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Last Updated: Tuesday, 6 December 2005, 23:43 GMT
Antarctica's ice bottom exposed
By Jonathan Amos
BBC News science reporter, San Francisco

The data will now be fed into computer models

British and US scientists have produced a remarkable map of the underside of the West Antarctic Ice Sheet (WAIS).

It shows in unprecedented detail the mountains, troughs and lakes that lie under the great ice mass.

Researchers have long wondered about the stability of the WAIS given that so much of it is grounded below sea level.

The new radar data gathered by scientific planes will now be used to model with greater confidence how the region might respond to a warmer world.

If all the surveyed zone were to melt, it would produce a global ocean rise of 1.3m.

What we really need to know is how fast sea levels will go up
David Vaughan, Bas
"These data are critically important," said Don Blankenship from the University of Texas (UT), Austin.

"Without them we wouldn't be able to asses the contribution of the WAIS to global sea level rise," he told the BBC News website.

And co-principal investigator David Vaughan, from the British Antarctic Survey (Bas), added: "What we really need to know is how fast sea levels will go up.

"If you talk to people that assess the impacts of climate change, it is the rate of rise they're most interested in - be that 10cm a century, or 50cm or 100cm. These data will now let us move forward on that."

Long flights

The project pulled together two airborne surveys based deep in the interior of the WAIS.

The British segment went by the acronym BBas; the Americans were known as Agasea. Both their planes were fitted with gravity meters, magnetometers, equipment to measure surface elevation, and ice-penetrating radar.

Survey planes on the ice (Bas)
The UK and US planes flew two overlapping grids to map the region
It is the radar map of the rock bed under the WAIS that has been reported here at the Fall Meeting of the American Geophysical Union.

To obtain it, the aircraft had to fly up and down some 80,000km (50,000miles) of grid lines in the two months from December 2004.

"It was a remarkable job to complete it all in the one season," said co-principal investigator Jack Holt, also from UT.

"The isolation was the most noticeable thing; it's pretty stressful - the waiting and not knowing what the weather is going to do and if the fuel is going to arrive to go flying."

The radar sees through about 4km of ice thickness in some locations, down to bedrock that sits 3km below sea level. It reveals in great clarity the deep troughs, valleys, mountains and sub-glacial lakes that mark the region.

Retreat potential

It gives a detailed picture of the topography over which two big glaciers - Pine Island Glacier (PIG) and the Thwaites Glacier - must slide to make it to the Amundsen Sea and calve bergs.

These "rivers of ice" carry prodigious volumes. PIG alone transports some 69 cubic km of ice each year from about 10% of the WAIS.

Glaciologists are keen to know how the behaviour of these glaciers might change in a warmer climate - and whether their melting and retreat would accelerate the ice discharge from the interior.

Map shows "drainage" areas feeding Pine Island and Thwaites Glaciers (BBC)
The new data show clearly that the PIG flows over a huge ridge some 250km inland from the front of the glacier, and this would likely impede any catastrophic retreat.

Thwaites, on the other hand, has a relatively consistent slope all the way into the WAIS interior and its behaviour would, as a consequence, be very different.

"Pine Island looks like quite a substantial threat over a short period - in the next 50-100 years we might be getting a measurable amount of sea-level rise from Pine," Dr Vaughan said.

"Over the longer period - on the multi-century scale - Thwaites is probably more of a threat because of this slope all the way down, and the retreat could get into the heart of West Antarctica and destabilise the region."

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