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Floating Ice Shelves Key to Ice Sheet Stability

In news that promises to rekindle debate over the importance of ice shelves in stabilizing the larger ice sheets on the Antarctic continent, researchers report that satellite images and aerial mapping surveys have revealed surges in the seaward flow rates of glaciers on the Antarctic Peninsula. The work, published in the current issue of the journal Science, provides the first concrete evidence in support of a decades-old idea that calving coastal ice shelves may set in motion a chain of events that could throw global sea level rise into high gear.

The 30-year-old theory holds that following the loss of floating ice shelves that cling to the much larger and more stable continental ice sheets, the seaward flow of glaciers sharply increases, or surges. The fast-moving ice streams thus drain the hulking West Antarctic Ice Sheet into the sea. If this were to occur, the tremendous volume of ice splashing down could eventually raise the global sea level by five to seven meters--enough to drown the lower third of Florida. Over the past 10 years, however, glaciologists have presented mathematical models discrediting this notion.

Now Hernan De Angelis and Pedro Skvarca of the Instituto Antartico Argentino in Buenos Aires report having documented glacial surges over a period of nearly eight years, following the start of the disintegration of Antarctica¿s Larsen Ice Shelf. In the mid-1990s, the team flew over the region and noticed ice terraces, or slices of ice attached to solid rock, suggesting that a glacier had recently ripped away from the surface. During the next several years, they conducted aerial Global Positioning System (GPS) mapping surveys, analyzed satellite imagery and identified five major glaciers on the northern end of the peninsula where surges had taken place. They found that over a period of four years, the flow velocity of one of the glaciers increased three-fold and dumped 24 square kilometers of ice into the Weddell Sea. A second GPS survey in early 2002 revealed that the glacial surge had ended and the ice was retreating inland--a sign of warming.

"The discovery is of global importance because it shows that five major glaciers affected by ice-shelf removal are rapidly contributing to the sea level rise," the researchers explain. "Although this affected only a small portion of the Antarctic Peninsula, it demonstrated that ice shelves can play an important role on the force balance of some glaciers."

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