Warming ocean waters are melting the Antarctic ice shelves from the bottom up, researchers said Thursday in the first comprehensive study of the thick platforms of floating ice.

Scientists have long known that basal melt, the melting of ice shelves from underneath, was taking place and attributed the trend to icebergs breaking off the platforms.

But the new study, to be published in Friday's issue of the journal Science, said most of the lost mass came from the bottom, not the top.

"Our study shows melting from below by the ocean waters is larger, and this should change our perspective on the evolution of the ice sheet in a warming climate," said lead author Eric Rignot of NASA's Jet Propulsion Laboratory and the University of California, Irvine.

Overall, Antarctic ice shelves lost 2,921 trillion pounds (1,325 trillion kilograms) of ice per year in 2003 to 2008 through basal melt, compared to 2,400 trillion pounds lost due to iceberg formation.

During the process known as calving, large chunks of ice break off from the part of the ice shelf facing the sea.

The researchers also made the surprising discovery that the three giant ice shelves that make up two thirds of the entire Antarctic ice shelf area only account for 15 percent of basal melting.

The melted ice shelves are also distributed unevenly across the continent.

Ice shelves tend to lose mass twice as fast as the Antarctic ice sheet on land over the same period, according to the study.

"Ice shelf melt doesn't necessarily mean an ice shelf is decaying; it can be compensated by the ice flow from the continent," Rignot said.

"But in a number of places around Antarctica, ice shelves are melting too fast, and a consequence of that is glaciers and the entire continent are changing as well."

Antarctica holds about 60 percent of Earth's freshwater inside its huge ice sheet.

The researchers said that understanding how ice shelves melt will help improve projects of how the Antarctic ice sheet will respond to a warming ocean and raise sea levels.