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Arctic icebergs are chunky free-floaters, like the one that put a tragic end to the maiden voyage of the Titanic. At the other extreme of the planet lurk the behemoths of berg-dom: island-sized slabs that clunk and bump around the periphery of Antarctica, sometimes for decades, rearranging geography and wildlife habitat as they go.

Eye in the Sky

Icebergs such as C-16, shown here in this photograph from November 2000, may threaten shipping lanes.
Icebergs such as C-16, shown here in this photograph from November 2000, may threaten shipping lanes.

Scientists have always been fascinated with icebergs, but until recently the extremes of polar weather have made it impossible to observe them up close or even very often.

Today, using images from polar-orbiting satellites scientists are watching icebergs in virtual realtime as they separate from parent ice. Once liberated, they can be tracked with precision.


Two iceberg events in Antarctica this past year have attracted particular attention. In January the Ninnis Glacier lost its “tongue,” the 350-square-mile (900-square-kilometer) extension that had floated at the seaward end of the tidewater glacier since early in the century. When it broke off and floated off into the Indian Ocean, the coastline of eastern Antarctica changed significantly.

The tongue was first observed between 1911 and 1914 by members of the Australasian Antarctic Expedition, who described it as extending 87 miles (140 kilometers) “out into the ice-strewn sea.” Over the decades the tongue lost about a third of its size. Shearing was evident in Landsat images in 1989 but separation didn’t occur until more than a decade later. Paul Massom, a research scientist with the Antarctic Cooperative Research Centre at the University of Tasmania in Australia, noticed the channel forming while studying satellite images.

“The movement of the berg followed soon afterwards,” says Massom in a NASA report. The separation and “the resultant change in the coastal configuration may have significant impact on sea-ice and polynya behavior in the region.” Polynyas are open-water areas surrounded with sea ice that scientists believe have significance for fish, seabirds and marine mammals.

Three months after the Ninnis Glacier lost its tongue, the largest iceberg ever recorded broke away from the Ross Ice Shelf. When “B-15” drifted off into the Ross Sea, it was more than 4,200 square miles (11,000 square kilometers)—about the size of the Connecticut—above the water and ten times that size below. Since March the berg has fractured into several pieces. Scientists at the University of Wisconsin’s Antarctic Research Center plan to install weather stations and global positioning transponders on the sections to monitor their changing positions.

Unlike Arctic icebergs that move south on prevailing ocean currents, Antarctic icebergs rarely threaten ships. They tend to hug the continent, thumping along, sometimes even grounding themselves in a new place. Because these bergs are so big, they often profoundly affect wildlife as they move: relocating communities of sea life, obstructing the movement of sea ice and affecting the formation of “fast ice,” the thick layer that binds the sea surface in winter. Fast ice provides marine mammals a platform on which to interact, mate, and forage.

Iceberg movements have even erased history. Sometime during the 1960s a huge piece of sheet ice broke away from the Ross Ice Shelf and carried off whatever remnants there were of the base camp built by Roald Amudsen during his expedition to the South Pole in 1911-12. At the same time it swept away the five Little America camps set up by Admiral Richard Byrd during his expeditions between 1929 and the mid-1950s.


Scientists downplay the role of global warming in the formation of icebergs. Icebergs have been common in the North Atlantic since the early days of ocean travel, long before global warming was an issue. And Antarctica’s ice sheets have been observed retreating and advancing as far back as 1841, when British explorer Sir James Clark Ross first described the “great ice barrier.” Glaciologists believe that rising and falling tides and storm waves are the major causes of cracks that eventually work through ice as thick as 3,300 feet (1,000 meters). Antarctica’s ice sheets maintain themselves by breaking off every 50 to 100 years—rather like keeping one’s nails clipped.

Icebergs won’t cause sea levels to rise. Because they were part of ice that was floating to begin with, they will not add volume once separated. However, there is some concern about the potential of loss of too much of Antarctica’s ice sheet. The sheet is believed to restrain the glacial ice that covers the continent. If enough of it disappeared, the ancient ice would begin drifting into the sea. Within 250 years the melting of the West Antarctic Land Sheet, scientists say, could raise sea levels by 20 feet (6 meters).

Looking on the bright side, futurists view icebergs as a possible answer to the world’s growing freshwater needs. As chucks of ancient glacial ice, icebergs consist of pure fresh water. A behemoth the size of, say, B-15 could be towed to Los Angeles where it could supply the city’s water needs for hundreds of years.

Eye in the Sky is a weekly series that brings you the story behind the headlines using satellite imagery, remote sensing, aerial photography, and maps. This feature is developed by National Geographic News with the sponsorship of the National Imagery and Mapping Agency (NIMA) and Earth-Info. Check out maps and imagery at

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More Information
•  “Ice sheets,” found only in Antarctica and Greenland, are enormous masses of glacial ice and snow that cover land areas of 19,300 square miles (50,000 square kilometers) or larger.
•  “Ice shelves” are extensions of ice sheets into the sea.
•  Antarctica is nearly surrounded with ice shelves, which create island-sized bergs when they calve.
•  Most North Atlantic icebergs break off of tidewater glaciers, which flow through valleys into the sea.
•  An ice tongue is the floating extension of a glacier out into the sea.

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“Iceberg Alley”

Ships beware. During iceberg season—February to July —the eastern edge of the Grand Banks becomes one of the most treacherous places in the North Atlantic. In this area of the ocean east of Newfoundland- also known as Iceberg Alley—huge chunks of ice litter the sea surface, lingering sometimes well into late summer. From Iceberg Alley came the bergs that sank R. M. S. Titanic in 1912 and the S.S. Hanshedtoft in 1959. The two tragedies cost more than 1,600 lives.

Most of Iceberg Alley’s bergs originate from a hundred or so tidewater glaciers along the western coast of Greenland. Together they calf 10,000 to 15,000 icebergs each year, according to the International Ice Patrol. Glaciers on Alaska’s Ellesmere Island and along the eastern coast of Greenland contribute the rest. The Labrador Current funnels these bergs south, sometimes into major shipping lanes.

The largest North Atlantic iceberg on record rose 550 feet out of the ocean—almost as tall as the Washington Monument. But size isn’t what counts. Smaller icebergs can be more dangerous because they are less likely to show up on a ship’s radar. Those with flat tops are more difficult to see. Because seven-eighths of an iceberg is below water, even small ones can cause great damage to a ship’s hull.

Icebergs melt rapidly once they move south of 41 N latitude, where they meet the warmer waters of the Gulf Stream. But occasionally bergs are spotted farther south. In 1926 a small iceberg was spotted off Bermuda.