Andreas Muenchow, professor of ocean science and engineering at the University of Delaware, said satellite images have revealed that the glacier has lost about a quarter of its 43-mile-long floating ice shelf.
The last time such a large ice island formed was in 1962 when the Canadian Ward Hunt Ice Shelf calved an island. Smaller pieces of that chunk became lodged between real islands inside the Nares Strait.
Muenchow said he had expected an ice chunk to break off from Petermann, one of the two largest remaining glaciers in Greenland, because it had been growing in size for seven or eight years. But he said he did not expect it to be so large.
“The freshwater stored in this ice island could keep the Delaware or Hudson Rivers flowing for more than two years,” said Muenchow, whose research in the area is supported by the National Science Foundation.
“It could also keep all US public tap water flowing for 120 days.”
He said it was hard to judge whether the event occurred due to global warming because records on the sea water around the glacier have only been kept since 2003.
“Nobody can claim this was caused by global warming. On the other hand nobody can claim that it wasn’t,” Muenchow said, adding that the flow of sea water below the glaciers is one of the main causes of ice calvings off Greenland.
Regine Hock, a glacial geophysicist at the University of Alaska Fairbanks, told the National Geographic that the breakup of ice shelves is “a normal process that happens all the time”.
But she said that such a “huge, huge piece of ice … is very unusual”.
Scientists have said the first six months of 2010 were the hottest globally on record. The El Niño weather pattern has contributed to higher temperatures, but many scientists say elevated levels of man-made greenhouse gases are pushing temperatures higher.
The initial discovery of the breakaway island was made by Trudy Wohlleben of the Canadian Ice Service.
Experts believe the island could fuse to land, break up into smaller pieces, or slowly move south where it could block shipping.
Petermann Glacier spawned smaller ice islands in 2001 (34 square miles) and 2008 (10 square miles).