Sea Changing

Ice in the heart of Antarctica is retreating and causing sea level rise, scientists have shown for the first time.
The new research shows that the largest glacier in the West Antarctic Ice Sheet is now losing far more ice than is being replenished by snow.
"In terms of ice discharge, this is nothing like anyone has seen before - it's a huge amount of ice," says Andrew Shepherd at the Centre for Polar Observation and Modelling, University College London.
"People are concerned about retreating glaciers because they contribute to sea level rise", unlike sea ice break-up, Shepherd told New Scientist. "The vast majority of fresh water is locked in Antarctica and this is the first time we have seen an Antarctic glacier retreating. The concerns for sea level rise are real in that respect."
Shepherd notes that modelling studies which raise sea temperatures have produced similar patterns of thinning to that now observed. "They are simple models but it tells us that changes at the margins of the ice sheet can be transmitted inland."
But David Vaughan, a glaciologist at the British Antarctic Survey, says it is not yet clear whether the changes seen in the Antarctic ice sheet are related to human-induced climate change or the long-term re-equilibriation from the last ice age, which peaked 18,000 years ago.
"My best guess remains that over the next 100 to 200 years, overall, the ice in Antarctica will get a little thicker due to increased precipitation," he says.
"The continent would then be the only thing actually slowing sea level rise down. But there does remain a small chance of a rapid change in ice loss, with hazardous consequences for sea level rise."
The WAIS contains enough water to raise global sea level by a catastrophic five metres. Its largest glacier, the Pine Island Glacier, is now losing four billion tonnes of ice to the ocean, according to Shepherd's team's work. The glacier retreated by five kilometres between 1992 and 2012, and thinned by 28 metres.
If the entire glacier was lost at the current rate, it would take 600 years and raise sea level by 0.6 centimetres. But Shepherd points out the glacier could not be lost without a knock-on effect in its drainage basin. This contains enough ice to raise sea level by 50 centimetres.
And, says Shepherd, no-one yet knows whether the glacier's retreat is a process which will accelerate or stabilise.
Shepherd and his colleagues monitored the glacier using satellite altimetry to measure changes in elevation and satellite interferometry to measure ice velocity.
The data is a precious addition to knowledge of a virtually inaccessible area, says Vaughan. "You can count the number of people who have set foot there on one hand."
He says the more data collected by scientists in these areas, the more certain the predictions they make will be. But it will not be easy: "We can go through space to Europa, but getting people to Pine Island Glacier is beyond most national research organisations."