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article image‘Unstoppable' loss of Antarctic glaciers prompts sealevel warning

By Robert Myles     May 13, 2014 in Environment
Irvine - The rapidly melting West Antarctic Ice Sheet is in an irreversible state of decline according to the latest NASA study. As a result, the "ice-dam" preventing glaciers in the area melting into the sea will ultimately disappear.
West Antarctic glaciers already contribute significantly to sea level rise. They release as much ice into the ocean annually as the entire Greenland Ice Sheet and contain enough ice to increase global sea levels by 4 feet (1.2 meters). The West Antarctic Ice Sheet is melting faster than most scientists had expected, meaning that current predictions of sea level rise will need to be revised upward.
Researchers from NASA and the University of California, Irvine used 40 years of observations in their study published Monday. Lead author Eric Rignot, of UC Irvine and NASA's Jet Propulsion Laboratory, Pasadena said the multiple lines of evidence considered by the researchers lead them to conclude that glaciers in the Amundsen Sea sector of West Antarctica "have passed the point of no return."
The NASA findings will be reinforced later this week when a different research team, based at the University of Washington, Seattle, publishes a separate study in Science journal focusing on the Thwaites Glacier, part of the West Antarctic Ice Sheet. The Seattle team predict that the collapse of the Thwaites Glacier, one of the largest single contributors to sea level rise, is inevitable as warmer ocean waters weaken the ice. As the ice at the point where the glacier loses contact with the land surface or sea-bed weakens — known as the grounding line — so the glacial ice floats rendering it more prone to rapid melt.
The grounding line of a glacier is a key factor in understanding the rapidly increasing melt. Glaciers flow out from land to the ocean, with their leading edge floating on seawater. Nearly all glacial melt stems from the underside of the glacier beyond the grounding line, on the section floating on seawater.
In the same way that a beached boat can be floated off in shallow water if it’s made lighter, so a glacier can float over an area of sea where it was formerly grounded when melt causes it to become thinner and lighter. That allows warmer ocean currents greater contact with the glacier ice, leading to a feedback loop: the greater the melt, the less weight; the less the weight, the greater the flotation; greater flotation means more glacial ice exposed to warmer ocean currents, and so on.
The NASA researchers found the glaciers studied had thinned so much that they now float above points where they formerly sat solidly on land, meaning their grounding lines are retreating inland.
One of the challenges faced by Rignot’s team was in trying to determine what was going on with grounding lines that were sometimes buried under a thousand or more meters of ice. To overcome this, satellite data gathered over a period of almost 20 years from the European Earth Remote Sensing (ERS) satellites was used. This enabled the scientists to map the grounding lines and topography at great depths beneath the glaciers to an accuracy of less than a quarter of an inch.
These measurements pointed to glaciers flowing more quickly and grounding lines retreating, two factors that reinforce each other. As the flow speed of glaciers increases, glaciers stretch out and thin, in turn reducing their weight and lifting them further off the bedrock. As the grounding line retreats, and more of the glacier becomes waterborne, just as melting ice with a layer of water on top becomes even slippier, so the glacier experiences less resistance thereby accelerating its flow.
But there remained the possibility that the underlying topography of the Antarctic bedrock would act as a brake on the glaciers, slowing their flow.
Just as studded winter tyres on a vehicle offer better grip in snow and ice, so bumps or hills below the glaciers might cause them the get snagged up, halting or at least slowing their eventual demise.
Using principally ice velocity data from ERS coupled with ice thickness data from NASA's Operation IceBridge mission that studies Earth’s Polar Caps, the researchers produced accurate topographical maps of bedrock elevation below the glaciers.
These maps revealed that five of the six glaciers studied had no "studs" or pinning points upstream from present grounding lines that would offer the prospect of some slowdown in the flow of the glaciers into the ocean. Only Haynes Glacier has major bedrock obstructions upstream, but it drains only a small sector of the West Antarctic Ice Sheet and is retreating as rapidly as the other glaciers studied.
Adding to the gloomy predictions, the bedrock topography showed all the glacier beds slope deeper below sea-level the further they extend inland. In effect, as the glaciers retreat, there’s nothing to stop the warm waters of the ocean flowing in. In a situation somewhat akin to what might happen in much of the Netherlands if the North Sea were to overcome Holland’s dyke defences, in the Antarctic, warm ocean waters would inundate the low-lying Antarctic interior causing ever more rapid melting of the glaciers.
A combination of accelerating flow rates, lack of pinning points and sloping bedrock all point to the same conclusion. According to Rignot, "The collapse of this sector of West Antarctica appears to be unstoppable," adding, "the fact that the retreat is happening simultaneously over a large sector suggests it was triggered by a common cause, such as an increase in the amount of ocean heat beneath the floating sections of the glaciers. At this point, the end of this sector appears to be inevitable."
If there can be said to be a bright spot in NASA’s predictions, it concerns the timescale — Rignot said, "A conservative estimate is it could take several centuries for all of the ice to flow into the sea."
The timescale appears to be confirmed by the University of Washington’s study of the Thwaites Glacier. It will likely be gone in a matter of centuries, say researchers, in the process causing a sea level rise of 2 feet from one glacier alone.
But disappearance of the Thwaites Glacier would be like removing a bottle-stopper. It acts as a lynch-pin for the rest of the West Antarctic Ice Sheet. If it goes, then the rest of the ice sheet contains enough water to cause another 10 to 13 feet (3 to 4 meters) of global sea level rise.
Commenting, lead author Ian Joughlin, a glaciologist at University of Washington’s Applied Physics Laboratory, said, “Previously, when we saw thinning we didn’t necessarily know whether the glacier could slow down later, spontaneously or through some feedback,” adding, “In our model simulations it looks like all the feedbacks tend to point toward it actually accelerating over time; there’s no real stabilizing mechanism we can see.”
NASA scientist Tom Wagner summed it up, telling reporters at yesterday’s press conference, "We finally have hit this point where we have enough observations to put this all together, to say, 'Wow, we really are in this state.'"
No kidding.
More about West Antarctic Ice Sheet, Antarctic glaciers, Melting glaciers, Rising sea levels, Sea level rise
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