BioEd Online

Antarctic ice is protected from the sea by rocky wedges of debris that act as 'sandbags' to protect glaciers from rising waters, a survey of one of the continent's major ice flows has revealed. If much of Antarctica's ice is protected in this way, it may help to fend off ice melting as a result of rising sea levels.

Researchers led by Sridhar Anandakrishnan of Pennsylvania State University in University Park, conducted a radar survey of the final 25 kilometres of the Whillans Ice Stream, a 500-kilometre-long glacier that sprawls towards the Ross Ice Shelf in West Antarctica. They focused on the 'grounding line' — the region where the ice stops flowing over land and passes out onto the floating ice shelf. Underneath this grounding line they found a pile of debris up to 31 metres thick, on top of which is a 10-metre-thick bulge of ice.

If that rocky wedge were not there, rising sea levels might be expected to melt their way up the gentle slope of the underside of the ice, lifting the glacier's mouth off the land and speeding its melting.

But instead it looks like there's a sandbag in the way, which should protect the ice undersides from encroaching waters, Anandakrishnan explains. "Until the sea level rises sufficiently to overcome the added height, there won't be any retreat," he says.

Batten the hatches

The wedge was created by the glacier flow carrying chunks of rock and depositing them in a pile, explains Anandakrishnan's colleague Richard Alley, who led a computer-modelling project to study the process. Both studies are published online in Science^1,2.

"The stabilizer is not perfect, but it does provide some stability," Alley says. Because the sandbag is a bit leaky — water can still get in around the edges — they think the glacier could resist a sea-level rise of up to 10 metres before beginning to melt.

The Intergovernmental Panel on Climate Change (IPCC) recently predicted that global sea levels would rise by no more than 59 centimetres this century, although its report noted that unpredictable ice melting could add dramatically to this rise.

A survey of the Ross Sea also reveals piles of rock similar to the Whillans wedge on the sea floor, suggesting that many previous glaciers may have had similar 'sandbags' that ended further out than the current shoreline. This raises hopes that many other glaciers today may also be protected in this way.

No news is all good

The researchers do not yet know exactly how much ice is protected by the Whillans Ice Stream's wedge. But the glacier is a major feature of West Antarctica — it is 500 kilometres long, 100 kilometres wide and a kilometre thick in places.

But Alley warns that the existence of these wedges may not be cause for unbridled optimism. "At first glance, this might be considered an encouraging indication," he says. "However, you could also spin this in a pessimistic way."

"We know the ice sheets have changed a lot in the past, so if the ice is less sensitive to sea level than we thought, then it must be more sensitive to temperature than we thought," he explains. "As we expect future changes in temperature, maybe more so than in sea levels, perhaps things are not as optimistic as you might think."