Antarctic Ice Sheet
Steadily Melting

WASHINGTON (Reuters) - A big chunk of Antarctica has been melting for thousands of years and will likely continue to melt until it swamps millions of miles (km) of coastland, scientists said on Thursday. They said there was nothing anyone could do about it -- and unlike other areas of Antarctica that were melting, global warming was probably not to blame.
"During the last ice age, the West Antarctic Ice Sheet (WAIS) was 1,300 km (650 miles) more extensive than it is now in the Ross Sea Embayment," Brenda Hall of the University of Maine, who helped work on the study, said in an interview conducted by e-mail.
"Our data would suggest that it has been retreating ever since the end of the Ice Age -- probably the last 10,000 years."
Reporting in the journal Science, they said its complete collapse would raise the global sea level by 15 to 20 feet (5 to 6 metres).
"Continued recession and perhaps even complete disintegration of the WAIS within the present interglacial period could well be inevitable," they wrote. The interglacial period is the time between the last ice age, which ended 11,000 years ago, and the next one. No one is sure when that will occur.
Howard Conway, a geophysicist at the University of Washington who led the study, said that although human-caused global warming might be speeding up the process, there was little people could do to stop it.
"Collapse appears to be part of an ongoing natural cycle, probably caused by (a) rising sea level initiated by the melting of the Northern Hemisphere ice sheets at the end of the last ice age," he said in a statement.
"But the process could easily speed up if we continue to contribute to warming the atmosphere and oceans."
It is clear in some areas that the ice has been melting since human activity such as burning fossil fuels started sending average temperatures higher at the beginning of the century.
But the causes in other areas that are melting have not been so clear. Sometimes changes in currents, or in the way water washes under the ice, can affect melting.
Hall, Conway and colleagues made dozens of visits to the West Atlantic Ice Sheet to measure the rate of melting. They looked for clues about how big the ice sheet, which currently covers about 360,000 square miles (932,300 sq km), was in the past.
Clues included deposits of penguin guano. "If the penguins lived there, there couldn't have been any glacial ice at that site or offshore in the ocean. They need open water," Hall said.
They also looked at radar imaging of subsurface ice structures and of ground-level ice. Ice weighs land down, and the earth springs back when the ice melts. The researchers used carbon-14 dating to tell how long ago beaches now 90 feet (28 metres) above sea level were once under ice.
Roosevelt Island in the Ross Sea provided other clues. "It is an ice island -- a big block of ice that is so thick that it rests on the sea floor, even though the surrounding ice is all floating," Hall said.
"We used radar to look at the layers of ice inside Roosevelt Island." Their measurements show it was once about 1,600 feet (488 metres) thicker.
In another study, Ian Joughin and colleagues at the Jet Propulsion Laboratory and California Institute of Technology in Pasadena found water is pouring off the ice sheet in ice streams that slide quickly to the ocean and break off into giant icebergs.
Hall said she hoped the studies would not give ammunition to critics of the global warming theory who say human activity is having no effect whatsoever.
"Although we say that the current retreat of the ice sheet may not be the result of global warming, that doesn't mean that global warming doesn't exist," she said.
She noted that collapse of the WAIS would not be the only effect of global warming. It could also shift ocean circulation and weather patterns, bring drought, severe storms and the wider spread of tropical diseases.