Washington: Powerful ocean currents beneath West Antarctica's Pine Island Glacier Ice Shelf are eroding the ice from below, speeding the melting of the glacier as a whole, a new study has found.

According to the study, published in the journal Nature Geoscience, a growing cavity beneath the ice shelf has allowed more warm water to melt the ice -- a process that feeds back into the ongoing rise in global sea levels.

The glacier is currently sliding into the sea at a clip of 4km (2.5 miles) a year, while its ice shelf is melting at about 80 cubic kilometres a year -- 50 percent faster than it was in the early 1990s, the researchers said.

"More warm water from the deep ocean is entering the cavity beneath the ice shelf, and it is warmest where the ice is thickest," said Stan Jacobs, an oceanographer at Columbia University's Lamont-Doherty Earth Observatory and the lead author of the study.

In 2009, Jacobs and an international team sailed to the Amundsen Sea aboard the icebreaking ship Nathaniel Palmer to study the region's thinning ice shelves -- floating tongues of ice where land bound glaciers meet the sea.

One goal was to study oceanic changes near the Pine Island Glacier Ice Shelf, which they had visited in an earlier expedition, in 1994.

The researchers found that in 15 years, melting beneath the ice shelf had risen by about 50 percent. Although regional ocean temperatures had also warmed slightly, by 0.2 degrees C or so, that was not enough to account for the jump.

The local geology offered one explanation. On the same cruise, a group led by Adrian Jenkins, a researcher at British Antarctic Survey and study co-author, sent a robot submarine beneath the ice shelf, revealing an underwater ridge.

The researchers found that the ridge had once slowed the glacier like a giant retaining wall. When the receding glacier detached from the ridge, sometime before the 1970s, the warm deep water gained access to deeper parts of the glacier.

Over time, the inner cavity grew, more warm deep water flowed in, more melt water flowed out, and the ice thinned. With less friction between the ice shelf and seafloor, the land bound glacier behind it accelerated its slide into the sea.
       
Other glaciers in the Amundsen region have also thinned or widened, including Thwaites Glacier and the much larger Getz Ice Shelf.
       
In recent decades, researchers have found evidence that Antarctica is getting windier, and this may also help explain the changes in ocean circulation.
       
Stronger circumpolar winds would tend to push sea ice and surface water north, said Jacobs. That in turn, would allow more warm water from the deep ocean to upwell onto the Amundsen Sea's continental shelf and into its ice shelf cavities.
       
Pine Island Glacier, among other ice streams in Antarctica, is being closely watched for its potential to redraw coastlines worldwide. Global sea levels are currently rising at about 3mm (.12 inches) a year. By one estimate, the total collapse of Pine Island Glacier and its tributaries could raise sea level by 24 cm (9 inches).

(Agencies)