The Pine Island Glacier and, “the Thwaites Glacier, sit at the outer edge of one of the most active ice streams on the continent”. They provide a buttressing effect to the ice stream, by creating a backward stress that balances the downward stress of the ice trying to flow out to sea.
In 2015, a massive chunk of ice broke off of the Pine Island Glacier. AGU notes that it wasn't until they started testing for new image processing software recently “that they noticed a crack had formed at the very base of the ice shelf nearly 20 miles inland in 2013. The rift propagated upward over two years, until it broke through the ice surface and set the iceberg adrift over 12 days in late July and early August 2015.
The AGU reports:
“It’s generally accepted that it’s no longer a question of whether the West Antarctic Ice Sheet will melt, it’s a question of when,” said Ian Howat, associate professor of Earth sciences at Ohio State and lead author of the new study. “This kind of rifting behavior 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.”
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“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 center of the ice shelf and propagated out to the margins. This implies that something weakened the center 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” in the ice shelf 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 West Antarctic Ice Sheet is particularly vulnerable to global warming caused by our burning of fossil fuels. It’s collapse will be catastrophic. Pine Island and Thwaites glaciers have 10 feet of sea level rise currently locked up in ice, the rapid flow of that ice into the ocean would submerge most coastal cities throughout the world.
From the study abstract:
Pine Island Glacier has undergone several major iceberg calving events over the past decades. These typically occurred when a rift at the heavily fractured shear margin propagated across the width of the ice shelf. This type of calving is common on polar ice shelves, with no clear connection to ocean-ice dynamic forcing. In contrast, we report on the recent development of multiple rifts initiating from basal crevasses in the center of the ice shelf, resulted in calving further upglacier than previously observed. Coincident with rift formation was the sudden disintegration of the ice mélange that filled the northern shear margin, resulting in ice sheet detachment from this margin. Examination of ice velocity suggests that this internal rifting resulted from the combination of a change in ice shelf stress regime caused by disintegration of the mélange and intensified melting within basal crevasses, both of which may be linked to ocean forcing.
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