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Big ice shelf's disappearing act

Sediments extracted from the Antarctic seafloor show the world's largest ice shelf has disintegrated and reappeared many times in the past.

Fluctuations in the Ross Ice Shelf are revealed by an early look at cores drilled from the seabed underneath the giant ice slab.

The investigation is being carried out by scientists drilling near the US and New Zealand bases on Ross Island.

The team wants to link the data to what is already known about past climate.

The long-term aim of the scientists is to find out what the Ross Ice Shelf - a floating slab of ice the size of France - and the West Antarctic Ice Sheet have been doing over the past 10 million years.

The researchers are part of the first team to drill as part of the international Andrill project to investigate the geological history of Antarctica.

"We're seeing numerous cycles of the ice shelf or ice sheet being present at the site and then being absent," said Dr Tim Naish, a palaeoclimatologist at New Zealand's Institute of Geological and Nuclear Sciences and one of the chief scientists for the Antarctica Geological Drilling project (Andrill).

"These are dramatic fluctuations."

"The big question is how stable is this enormous ice shelf, the Ross Ice Shelf, which is being fed by the West Antarctic Ice Sheet," said Dr Naish.

Early signs

Already, just weeks into the project and more than 600m into the seafloor off Antarctica, they've been able to show that this part of the icy continent has at earlier times been open water, and at times, has been covered by an ice sheet.

"When the ice sheet is there, the sediments you get under it are very rubbly. They are the sort of sediments that you would see at the front of [glaciers]," explains Naish.

"When the ice lifts a bit, so water can flow underneath, and it becomes an ice shelf, you still get those rubbly bits but you also get sediments that tell you water was around, that water was flowing back and forth.

"When the ice shelf disappears and you've got completely open water, then you've got a completely different situation where you have high biological productivity and a lot of microfossils preserved."

The scientists also want to determine, by dating the sediments, just when the ice disappeared in order to link that information to what is known about the climate back then.

Warmer world

The aim, said Dr Naish, is provide from this sedimentary snapshot an analogue for what might happen in the warmer climate the Intergovernmental Panel on Climate Change (IPCC) predicts.

The IPCC, the body responsible for collating and analysing climate data for the global community, predicts average global temperatures will rise by between 1.4C and 5.8C by 2100.

"If we drill back in time, we can get a record of how [the Ross Ice Shelf] behaved, during times, certainly in the last million years, when we know the temperatures from the ice cores suggest that the planet was two to three degrees warmer," the Andrill project member explained.

Scientists know ice shelves are the most vulnerable part of the Antarctic. On the Antarctic Peninsula, where temperatures have risen 2.5C in the past 50 years, there have been spectacular collapses such as the demise in 2002 of the Larsen B shelf.

The collapse of an ice shelf can lead to further loss of ice from the Antarctic continent itself.

Dr Naish explained: "One of the things we've learnt from the collapse of the ice shelves around the Antarctic Peninsula is that once the ice shelf goes, the glaciers feeding it speed up and you start to lose ice mass off the continent much faster because the ice shelves essentially buttress the glaciers that are feeding them."

Shifting ice

If the West Antarctic Ice Sheet and its northern counterpart on Greenland, were both to melt, sea levels around the world would rise about 10 to 12m. And there is some evidence, says Dr Naish, that the West Antarctic Ice sheet - and not just the Ross Ice Shelf - has also disappeared during warmer periods in the past.

Previous drilling has showed that ice sheets were quite dynamic, collapsing and reforming in line with the Earth's Milankovitch cycles. These are small "wobbles" in the Earth's orbit that are known to happen roughly every 20,000, 40,000 and 100,000 years.

But said Dr Naish, "during all those natural cycles, carbon dioxide never got above 300 parts per million. So in the last 200 years, we've had this geologically unprecedented increase in CO2 - it's 30% higher than it has been over the last several million years and it's occurred at a rate we've never seen geologically."

To understand more, the Andrill team will study exhaustively the core extracted from beneath the Ross Ice Shelf. So far, the team has drilled more than 600m and expects eventually to reach 1200m beneath the seafloor.

But already, the evidence in the sediments of what happened to the Ross Ice Shelf and the West Antarctic Ice Sheet is significant.

Dr Naish muses: "If they collapsed in the past without the present level of CO2 and the Earth was two to three degrees warmer, what's going to happen with the doubling of CO2 and potentially much higher temperatures?"

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