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Melting ice, global warning
02 October 2004
From New Scientist Print Edition.
Bob Holmes

Enlarge Figure 1

Antarctic glaciers are thinning and flowing faster than ever into the sea, according to three reports published last week. If the process continues, the global sea level could rise by a metre or more within decades.

While no direct link can yet be made to global warming, the warning signs are clear. Rising ocean temperatures, warmer air and changing currents - all aspects of climate change - can break up the large floating platforms known as ice shelves. "In a warming climate, the ice shelves become vulnerable. Not only can they melt from the top, but they can melt from the bottom," says glaciologist Robert Thomas, a NASA contractor in Wallops Island, Virginia, and leader of one of the studies. Several ice shelves have vanished in recent years.

The disintegration of floating ice shelves does not itself raise the sea level. But ice shelves serve as bulwarks for many glaciers on the Antarctic land mass, particularly those in western Antarctica, and the studies found evidence that the break-up of shelves is dramatically affecting the glaciers.

Two teams that observed the break-up of the Larsen B ice shelf off the Antarctic Peninsula in 2002 have been tracking the movement of glaciers that feed into the ice shelf, using satellites and radar. Thomas and his colleagues found that five of these glaciers were moving 3 to 8 times as fast as they were just before the ice shelf collapsed. Two glaciers slightly further south that have intact ice shelves showed no such increase (Geophysical Research Letters, vol 31, p L18401).

"The ice shelf is behaving a bit like a cork in a bottle," Thomas says. Pull out the cork, and the pent-up glaciers start surging towards the sea. As they do so, the glaciers elongate and should thin by tens of metres per year, the researchers calculated.

The calculations are borne out by laser altimetry measurements by a team led by Ted Scambos of the University of Colorado, Boulder. As well as confirming that the glaciers flowing into the broken-up Larsen B ice shelf are moving faster, Scambos's team found that they have thinned by as much as 38 metres in six months, a year after the break-up of the ice shelf (Geophysical research Letters, vol 31, p L18402).

The glaciers of the Antarctic Peninsula are small and relatively insignificant on a global scale. But two other studies suggest that a similar fate could befall the massive glaciers of western Antarctica. The region contains enough water to raise the world's sea level by 5 metres or more. These glaciers are penned in on two sides by the gigantic Ross and Ronne ice shelves, which are in no danger of breaking up. But on the third side, the west Antarctic ice sheet is fronted by the much smaller ice shelf of the Amundsen Sea. "It's like the back door to the ice sheet," says Thomas. "Scientists have been focusing all their attention on the front door, which is well bolted, but they have ignored the back door."

The glaciers emptying into the Amundsen Sea received little attention until two years ago, when Thomas and his colleagues began measuring the thickness of six glaciers using laser altimetry and radar from aircraft. They calculate that the glaciers have discharged about 250 cubic kilometres of ice each year - 60 per cent more than is being replenished by snow falling onto them (Science, DOI: 10.1126/science.1099650). That translates into a global sea level rise of 0.2 millimetres a year, or 10 per cent of the current rise in sea level.

The thinning of glaciers flowing to the Amundsen Sea was apparent not just near the coast, but hundreds of kilometres inland. About 300 kilometres inland the Pine Island glacier has thinned twice as fast in the last two years compared to its rate of thinning in the 1990s. And it has sped up by about 25 per cent over the last 30 years, suggesting that glaciers are also accelerating towards the Amundsen Sea.

The team's radar depth soundings also revealed that these glaciers are in places hundreds of metres thicker than previously thought, and therefore carry much more ice. "It's a bigger leaky hole," Thomas says. Moving at the same speed, thicker glaciers empty more ice into the sea than thinner ones.

If the Amundsen Sea ice shelf ever breaks up, much of the west Antarctic ice sheet could drain out in this direction even if the Ross and Ronne ice shelves remain intact, says Robert Bindschadler, a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who is not part of any of the research teams. Though the flow of the west Antarctic ice sheet would have to change direction dramatically, this is not out of the question. Martin Siegert of the University of Bristol, UK, and his colleagues have discovered a previously unknown fold in the ice sheet which appears to show that it changed direction a few thousand years ago (Science, vol 305, p 1948).

But it's not yet known whether the speed-up in the flow of glaciers now being observed will continue. They could surge briefly as ice shelves break up and then settle into a new equilibrium. They could even begin to grow again. Some of the glaciers released by the Larsen B ice shelf appear to have slowed their advance a year later, Thomas's group found.

But that may be less likely with glaciers flowing into the Amundsen Sea. Thomas's team found that the bottom of these thick west Antarctic glaciers lies well below sea level. If their ice shelves break up, and the glaciers start thinning, they could at some point float free of the bedrock and become ice shelves themselves, potentially increasing their melt rate. "It is possible - some of us would think it is probable - that if these glaciers continue to accelerate, they might reach a point of no return," Thomas says.

Bindschadler agrees that the speed-up of glaciers is disturbing. "Not only is it happening, but it appears to be happening at an accelerated rate," he says. "I call that the fingerprint of collapse."

Learning more about the rock beneath the west Antarctic glaciers could provide more clues about their fate, as ridges there could halt what would otherwise be a rapid slide.

Scambos believes that global warming is probably responsible for the changes being seen in Antarctica. But he says that not enough is known about how global warming affects the weather patterns, ocean temperatures and currents around Antarctica and how these, in turn, affect the rate of melting of ice shelves.

"By and large, Antarctica is still cold and stable," Scambos says. "The effects could be construed as being slow enough that future generations will be able to handle it. But I don't know."

From issue 2467 of New Scientist magazine, 02 October 2004, page 8

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