By Alister Doyle, Environment Correspondent
OSLO (Reuters) - A robot submarine has found clues to rising world sea levels by making trips deep beneath an ice shelf in Antarctica, scientists said on Tuesday.
The 7-meter (22-ft) submarine was making the first inspection of the underside of the shelf off the Pine Island glacier, in a U.S.-British mission.
"Because so little is known about ice sheet behavior, this research will take us a step further in understanding how ice sheets will contribute to sea level rise," Stan Jacobs, the U.S. lead scientist from Columbia University, said in a statement.
World sea levels would rise by about 5 meters if the ice of West Antarctica, including the Pine Island Glacier, melted, he said. Experts fear global warming could trigger a thaw.
The Pine Island glacier is the fastest-moving in Antarctica and its slide toward the ocean is accelerating. It brings more water to the sea than Europe's Rhine River and is adding about 0.25 mm (0.01 inch) a year to global sea levels.
Making 6 trips up to 70 km (44 miles) into the cavity, once surviving a collision with ice, the battery-powered "Autosub" took sonar measurements of the ice and studied the salinity of the water as part of a 53-day research cruise this year. The submarine traveled a total of 500 kms.
OCEAN CURRENTS
A little-understood shift in ocean currents may be bringing in more water and melting ice from below, scientists said. It is unknown if the shifts are linked to climate change blamed by the U.N. Climate Panel on emissions of greenhouse gases.
"My hypothesis is that more water seems to be coming toward the glacier rather than the temperature of the water is rising," said Adrian Jenkins, the lead British scientist at the British Antarctic Survey.
Water in the area was about 1 Celsius (33.80F).
Jenkins told Reuters past studies of ice shelves measured water flows at the front, without understanding underwater currents, the shape of the seabed and the ice.
"That means there remains a black box -- you don't now where the melting is happening," he said. The flow of the Pine Island glacier has accelerated to 3.7 km (2.3 miles) a year from 2.4 in the mid-1990s.
"We were able to map the shape of the cavity and measure the water properties so we can say with more certainty how the ocean heat gets to the ice base to do the melting," he said.
The fear is that Antarctica's ice will slide toward the sea far faster if ice shelves floating on the water -- and acting as a brake -- break up and disappear. Several ice shelves on the Antarctic Peninsula to the north have broken up in recent years.
Jenkins said it would take several months to go through the data compiled by the submarine.
"We're really happy with the way the submarine performed," said Steve McPhail, lead engineer of the mission at the National Oceanography Center in Southampton, England.
He said it was nerve-racking waiting for it to return from a pre-programed route that could take 24 hours to cover, not knowing if it had broken down or crashed.
(Editing by Janet Lawrence)