The high-elevation interior gained 54 gigatons (14 cubic miles) annually from excess snowfall, they said.

This is a change from the 1990s, when ice gains approximately equaled losses, said Scott Luthcke of NASA's Planetary Geodynamics Laboratory outside Washington.

"That situation has now changed significantly, with an annual net loss of ice equal to nearly six years of average water flow from the Colorado River," Luthcke said.

Luthcke and his team reported their findings in Science Express, the advance edition of the journal Science.

The ice mass loss in this study is less than half that reported in other recent research, NASA said in a statement, but it still shows that Greenland is losing 20 percent more mass than it gets in new snowfall each year.

The Greenland ice sheet is considered an early indicator of the consequences of global warming, so even a slower ice melt there raises concerns.

"This is a very large change in a very short time," said Jay Zwally, a co-author of the study. "In the 1990s, the ice sheet was growing inland and shrinking significantly at the edges, which is what climate models predicted as a result of global warming.

"Now the processes of mass loss are clearly beginning to dominate the inland growth, and we are only in the early stages of the climate warming predicted for this century," Zwally said.