By Martin Redfern
Rothera Research Station, Antarctica
UK scientists working in Antarctica have found some of the clearest evidence yet of instabilities in the ice of part of West Antarctica.
The UK work is discovering just how fast the ice is moving
If the trend continues, they say, it could lead to a significant rise in global sea level.
The new evidence comes from a group of glaciers covering an area the size of Texas, in a remote and seldom visited part of West Antarctica.
The "rivers of ice" have surged sharply in speed towards the ocean.
David Vaughan, of the British Antarctic Survey, explained: "It has been called the weak underbelly of the West Antarctic Ice Sheet, and the reason for that is that this is the area where the bed beneath the ice sheet dips down steepest towards the interior.
"If there is a feedback mechanism to make the ice sheet unstable, it will be most unstable in this region."
There is good reason to be concerned.
Satellite measurements have shown that three huge glaciers here have been speeding up for more than a decade.
The biggest of the glaciers, the Pine Island Glacier, is causing the most concern.
Julian Scott has just returned from there. He told the BBC: "This is a very important glacier; it's putting more ice into the sea than any other glacier in Antarctica.
"It's a couple of kilometres thick, its 30km wide and it's moving at 3.5km per year, so it's putting a lot of ice into the ocean."
It is a very remote and inhospitable region. It was visited briefly in 1961 by American scientists but no one had returned until this season when Julian Scott and Rob Bingham and colleagues from the British Antarctic survey spent 97 days camping on the flat, white ice.
The team drove its skidoos for thousands of km across the ice
At times, the temperature got down to minus 30C and strong winds made work impossible.
At one point, the scientists were confined to their tent continuously for eight days.
"The wind really makes the way you feel incredibly colder, so just motivating yourself to go out in the wind is a really big deal," Rob Bingham told BBC News.
When the weather improved, the researchers spent most of their time driving skidoos across the flat, featureless ice.
"We drove skidoos over it for something like 2,500km each and we didn't see a single piece of topography."
Rob Bingham was towing a radar on a 100m-long line and detecting reflections from within the ice using a receiver another 100m behind that.
The signals are revealing ancient flow lines in the ice. The hope is to reconstruct how it moved in the past.
Julian Scott was performing seismic studies, using pressurised hot water to drill holes 20m or so into the ice and place explosive charges in them. He used arrays of geophones strung out across the ice to detect reflections, looking, among other things, for signs of soft sediments beneath the ice that might be lubricating its flow.
He also placed recorders linked to the global positioning system (GPS) satellites on the ice to track the glacier's motion, recording its position every 10 seconds.
The Pig - Pine Island Glacier - is a major draining feature on the Wais
Throughout the 1990s, according to satellite measurements, the glacier was accelerating by around 1% a year. Julian Scott's sensational finding this season is that it now seems to have accelerated by 7% in a single season, sending more and more ice into the ocean.
"The measurements from last season seem to show an incredible acceleration, a rate of up to 7%. That is far greater than the accelerations they were getting excited about in the 1990s."
The reason does not seem to be warming in the surrounding air.
One possible culprit could be a deep ocean current that is channelled onto the continental shelf close to the mouth of the glacier. There is not much sea ice to protect it from the warm water, which seems to be undercutting the ice and lubricating its flow.
Julian Scott, however, thinks there may be other forces at work as well.
Much higher up the course of the glacier there is evidence of a volcano that erupted through the ice about 2,000 years ago and the whole region could be volcanically active, releasing geothermal heat to melt the base of the ice and help its slide towards the sea.
David Vaughan believes that the risk of a major collapse of this section of the West Antarctic ice sheet should be taken seriously.
Geothermal activity may be playing its part, says Julian Scott
"There has been the expectation that this could be a vulnerable area," he said.
"Now we have the data to show that this is the area that is changing. So the two things coinciding are actually quite worrying."
The big question now is whether what has been recorded is an exceptional surge or whether it heralds a major collapse of the ice. Julian Scott hopes to find out.
"It is extraordinary and we've left a GPS there over winter to see if it is going to continue this trend."
If the glacier does continue to surge and discharge most of it ice into the sea, say the researchers, the Pine Island Glacier alone could raise global sea level by 25cm.
That might take decades or a century, but neighbouring glaciers are accelerating too and if the entire region were to lose its ice, the sea would rise by 1.5m worldwide.