As has been widely reported in the media, massive icebergs have sheared off Antarctica's West Ice Sheet in recent years. An iceberg forms when a large chunk of ice breaks free, or calves, from an ice shelf -- a floating extension of a landed ice sheet. As with an ice cube floating in a glass of water, an iceberg displaces the same volume of water that it contains. So, if it were to melt, the sea level would stay exactly the same.

The fact that sea ice doesn't affect sea level when it melts doesn't mean that the breakup of an ice shelf is inconsequential. An ice shelf acts as a buttress that prevents a landed ice sheet -- whose melting would cause sea level to rise -- from sliding into the sea and increasing the volume of water. In fact, if you divided the volume of the water frozen in the entire West Sheet by the total surface area of the seas in which it would melt, you would determine that its demise would raise the global sea level by as much as 6 meters. It is estimated that a sea-level rise of less than half a meter would inundate up to 300 meters of seashore, and that such flooding would displace millions of people in coastal cities and low-lying islands worldwide.

Rising global temperatures are thought to be responsible for accelerated glacial movement. Climate records over the past 50 years show that air temperatures have increased 2.5°C in the Antarctic Peninsula, which juts off the West Sheet. This is about five times greater than warming measured for the rest of the world. Other evidence, such as the increased incidence of ice stream formation, corroborates the proposed connection between increased temperatures and decreased ice in Antarctica.

After the observed Larsen Ice Shelf collapse in 2002, measurements of glacial movement on the West Sheet indicate that some glaciers are moving eight times as fast as before this breakup, dumping nearly 25 cubic kilometers more ice into the ocean each year. These results demonstrate that not all parts of the globe respond to increased temperatures equally, and that the ice-covered regions of the Antarctic appear to be particularly sensitive to even small changes in global temperature.

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