Alister Doyle, Reuters 4 Apr 09;
OSLO (Reuters) - An ice bridge which had apparently held a vast Antarctic ice shelf in place during recorded history shattered on Saturday and could herald a wider collapse linked to global warming, a leading scientist said.
"It's amazing how the ice has ruptured. Two days ago it was intact," David Vaughan, a glaciologist with the British Antarctic Survey, told Reuters of a satellite image of the Wilkins Ice Shelf on the Antarctic Peninsula.
The satellite picture, from the European Space Agency (ESA), showed that a 40 km (25 mile) long strip of ice believed to pin the Wilkins Ice Shelf in place had splintered at its narrowest point, about 500 meters wide.
"We've waited a long time to see this," he said.
The Wilkins, now the size of Jamaica or the U.S. state of Connecticut, is one of 10 shelves to have shrunk or collapsed in recent years on the Antarctic Peninsula, where temperatures have risen in recent decades apparently because of global warming.
The ESA picture showed a jumble of huge flat-topped icebergs in the sea where the ice bridge had been on Friday, pinning the Wilkins to the coast and running northwest to Charcot Island.
"Charcot Island will be a real island for the first time in history," Vaughan said.
Vaughan, who landed on the flat-topped ice bridge on the Wilkins in January in a ski-equipped plane with other scientists and two Reuters reporters, said change in Antarctica was rarely so dramatic. It was the first -- and last -- visit to the area.
The loss of the ice bridge, jutting about 20 meters out of the water and which was almost 100 km wide in 1950, may now allow ocean currents to wash away far more of the Wilkins shelf.
"My feeling is that we will lose more of the ice, but there will be a remnant to the south," said Vaughan. Ice shelves float on the water, formed by ice spilling off Antarctica, and can be hundreds of meters thick.
Nine other shelves have receded or collapsed around the Antarctic Peninsula in the past 50 years, often abruptly like the Larsen A in 1995 or the Larsen B in 2002 further north.
DISAPPEAR FROM MAP
Cores of sediments on the seabed indicate that some of these ice shelves had been in place for at least 10,000 years. Vaughan said an ice shelf would take many hundreds of years to form.
In January, the remaining ice bridge had been surrounded by icebergs the size of shopping malls, many of them trapped in sea ice. A few seals were visible lolling on sea ice in the low Antarctic sunshine.
On that visit, Vaughan put up a GPS satellite monitoring device and predicted the ice bridge would break within weeks. The plane left quickly, in case the ice was unstable on a part of the world about to disappear from the map.
Temperatures on the Antarctic Peninsula have risen by up to about 3 Celsius (5.4 Fahrenheit) in the past 50 years, the fastest rate of warming in the Southern Hemisphere.
"We believe the warming on the Antarctic Peninsula is related to global climate change, though the links are not entirely clear," Vaughan said. Antarctica's response to warming will go a long way to deciding the pace of global sea level rise.
About 175 nations have been meeting in Bonn, Germany, since March 29 as part of a push to agree by the end of 2009 a new U.N. treaty to combat climate change. The talks end on April 8.
The loss of ice shelves does not affect sea levels -- floating ice contracts as it melts and so does not raise ocean levels. But their loss can allow glaciers on land to slide more rapidly toward the sea, adding water to the oceans.
The Wilkins does not have much ice pent up behind it. But bigger ice shelves to the south on the frozen continent, where no major warming has been detected, hold back far more ice.
Ice bridge ruptures in Antarctic
BBC News 4 Apr 09;
An ice bridge linking a shelf of ice the size of Jamaica to two islands in Antarctica has snapped.
Scientists say the collapse could mean the Wilkins Ice Shelf is on the brink of breaking away, and provides further evidence or rapid change in the region.
Sited on the western side of the Antarctic Peninsula, the Wilkins shelf has been retreating since the 1990s.
Researchers regarded the ice bridge as an important barrier, holding the remnant shelf structure in place.
Its removal will allow ice to move more freely between Charcot and Latady islands, into the open ocean.
European Space Agency satellite pictures had indicated last week that cracks were starting to appear in the bridge. Newly created icebergs were seen to be floating in the sea on the western side of the peninsula, which juts up from the continent towards South America's southern tip.
Professor David Vaughan is a glaciologist with the British Antarctic Survey who planted a GPS tracker on the ice bridge in January to monitor its movement.
He said the breaking of the bridge had been expected for some weeks; and much of the ice shelf behind is likely to follow.
"We know that [the Wilkins Ice Shelf] has been completely or very stable since the 1930s and then it started to retreat in the late 1990s; but we suspect that it's been stable for a very much longer period than that," he told BBC News.
"The fact that it's retreating and now has lost connection with one of its islands is really a strong indication that the warming on the Antarctic is having an effect on yet another ice shelf."
While the break-up will have no direct impact on sea level because the ice is floating, it heightens concerns over the impact of climate change on this part of Antarctica.
Over the past 50 years, the peninsula has been one of the fastest warming places on the planet.
Many of its ice shelves have retreated in that time and six of them have collapsed completely (Prince Gustav Channel, Larsen Inlet, Larsen A, Larsen B, Wordie, Muller and the Jones Ice Shelf).
Separate research shows that when ice shelves are removed, the glaciers and landed ice behind them start to move towards the ocean more rapidly. It is this ice which can raise sea levels, but by how much is a matter of ongoing scientific debate.
Such acceleration effects were not included by the UN's Intergovernmental Panel on Climate Change (IPCC) when it made its latest projections on likely future sea level rise. Its 2007 assessment said ice dynamics were poorly understood.
