Ice Shelf
*Zubchatyy Ice Shelf

Ice shelf disruption

In the last several decades, glaciologists have observed consistent decreases in ice shelf extent through melt, calving, and complete disintegration of some shelves. [ [http://news.yahoo.com/s/afp/20080710/sc_afp/warmingantarcticaice "Antarctic ice shelf 'hanging by thread': European scientists"] . July 10, 2008. "Yahoo! News".]

The Ellesmere ice shelf reduced by 90 percent in the twentieth century, leaving the separate Alfred Ernest, Ayles, Milne, Ward Hunt, and Markham Ice Shelves. A 1986 survey of Canadian ice shelves found that 48 km². (3.3 cubic kilometers) of ice calved from the Milne and Ayles ice shelves between 1959 and 1974. [Jeffries, Martin O. [http://pubs.aina.ucalgary.ca/arctic/Arctic39-1-15.pdf Ice Island Calvings and Ice Shelf Changes, Milne Ice Shelf and Ayles Ice Shelf, Ellesmere Island, N.W.T.] . "Arctic" 39 (1) (March 1986)] The Ayles Ice Shelf calved entirely on August 13, 2005. The Ward Hunt Ice Shelf, the largest remaining section of thick (>10 m) landfast sea ice along the northern coastline of Ellesmere Island, lost 600 square km of ice in a massive calving in 1961-1962. [Hattersley-Smith, G. The Ward Hunt Ice Shelf: recent changes of the ice front. "Journal of Glaciology" 4:415-424. 1963.] It further decreased by 27% in thickness (13 m) between 1967 and 1999. [Vincent, W.F., J.A.E. Gibson, M.O. Jeffries. [http://www.cen.ulaval.ca/warwickvincent/PDFfiles/149.pdf Ice-shelf collapse, climate change, and habitat loss in the Canadian high Arctic] . "Polar Record" 37 (201): 133-142 (2001)] In summer 2002, the Ward Ice Shelf experienced another major breakup. [cite web|url=http://earthobservatory.nasa.gov/Study/wardhunt/ |title=Breakup of the Ward Hunt Ice Shelf |author=NASA Earth Observatory]

Two sections of Antarctica's Larsen Ice Shelf broke apart into hundreds of unusually small fragments (100's of meters wide or less) in 1995 and 2002.

The breakup events may be linked to the dramatic polar warming trends that are part of global warming. The leading ideas involve enhanced ice fracturing due to surface meltwater and enhanced bottom melting due to warmer ocean water circulating under the floating ice.

The cold, fresh water produced by melting underneath the Ross and Flichner-Ronne ice shelves is a component of Antarctic Bottom Water.

It is a common misconception that the melting of floating ice shelves will not raise sea levels at all. However, there is a small effect. Because ice shelves are fresh ice, when melted their water is less dense than sea water; i.e they have greater volume for given mass than sea water. The volume of the sea water needed to displace the ice shelf is less than the volume of the water contained in the ice shelf. If the ice is melted, a small fraction of the volume of the ice that is above sea level is added the volume of the seas, increasing sea level. [ [http://www.physorg.com/news5619.html Peter Noerdlinger, PHYSORG.COM "Melting of Floating Ice Will Raise Sea Level"] ]

References

ee also

Ice sheet dynamicsWikinews
*

External links

* http://www.antdiv.gov.au/default.asp?casid=1547 - from the Australian Antarctic Division
* http://nsidc.org/iceshelves - from the U.S. National Snow and Ice Data Center
* http://www.cnn.com/2006/TECH/science/12/29/canada.arctic.ap/index.html - CNN story about the Canadian Ayles ice shelf break up in August 2005
* http://ice-glaces.ec.gc.ca/ - from the Canadian Ice Service