I'LL HAVE DOUBLE THE ICE WITH THAT

The Amery Ice Shelf in Antarctica is nearly twice as large as previously thought.

A study by a team at the Antarctic Cooperative Research Centre and Australian Antarctic Division found that the shelf's grounding zone, where floating ice connects to the land, is further upstream than expected.

The study appeared in this week's Journal of Geophysical Research.

"It is important because what happens is you have seawater under ice shelves, which melts the ice more quickly where the ice starts to float," explained Mr Mike Craven, a glaciologist with Amery Ice Shelf Ocean Research (AMISOR), part of the Australian Antarctic Division.

"So it's important to know where that grounding line is."

The finding alters current understanding of the shape and dimension of the ocean cavity beneath the ice shelf. This has implications for estimating ocean circulation, tides, and modeling studies of the freezing and melting influences of ice shelves.

The team used satellite radar altimetry (which measures characteristics of the surface of the earth) and found that floating ice extends more than 240 kilometers further towards the pole than any researchers had predicted.

"If you know the elevation and the thickness of the ice, you can work out how high the ice shelf should be floating. If it is much higher than you expect, then it is almost certainly on land," said Mr Craven

Several other methods, including global positioning and imagery techniques, confirmed their results.

The ice at the calving front - where it is breaking off into the ocean – is about 250 metres thick.

"But when you get back to its true grounding line – where the ice begins to flow off the ground - we are talking 2,500 metres," he said.

The new calculations suggest that the Amery Ice Shelf is nearly 72,000 square kilometers. The previous estimate for the shelf was approximately 40,000 square kilometers.

In the 1960s and 1970s, researchers conducted surveys on the surface of the shelf using standard surveying techniques like electronic distance machines.

"They came across an area where there was a reasonable elevation change over a reasonably short horizontal distance – a matter of tens of metres over a few kilometres," he said. The researchers thought that elevation change represented where the shelf was grounded.

Now larger surveys have been done over the whole of the continent. "It actually was a reasonably small sticking point," said Mr Craven.

Ice is continually flowing away from a number of domes at high points on the Antarctic continent.

"It flows fairly slowly at first because the plateau is such a flat region, so it might be moving at only an order of 10 metres per year, but as it gets towards the coast and the slopes start to increase, it can move up to 500 to 600 metres a year. So the ice is continuously oozing off Antarctica all the time.”

The Amery Ice Shelf is the third-largest shelf in the Antarctic. Yet it is dwarfed by the massive Filchner-Ronne and Ross shelves. It sits south of India, on the opposite side of Antarctica to the Larsen ice shelf (on the Antarctic Peninsula), part of which recently collapsed.

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