Pine Island glacier may disappear within 100 years

14 August 2009

One of Antarctica's greatest glaciers is thinning so quickly it could disappear within 100 years. This is 500 years sooner than previously estimated and jeopardises a volume of ice that could raise global sea levels by around 25cm.

Antarctic field camp

British Antarctic Survey field camp on Pine Island Glacier

Researchers reported just eight years ago that Pine Island Glacier in West Antarctica could be lost within 600 years, but now they say satellite data covering a longer period of time means they are able to make a more accurate estimate.

Research led by Professor Duncan Wingham of University College London suggests that the rate at which the glacier is thinning has accelerated and spread inland. Wingham and his team calculate that the central 'trunk' of the glacier lost four times as much ice in 2006 than it did in 1995: around 10·2 cubic kilometres compared with 2·6 cubic kilometres.

Pine Island Glacier is one of a handful of glaciers that transport huge amounts of ice from the West Antarctic Ice Sheet to the sea. It moves more ice than any other Antarctic glacier and is the fastest moving in the entire continent.

This is perhaps the greatest signal of change in the cryosphere today, and without a continuous record of satellite observations we wouldn't even know it was happening.

- Professor Andrew Shepherd, University of Leeds

Although scientists have considered the glacier to be a region of potential instability for several decades, it lies in an inhospitable region of West Antarctica that has proved difficult to reach. This means researchers have had to wait for recent satellite measurements to find out exactly what's happening to the glacier.

Wingham's team report in the journal Geophysical Research Letters how they used European Space Agency satellites, Envisat and ERS-2, to analyse how the glacier has changed from 1995 and 2008. The satellites measured the height of the glacier's surface every 35 days for almost 15 years.

Back in 2001, co-author Professor Andrew Shepherd of the University of Leeds, used a shorter five-year record to estimate that the central trunk of the glacier would be lost in the next 600 years.

Now the longer dataset means the team can be more accurate in their estimate. Shepherd explains, "The five year dataset gave us a straight line, but over 15 years, we see a definite curvature which we interpret to mean the glacier is thinning at a constantly accelerating rate."

"As the glacier thinning speeds up, more and more ice is lost to the oceans and the supply of snowfall just cannot keep pace" he adds.

Sea level rise

The most recent assessment report of the Intergovernmental Panel on Climate Change used linear trends to predict the future contribution of Pine Island Glacier to sea level rise. "Our new data show that such assumptions are no longer reliable because the rate of thinning is quadrupling each decade," says Shepherd.

Sea water in this region is about 0·5 degrees Centigrade warmer than scientists think it should be. And because Pine Island Glacier flows straight into the sea, it's much more vulnerable to changes in water temperature than most other Antarctic glaciers which are buttressed by floating ice shelves.

"We know that warm water is the most likely candidate for the cause of Pine Island Glacier thinning. What we need now is for oceanographers to tell us why the ocean temperature is so high: although the change is in line with the rate at which the global oceans have warmed over the twentieth century, it could equally signal a southwards migration of the Antarctic Circumpolar Current into Pine Island Bay which may be due to an entirely different cause," he adds.

Some researchers suggest that warm water at the coastal end of the glacier may be melting the underside of the glacier's floating ice shelf at an unsustainable rate. They say that this may have ultimately led to accelerated thinning inland.

"This is perhaps the greatest signal of change in the cryosphere today, and without a continuous record of satellite observations we wouldn't even know it was happening," says Shepherd.

"Most of the changes in Antarctica are taking place near to its coast, which is where satellites have traditionally worked least well. Because Cryosat-2 [the brainchild of Wingham] - scheduled for launch in December this year - will be able to see these steep coastal areas better, our estimates of Antarctic imbalance will be even more accurate," adds Wingham.

The underlying danger is that most of the West Antarctic Ice Sheet lies on bedrock that rests well below sea level. If the Pine Island Glacier continues to thin and retreat it will eventually become afloat, leading to drawdown of ice from the deep interior of Antarctica.

'Spatial and temporal evolution of Pine Island Glacier thinning, 1995-2006' - DJ Wingham, DW Wallis, and A Shepherd (2009), Geophysical Research Letters, doi:10.1029/2009GL039126, in press.
(accepted 5 August 2009)