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Ice Caps and Sea Levels

Ice Caps, Ice Sheets, and Ice Shelves: What's the Difference?

Glaciers in the Straits of Magellan in Southern Chile.

An ice sheet is a chunk of glacier ice that covers the land surrounding it and is greater than 50,000 kilometers (20,000 miles) wide. An ice sheet is also known as a continental glacier. During the last glacial period the Laurentide ice sheet covered much of Canada and North America, the Weichselian ice sheet covered Northern Europe, and the Patagonian ice sheet covered much of Southern South America. Currently, the only ice sheets on Earth are in Antarctica and Greenland.

Ice sheets are usually warmer at their base than on the surface. That is because of geothermal heat. When melting occurs, the water lubricates the sheet and it flows more quickly. This produces fast-flowing channels called ice streams.

Antarctic Ice Sheet

The Antarctic ice sheet is the largest single mass of ice on Earth. It covers almost 14 million square kilometers, has 30 million cubic kilometers of ice, and holds approximately 90 percent of the freshwater on our planet. If melted, the Antarctic ice sheet would raise sea levels by 61.1 meters.

The Antarctic ice sheet is divided by the Transantarctic Mountains. On one side is the East Antarctic ice sheet (EAIS), with the West Antarctic ice sheet (WAIS) on the other side of the mountains. Interestingly, the West Antarctic ice sheet lies below sea level, meaning that the WAIS would be seabed if the ice sheet was not there. The WAIS is next to the Ross and Ronne ice shelves.

Greenland Ice Sheet
Map of Greenland Ice Sheet
Map of the Greenland ice sheet.

The Greenland ice sheet covers approximately 82 percent of Greenland’s surface. If melted, this ice sheet would raise sea levels by 7.2 meters. NASA’s Gravity and Recovery and Climate Experiment results estimate that 239 cubic kilometers (57.3 cubic miles) of the ice sheet are melting each year.

Ice Caps

Ice caps cover less than 50,000 square kilometers and usually feed a series of glaciers around its edges. While not hemmed in by any surface features (they lie on top of mountains), they are usually centered on a highest point (called a massif). Ice flows away from this highest point toward the cap’s edges.

If a glacier moves or retreats, distinctive erosional features are formed. The Great Lakes in North America were formed by glacial action.

Polar ice caps are high-latitude regions covered in ice. They are not strictly an ice cap (because they are usually larger than the 50,000 square kilometer limit used to define ice caps), but most people refer to these areas as ice caps anyway.

Ice Shelves

An ice shelf is a thick, floating slab of ice that forms where a glacier or ice flows down a coastline. Ice shelves are found only in Antarctica, Greenland, and Canada. Thicknesses of floating ice shelves range from 100-1,000 meters.

Ice shelves are formed by forces of gravity from ice along the shore. Gravity constantly pressures the movement of ice from the land to the shelf. Ice shelves lose mass when chunks break off and slide into the ocean water. Shelves gain mass by snow accumulation on the upper surface.

The Ross Ice Shelf in 1997
The Ross ice shelf in 1997.

The world’s largest ice shelves are the Ross ice shelf and the Filchner-Ronne ice shelf in Antarctica. The Larsen ice shelf in Antarctica broke up into hundreds of small fragments in 1995 and 2002. Global warming trends may have been a factor in the breakup of the ice shelf.

Canadian ice shelves are attached to Ellesmere Island. The Ayles ice shelf broke up in 2005, the M’Clintock ice shelf broke from 1963 to 1966, and the Markham ice shelf broke up in 2008. The only Canadian shelves still existing are the Alfred Ernest, Milne, Ward Hunt, and Smith ice shelves.

The remaining ice shelves were formed with the Ellesmere ice shelf was reduced by 90 percent during the 20th century.