An ice shelf is a floating platform of ice, usually fed by mountain glaciers or ice sheets. Ice shelves advance over the ocean for several decades until they become unstable, at which point large icebergs break off (calve) from the ice shelf front. Advancing and retreating is a normal process for maintaining ice volume.

The enormous volume of ice within the Antarctic Ice Sheet - about 30 million cubic kilometers (13.5 million square kilometers or 5.3 million square miles) - contains about 91 percent of the total volume of glacier ice on Earth. If it melted, it would raise the sea level about 65 meters (213 feet). Ice shelves cover fifty percent of the Antarctic coast. The surface area of all ice shelves taken together is more than one tenth the size of the continent. The largest individual shelf is the Ross Ice Shelf in West Antarctica, also called the Great Ice Barrier. It is as big as France (an area of roughly 487,000 sq.km) and fed by no less than seven ice streams. It is about 800 km across and several hundred meters thick. The nearly vertical ice front to the open sea is more than 600 km long and between 15 and 50 meters high above the water surface. 90 percent of the floating ice is below the water surface.

Iceberg B-15 broke from the Ross Ice Shelf in Antarctica and it is among the largest ever observed, this new iceberg is approximately 183 miles long (295 kilometers) x 25 miles wide (37 kilometers). Its 4,250 square-mile area is nearly as large as the state of Connecticut. It extends about 900 feet below the surface and rises about 120 feet (30 meters) above the ocean. The iceberg was 'calved', or broke off from, the main ice sheet about 200 miles east of the National Science FoundationÃs (NSF) McMurdo Station as measured from its western edge

The iceberg was formed from glacial ice moving off the Antarctic continent and calved along pre-existing cracks in the Ross Ice Shelf near Roosevelt Island. The calving of the iceberg essentially moves the northern boundary of the ice shelf about 25 miles to the south, a loss that would normally take the ice shelf as long as 50-100 years to replace.

The shear face of an iceberg that broke off the Ross Ice Shelf in Antarctica
High-resolution radar map of Antarctica The red arrow points to the Ross Ice Shelf and the Ross Sea.(Courtesy: NASA and Ohio State University. As seen from the Canadian satelite RADARSAT in October 1999

Larsen Ice Shelf

(72°40′S 16°0′W) is an ice shelf about 250 miles long on the coast of Queen Maud Land

Every year, the edges of ice shelves break off, or calve, into icebergs as a result of seasonal warming. Rarely, a very large iceberg will fall into the ocean. During normal years, the total mass of calvings is an extremely small percentage of the ice cap, and the ice lost through calving equals the mass of snowfall on the continent. Therefore, the shape of Antarctica remains in equilibrium. Or so it has been until recently. Significant changes in the volume and dynamics of the ice sheet, therefore, would cause global environmental changes. In turn, changes in the temperature of the atmosphere and oceans can affect the volume and dynamics of the Antarctic Ice Sheet.

Location of the Larsen-B ice shelf.

Ice shelves and outlet glaciers, which move seaward on a decadal time scale, are floating extensions of a grounded ice sheet. They are composed of fresh water ice that originally fell as snow and are largely considered geographically permanent features. When an ice shelf or an outlet glacier extends far enough seaward to be affected by tides and currents, a tabular or irregular piece breaks off and floats away. While the calving process is usually part of a natural cycle, the calving of a large iceberg may be a sign of a more serious disintegration.

Beginning on January 31st 2002 and continuing over the course of the next thirty five days 3250km2 of the Larsen-B ice shelf in Antarctica disintegrated . The size and speed with which this huge piece of ancient ice collapsed shocked scientists and feared about the immediate consequences of global warming.

The bergs in the background are about 25 m high. Photo courtesy of Keith Nicholls, British Antarctic Survey

It has been discovered by scientists that the Larsen ice shelf is thinning due to warmer oceans around it. . The Larsen ice shelf was about 300 meters thick. When two previous sections of the shelf thinned to about 200 meters they quickly disintegrated. Current estimates suggested that the Larsen ice shelf would begin to disintegrate rapidly by about 2070, although that was likely to happen sooner if current warming trends continued. The disappearance of the ice shelf might also affect the local ice sheets, large bodies of ice trapped on land by the ice shelf.

Ice shelves act as a buttress, or braking system, for glaciers. Further, the shelves keep warmer marine air at a distance from the glaciers; therefore, they moderate the amount of melting that occurs on the glaciers' surfaces. Once their ice shelves are removed, the glaciers increase in speed due to meltwater percolation and/or a reduction of braking forces, and they may begin to dump more ice into the ocean than they gather as snow in their catchments. Glacier ice speed increases are already observed in Peninsula areas where ice shelves disintegrated in prior years.

This photo released by Greenpeace shows a crack in the Larsen B ice shelf in the Weddell sea, in Antarctica.

The average air temperature over the Antarctic Peninsula, a finger of land in the west jutting out toward South America, has risen 2.5° C in the last 50 years, much more rapidly than the rest of the world. The highly variable climate of the region is sensitive to the amount of winter sea ice, frozen sea water that forms around the Peninsula in the fall and which mostly melts in summer.

Average annual temperatures have now increased to -3° C, plant life is exploding, vegetation is reported to have increased 25-fold. The iceberg which broke off from the Larsen ice shelf on the east side of the Peninsula in January is 37 by 77 kilometers and 183 meters thick, the size of Rhode Island or Luxembourg. Without intact ice shelves to cool them, winds blowing over Antarctica will be warmer than usual this year and carry more moisture.

Over the next months and years the icebergs will drift north under the action of ocean currents and wind. The giant iceberg will melt rapidly once it crosses the Antarctic Convergence and enters warmer water. The calving of icebergs is normal within the lifecycle of ice shelves, which may be quiet for decades between calving events. Exceptionally the calving of one large iceberg may be a fore-runner of a more serious disintegration. It will take time before the implications are completely understood.

In theory, the breaking up of ice shelves could be a first step toward diminution of the West Antarctic Ice Sheet. If the rate of calving increases substantially, this could be a signal of global warming. The striking warming of the Peninsula that has taken place since 1940 may be part of a natural fluctuation; it is also possible that human-induced global warming could be speeding up the process.

The chart below shows some of the many calving events in the last 25 years. In 1986, three huge icebergs, larger than this year's, broke loose into Antarctic seas. Two were grounded nearby in shallow water and are still there. The third drifted north, reaching the tropical latitudes before melting completely, scientists believe.

During the past few years, ice shelves on the Antarctic Peninsula have been quickly melting. In fact, most of the major ice shelves on the Antarctic Peninsula have been fatally impacted. The Wordie collapsed in the late 1980s, and the Prince Gustav and Larsen A Ice Shelves disintegrated in 1995. As these shelves melt, they are permanently changing the shape of the continent. James Ross Island, once surrounded by Larsen, is free of ice and circumnavigable for the first time. Three more shelves, according to the National Snow and Ice Data Center, the National Oceanic and Atmospheric Administration, and the University of Colorado, have receded past a "point of no return." These three are the Larsen B, the Wilkins, and the George VI Ice Shelves.


AREA AND ESTIMATED VOLUME OF SELECTED CALVING EVENTS IN ANTARCTICA Source: R.S.Williams, Jr., & J.G.Ferrigno, USGS

The weight of Antarctica's ice is so enormous that it has literally pressed the continent two thirds of a mile into the earth. Under the massive forces of their own weight, the ice sheets deform and drag themselves outward. The interior ice flows slowly over rough bedrock.

The West Antarctic Ice Sheet (rests on a submerged volcanic archipelago about the size of the Philippine Islands) is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. It is bounded by the Ross Ice Shelf, the Ronne Ice Shelf (two major ice shelves toward the sea), and outlet glaciers that drain into the Amundsen Sea. It is estimated that it contains seven million cubic miles of ice. If the above sea level, grounded part of the West Antarctic Ice Sheet were to melt or float, sea level would rise about 6 meters (nearly 20 feet) around the world, with devastating effects, particularly on the Earth's coasts and low-lying islands.

Global warming could create an even more catastrophic scenario in East Antarctica, where a much larger ice sheet (the East Antarctic Ice Sheet), up to 4.8 kilometers (nearly 3 miles) thick, rests on a buried mountainous continent.

Data accumulated to date show that several ice shelves around the Antarctic Peninsula are shrinking; parts of the Larsen Ice Shelf are in an active state of disintegration. It has been reported a dense plume of ice fragments extended several hundred kilometers seaward. The Wordie Ice Shelf on the other side of the peninsula no longer exists.

Although B-15 is gigantic, its breakaway reduced the size of the Ross Ice Shelf by only two percent. The frozen ice sheet over the Ross Sea helps preserve the lifespan of the glacier that feeds it, for two reasons. It helps break the glacier's journey to the sea. And, it adds distance between the glacier and the open sea water, which has a warming effect. It gets as thick as 3000 feet thick in some areas, and melts slowly from the base, becoming gradually thinner by the time it reaches the sea (it’s about 250 feet thick at the drill site).

The East and West Antarctic ice sheets are reaching two and a half miles thick and containing 70% of the world's freshwater. Since ice shelves act as buttresses to the continental ice streams. So if the Ross Ice Shelf collapsed, the ice streams flowing out from West Antarctica would speed up and that would cause sea level to rise—something humans (especially the scientists) are keen to avoid.

Scientists believe that Ross Ice Shelf collapse would rise sea levels between 5 to 17 m. They want to get a pretty detailed history of the ice shelf, going abruptly from full glacial conditions to open ocean conditions.

The Larsen Ice Shelf collapse in 2002 had already proved that ice shelf can desintegrate "extremely quickly. Once dating of the sample was completed, researchers would be able to look at what the ice shelf was doing during periods when scientists knew from other evidence that it was 2 degree C to 4 degree C warmer than today.