The Heat Is Online

Antarctic Ice Shelf Break-Up Accelerates

Release from National Snow and Ice Data Center April 7, 1999

Two ice shelves on the Antarctic Peninsula known as the Larsen B and Wilkins are in "full retreat" and have lost nearly 3,000 square kilometers of their total area in the last year, say scientists in Colorado and the United Kingdom.

Researchers at the University of Colorado at Boulder's National Snow and Ice Data Center and the British Antarctic Survey attribute the retreats to a regional warming trend. The trend has caused the annual melt season to increase by two to three weeks over the last 20 years, they said.

Satellite photos monitored by NSIDC show that the Larsen B ice shelf has continued to crumble after an initial small retreat in spring 1998. In a series of events that began in November 1998, an additional 1,714 square kilometers of shelf area caved away, said Research Associate Ted Scambos of CU-Boulder's NSIDC.

On the opposite side of the peninsula, the Wilkins Ice Shelf retreated nearly 1,100 square kilometers in early March of last year, said Scambos.

Scientists looking at weather satellite imagery at that time suspected a breakup was underway and had their suspicions confirmed by radar satellite images.

"The radar images showed a large area of completely shattered ice, indicating an ice front 35 kilometers back from its previous extent," said Scambos. "The sudden appearance of thousands of small icebergs suggests that the shelves are essentially broken up in place and then flushed out by storms or currents afterward."

The British Antarctic Survey scientists had predicted one of these retreats, using computer models to demonstrate that the Larsen B was nearing its stability limit. With the small breakup observed last spring, the shelf had already retreated too far to continue to be supported by adjacent islands and shorelines.

Scientists at both institutes expected the two shelves to fail soon, but the current disintegration is occurring at an even faster rate than earlier breakups gave reason to anticipate.

"We have evidence that the shelves in this area have been in retreat for 50 years, but those losses amounted to only about 7,000 square kilometers," said David Vaughan, a researcher with the Ice and Climate Division of the British Antarctic Survey. "To have retreats totaling 3,000 square kilometers in a single year is clearly an escalation. Within a few years, much of the Wilkins ice shelf will likely be gone."

Ice shelves are floating plates of ice that are still attached to continents and which form when large glaciers flow toward the ocean in polar areas. Where they are supported by islands and sheltering coastline, they can become stable, long-term features, said Scambos.

Surface features on the Larsen B indicate that it has existed for at least 400 years. But as climate inches toward an average summertime temperature just above 0 degrees C -- the melting point of water -- the Larsen and Wilkins ice shelves have begun to disintegrate.

The Larsen B ice shelf is currently about 7,000 square kilometers -- about the size of Delaware. The Wilkins ice shelf is nearly twice that large, Scambos said.

The British researchers, who have monitored the peninsula's climate warming for decades, report an increase in mean annual temperature of about 2.5 degrees C or roughly 4.5 degrees F since the 1940's. Both groups concur that ice shelf breakup is a direct result of local climate warming.

According to Scambos, the recent warming trend has led to greater amounts of ponding melt on the shelf, weakening it. "Melt water at the surface acts to increase the extent of fracturing in the ice," he said. "The weight of the water essentially forces the cracks open, so a relatively small amount of climate warming can destroy a large, centuries-old ice shelf."

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The NSIDC is part of the Cooperative Institute for Research in Environmental Sciences, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration. Images of the Larsen B and Wilkins ice sheets are available at the following web sites:

Contact: Ted Scambos Theodore.Scambos@colorado.edu (303) 492-1113 University of Colorado at Boulder

http://www-nsidc.colorado.edu/NSIDC/ICESHELVES/lars_wilk_news

http://www.nerc-bas.ac.uk

From CNN website: April 8, 1999

Scientists: Antarctic ice shelves breaking up fast

WASHINGTON (Reuters) -- Two Antarctic ice shelves have broken up more quickly than anyone predicted, indicating that the effects of global warming may be accelerating, scientists said on Wednesday.

They published satellite images showing the Larsen B and Wilkins ice shelves in "full retreat," having lost nearly 1,100 square miles (3,000 square km) of their total area in the last year.

Ted Scambos of the University of Colorado at Boulder said his team and colleagues at the British Antarctic Survey in Cambridge had predicted the break-up would happen, but not this quickly.

"It happened much faster than we thought," Scambos said in a telephone interview. "Within this last calendar year we saw a retreat not only on Larsen but the Wilkins."

The Larsen Ice Shelf is on the eastern half of the peninsula, which is the part of the Antarctic that sticks up toward Argentina. The Wilkins is on the southwest side.

Alarmed by findings

"It was nearly as much activity in a single year as we've seen in 10 or 15 years up to now on average," Scambos said.

"To have retreats totaling 3,000 square kilometers in a single year is clearly an escalation," David Vaughan of the British Antarctic Survey said in a statement.

"Within a few years, much of the Wilkins ice shelf will likely be gone."

The researchers usually publish their findings in scientific journals and have submitted their findings to the Journal of Glaciology. But they were so alarmed by their findings that they decided to publicize them.

The effects will not be immediate. Ice shelves are floating on the ocean, so they do not cause sea levels to rise when they break up and melt.

But Scambos said the glaciers behind them could melt faster if the protective ice shelves disappear.

"Other ice shelves have huge glaciers behind them and large areas of ice to drain that are continental," Scambos said. That means the water locked up as ice in those glaciers would add to the sea level.

Sea level rising

According to the U.S. Geological Survey, global sea level has risen about 4 inches (10 cm) during the past century. It says if all the Earth's glaciers melted, which is unlikely, sea levels would rise by 260 feet (80 meters).

Antarctica, the fifth largest continent, contains about 90 percent of the world's glacial ice. Scambos thinks the satellite pictures have helped explain why the ice shelves are melting.

"Ice shelves are so large -- they are a thousand feet (300 meters) thick and many square miles (km) -- that warming at the top won't actually cause the ice to melt," he said.

"What we think instead is going on is that as these things crack naturally in the summer, the meltwater goes into the cracks."

Because the melted water is denser, it forces the cracks to open even wider.

"What we are seeing ... is an ice shelf that is essentially shattered, already being swept out. There are thousands of relatively small icebergs," he said.

Scientists believe the Larsen B ice shelf has existed for at least 400 years. But the local climate is inching toward an average summertime temperature just above 32 degrees Fahrenheit (0 degrees Celsius) -- the melting point of water.

The British Antarctic Survey reports an increase in mean annual temperature in the region of about 4.5 degrees Fahrenheit (2.5 degrees Celsius) since the 1940s.