Many Antarctic ice shelves have lost massive volume in recent decades due to the calving of icebergs, some of them enormous. Last year, for example, A-68a, the largest iceberg in the world, caused concern when it headed straight for the slightly smaller sub-Antarctic natural paradise of South Georgia. A-68a broke off the Larsen C ice shelf in the Weddell Sea on the Antarctic Peninsula in July 2017. Most other ice shelves in Antarctica are also losing stability due to break-offs of massive icebergs. In a new study, American scientists have now been able to show the underlying causes of the increasingly frequent calving.
Until now, the assumption has been that increased iceberg breakup resulted from hydrofracturing, where water from melt ponds at the surface seeps down through cracks in the ice shelf and expands as it freezes, much like frost weathering in rock. In the new study, which appeared in the journal Proceedings of the National Academy of Sciences, glaciologists from the University of California Irvine and NASA’s Jet Propulsion Laboratory used modeling to examine the dynamics that led to the breakup of A-68a.
Contrary to the existing theory, they found that the main reason for the breakup is the decrease of the so-called ice melange. This is a sort of ice slurry consisting of windblown snow, iceberg debris and frozen seawater that repairs cracks in icebergs like glue. According to the researchers, the circulation of ocean water beneath the ice shelf and warming from solar radiation cause the ice melange to gradually degrade over the decades.
These new findings have great significance for understanding the relationship between climate change and the stability of the ice shelf, which, according to current assumptions, buttresses the glaciers on land and slows their outflow toward the sea.
“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. 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.”
Eric Rignot, professor of earth system science at the University of California Irvine and co-author of the study
From hundreds of rifts in the Larsen C Ice Shelf, the researchers selected eleven vertically running rifts for closer examination and then created models to determine under which of three different scenarios the likelihood of rupture is greatest: (1) when the ice shelf thins due to melting processes; (2) when the ice melange thins; or (3) when both the ice shelf and the ice melange thin.
“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, a researcher at NASA JPL and leader of the group. However, the models instead showed that under scenario (1) – the decrease of the ice shelf, with no change to the melange – the rifts were able to heal, with the annual widening of the rifts dropping from 79 meters to 22 meters. Under scenario (3), the widening of the rifts also slowed, but to a lesser extent. Only scenario (2), the thinning of mélange, saw widening of rifts from an average of 76 meters to 112 meters per year.
According to Larour, this difference reflects the different nature of the substances. “The melange is thinner than ice to begin with,” Larour says. “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.” The ice melange doesn’t necessarily increase even in winter because the rifts extend to the base of the ice shelf, getting in contact with warmer ocean water.
“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 senior scientist at NASA JPL. “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.”
Along with other cryospheric scientists, Rignot has observed ice shelves on the Antarctic Peninsula breaking off due to a retreat that began decades ago. “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,” Rignot said. “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.”
Julia Hager, PolarJournal
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