‘Ice melange’ warning over Antarctica ice shelf collapse
"Ice melange" is a slushy concoction of wind-blown snow, iceberg debris and frozen seawater – and it could be key to understanding ice shelf collapse.
As the world warms, collapsing ice shelves could drive sea level rises – and the new findings about ice melange mean this could come sooner than expected.
The researchers believe ice melange may be key to why the iceberg A-68 broke off Antarctica’s Larsen C ice shelf after years of cracks spreading across the ice.
Ice melange normally "heals" cracks – but it’s thinning due to climate change.
New research shows that thinning of layers of ice melange could be a major driver of ice shelf collapse, according to a paper published in Proceedings of the National Academy of Sciences.
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"The thinning of the ice melange that glues together large segments of floating ice shelves is another way climate change can cause rapid retreat of Antarctica's ice shelves," said co-author Eric Rignot, UCI professor of Earth system science.
"With this in mind, we may need to rethink our estimates about the timing and extent of sea level rise from polar ice loss – i.e., it could come sooner and with a bigger bang than expected."
Using Nasa's Ice-sheet and Sea-level System Model, observations from the agency's Operation IceBridge mission, and data from Nasa and European satellites, the researchers assessed hundreds of rifts in the Larsen C ice shelf to determine which were most vulnerable to breaking.
They selected 11 top-to-bottom cracks for in-depth study, modelling to see which of three scenarios rendered them most likely to break: if the ice shelf thinned because of melting, if the ice melange grew thinner, or if both the ice shelf and the melange thinned.
"A lot of people thought intuitively, 'If you thin the ice shelf, you're going to make it much more fragile, and it's going to break,'" said lead author Eric Larour, Nasa JPL research scientist and group supervisor.
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Instead, the model showed that a thinning ice shelf without any changes to the melange worked to heal the rifts, with average annual widening rates dropping from 259 to 72 feet.
Thinning both the ice shelf and the melange also slowed rift widening but to a lesser extent.
But when modelling only melange thinning, the scientists found a widening of rifts from an average annual rate of 249 to 367 feet.
The difference, Larour explained, reflects the different natures of the substances.
Larour said: "The melange is thinner than ice to begin with. When the melange is only 10 or 15 meters thick, it's akin to water, and the ice shelf rifts are released and start to crack."
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Even in winter, warmer ocean water can reach the melange from below because rifts extend through the entire depth of an ice shelf.
"The prevailing theory behind the increase in large iceberg calving events in the Antarctic Peninsula has been hydrofracturing, in which melt pools on the surface allow water to seep down through cracks in the ice shelf, which expand when the water freezes again," said Rignot, who is also a Nasa JPL senior research scientist.
"But that theory fails to explain how iceberg A68 could break from the Larsen C ice shelf in the dead of the Antarctic winter when no melt pools were present."
He said he and others in the cryosphere studies community have witnessed ice shelf collapse on the Antarctic Peninsula stemming from a retreat that began decades ago.
Rignot said: "We have finally begun to seek an explanation as to why these ice shelves started retreating and coming into these configurations that became unstable decades before hydrofracturing could act on them.
"While the thinning ice melange is not the only process that could explain it, it's sufficient to account for the deterioration that we've observed."
Watch: The importance of ice shelves
