(AFP) - A massive iceberg twice the size of Manhattan has broken off of a glacier in Greenland, according to NASA satellite imagery, in what could be the latest indication of global warming.

The images released Wednesday show the massive chunk of ice breaking off of the Petermann Glacier on the northwestern coast of Greenland. The glacier produced a similar ice island twice as large in 2010.

NASA said the crack in the glacier had been visible since 2001, and that its polar-orbiting Aqua satellite had observed the break on July 16-17.

Oceanographer Andreas Muenchow of the University of Delaware said that while the icebergs seem large, most of the glacial melting takes place 600 meters (1,800 feet) below the surface, where the glacier meets bedrock and the ocean water is much warmer than at the surface.

"The dominant mass loss is via ocean melting from below. This dominant process is not visible, it is not possible to capture it with eye-catching images," he wrote on his blog at icyseas.org.

"Furthermore, and this appears counter-intuitive, the loss of area will have little direct effect on the ocean melting, because the 100-150 (-meter) thick floating ice shelf is bathed in ocean water near the freezing point."

Muenchow said the Atlantic waters melting the glacier appear to be warming, but that records only go back to 2003.

"Some changes are dramatically visible, such as the discharge of large ice islands," he wrote. "Some changes, perhaps more important, are not."