U.S. scientists monitoring Greenland's glaciers say they expect two of the largest glaciers to disintegrate within the next year.

Ohio State University Associate Professor Jason Box of the school's Byrd Polar Research Center says a massive 11-square-mile piece of the Petermann Glacier in northern Greenland broke away last month.

Box and graduate students Russell Benson and David Decker say they're even more concerned about what appears to be a massive crack farther back from Petermann's margin. They said that crack, seen in satellite images, might signal an imminent and much larger breakup.

"If the Petermann glacier breaks up back to the upstream rift, the loss would be as much as 60 square miles," said Box. That would represent a loss of one-third of the massive ice field.

At the same time, the scientists say the margin of the immense Jakobshavn glacier has retreated farther inland than it has during 150 years of observation. The researchers believe the glacier hasn't retreated to where it is now in at least the last 4,000 to 6,000 years.

The scientists are using data from National Aeronautics and Space Administration satellites and from cameras that monitor global warming effects on Greenland's glaciers.