July 16 (Bloomberg) -- A Manhattan-sized chunk of ice may break away from a glacier in northwestern Greenland and fall into the sea within about two months, Greenpeace said today.

A 5 billion-metric-ton piece of the Petermann Glacier may detach and float south along the coast of Canada’s Ellesmere Island in the Arctic, said Greenpeace’s Kieran Mulvaney, who directs the expedition of scientists monitoring the situation. The chunk is part of the Northern Hemisphere’s largest “ice tongue,” the part of a glacier extending over the sea.

“There appears to be a lot of activity, whirlpools and rivers forming on the glacier,” Mulvaney said today in an interview. “It’s clearly an environment in a state of change.”

Glaciers in Greenland and Antarctica are melting faster than predicted, accelerating their march to the sea and adding to the rising ocean levels that threaten coastal communities worldwide, according to many scientific studies.

The ice expected to break off Petermann this summer will dwarf the one-billion ton piece that fell away from the ice sheet last year, Greenpeace said in an e-mailed statement.

The forecast came from a team of independent scientists on a Greenpeace ship about 80 miles from the site, the environmental-advocacy group said. The boat was moored to the ice until a few days ago, Mulvaney said.

Sea-Level Threat

The floating ice tongue covers about 500 square miles, and researchers at the Byrd Polar Research Center at Ohio State University are monitoring a rift in the ice, according to NASA. Over a two-week period last summer, the glacier lost 11 square miles, NASA said.

Melting glaciers may contribute to sea-level rise of 18 to 59 centimeters (7 to 23 inches) this century, according to the United Nations’ International Panel on Climate Change.

The planet’s oceans have risen 0.2 meters over the past century, and global warming is expected to further add to that gain, the two UN organizations have said. As the earth heats up, water in oceans will increase in volume, a process known as thermal expansion that also adds to climbing sea levels.

To contact the reporter on this story: Jeremy van Loon in Berlin at jvanloon@bloomberg.net; Alex Morales in London at amorales2@bloomberg.net.

Last Updated: July 16, 2009 12:22 EDT