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Climate change: Seafloor ridges ‘stunning’ fast Antarctic retreat

Scientists are discovering how easily Antarctica ‘s ice margin will recede in a warming climate.

We also found structures on the seabed that suggest that at the height of the last ice age the ice layer was melting at speeds of up to 50 m a day.
That’s around 10 times quicker than what satellites have seen today.
The finding is significant as it imposes practical restrictions on the computer models used to predict potential regional change.

“In numerical models, you play with the parameters – and they can do very strange things,” said Prof Julian Dowdeswell. “But what these data are saying is that actually rates considerably higher than we get even in the satellite record today were possible in the not-far-distant geological past.”

The head of the Scott Polar Research Institute (SPRI) in Cambridge, UK, led an expedition to the Antarctic Peninsula’s Larsen area last year.
His team introduced autonomous underwater vehicles (AUVs) with high-resolution imaging capabilities to investigate the sediments at the western bottom of the Weddell Sea.
What the robots saw was a fragile ridge pattern that looked like a set of ladders where each rung was around 1.5 m high and 20-25 m apart spaced. These ridges are described by the scientists as characteristics produced at the ice grounding region.

This region is the stage that the ice streaming into the ocean near Antarctica becomes buoyant and starts to move. The rungs are formed when the ice constantly pats the sediments at this spot, as the tides rise and fall.
The ice would have been in retreat for the pattern to have been created and conserved (advancing ice will kill the ridges). And the tidal “clock” provides a figure for this setback.

Prof Dowdeswell explained: “We have a maximum of 90 of these rungs with a spacing of 20-25m – that gives us, if extrapolated, a rate of 40-50m per day. Again, if extrapolated – that’s a rate in excess of 10km per year of retreat. And the really interesting thing about that is it’s a rate that’s pretty much an order of magnitude higher than even the most rapid retreat of the grounding lines in the Pine Island-Thwaites system today.”

The Pine Island and Thwaites Glaciers are two of the fastest-changing ice structures in Antarctica whose ice borders are dissolved by warm ocean water flowing beneath them.
The ridges seen by the AUVs are approximately 40 km from today’s ice edge cliffs in the Larsen field. The traces are about 12,000 years old. Back then, the ice cap may have been much bigger than it is now, but like now, it would have undergone dramatic change when the world atmosphere recovered from the deepest freezes. Dr Alistair Graham of South Florida University , USA, is working with AUV info. He hadn’t been interested with this work.

He said he considered the explanation convincing, but in the group there will certainly be some doubt that the rungs developed with the tides on a regular basis.

“The data in the paper are absolutely stunning,” he told BBC News.

“The ability to map at a sub-metre resolution from an AUV really lifts a veil on the seafloor structure and composition. We have been able to get snapshots of these sorts of images before, by sending an AUV under Pine Island Glacier, for example, but these imagery from the Dowdeswell team are the best yet from anywhere around Antarctica.”

What this paper always shows to me is that there are always important things to be learnt by gazing at the poles’ previous ice sheet history. For something like Thwaites Glacier, where we are trying to predict its potential course, knowing what it does during the years and decades leading up to the discoveries we are making now would be a vital part of deciding how much ice is missing and how quickly it is moving forward.

“We do have similar high-resolution imagery from an AUV deployed at Thwaites Glacier last year that we are currently working on, and which will hopefully tackle some of these pressing questions.”

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