30th November 2016
A significant collapse of the West Antarctic ice sheet "within our lifetimes"
A massive iceberg that splintered away from West Antarctica was caused by a deep subsurface rift opening within the ice shelf, the first time this has been observed, say researchers. They warn that a significant collapse of the ice sheet is possible "within our lifetimes", with major consequences for sea level rise.
A major glacier in Antarctica is breaking apart from the inside out – suggesting that the ocean is weakening ice on the edges. The Pine Island Glacier – part of the ice shelf that bounds the West Antarctic Ice Sheet – is one of two glaciers that researchers believe are most likely to undergo rapid retreat, bringing more ice from the interior of the ice sheet to the ocean, where its melting would flood coastlines around the world.
In 2015, a nearly 225 square mile (582 square kilometre) iceberg broke away from the glacier. However, it wasn’t until researchers were testing some new image-processing software that they noticed something strange in satellite images taken before the event. In these images, they saw evidence that a rift formed at the very base of the ice shelf nearly 20 miles inland during 2013. Over the next two years, the rift propagated upward, until it broke through the ice surface and set the iceberg adrift over 12 days in late July and early August 2015.
"It's no longer a question of whether the West Antarctic Ice Sheet will melt, it's a question of when," said Ian Howat, an associate professor of earth sciences at Ohio State University. "This kind of rifting behaviour provides another mechanism for rapid retreat of these glaciers – adding to the probability that we may see significant collapse of West Antarctica in our lifetimes."
While this is the first time that researchers have witnessed a deep subsurface rift opening within Antarctic ice, they have seen similar breakups in the Greenland Ice Sheet – in spots where ocean water has seeped inland along the bedrock and begun to melt the ice from underneath. Howat said the satellite images provide the first strong evidence that these large Antarctic ice shelves respond to changes at their ocean edge in a similar way as observed in Greenland.
"Rifts usually form at the margins of an ice shelf, where the ice is thin and subject to shearing that rips it apart," he explained. "However, this latest event in the Pine Island Glacier was due to a rift that originated from the centre of the ice shelf and propagated out to the margins. This implies that something weakened the centre of the ice shelf, with the most likely explanation being a crevasse melted out at the bedrock level by a warming ocean."
Another clue: the rift opened in the bottom of a "valley" where the ice had thinned compared to the surrounding ice shelf. The valley is likely a sign of something researchers have long suspected: because the bottom of the West Antarctic Ice Sheet lies below sea level, ocean water can intrude far inland and remain unseen. New valleys forming on the surface would be one outward sign that ice was melting away far below.
The origin of the rift in the Pine Island Glacier would have gone unseen, too, except that the satellite images Howat and his team were analysing happened to be taken when the Sun was low in the sky. Long shadows cast across the ice drew their attention to the valley that had formed there.
"The really troubling thing is that there are many of these valleys further up-glacier," Howat said. "If they are actually sites of weakness that are prone to rifting, we could potentially see more accelerated ice loss in Antarctica."
More than half of the world's fresh water is frozen in Antarctica. The Pine Island Glacier and its nearby twin, the Thwaites Glacier, sit at the outer edge of one of the most active ice streams on the continent. Like corks in a bottle, they block the ice flow and keep nearly 10% of the West Antarctic Ice Sheet from draining into the sea.
Studies indicate that the West Antarctic Ice Sheet is particularly unstable, and could collapse within the next 100 years. This would lead to a sea level rise of nearly 3 metres (10 ft), engulfing major U.S. cities such as New York and Miami and displacing 150 million people living on coasts worldwide.
"We need to understand exactly how these valleys and rifts form, and what they mean for ice shelf stability," Howat said. "We're limited in what information we can get from space, so this will mean targeting air and field campaigns to collect more detailed observations. The U.S. and the U.K. are partnering on a large field science program targeted at that area of Antarctica, so this will provide another piece to the puzzle."
The study by Howat and colleagues – "Accelerated ice shelf rifting and retreat at Pine Island Glacier, West Antarctica" – is published this week in the journal Geophysical Research Letters.
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