ANTARCTICA — The 2017 calving of the A68 iceberg from Antarctica’s Larsen C ice shelf was likely caused by thinning ice melange[d], the mix of windblown snow, iceberg debris and frozen seawater that normally acts to glue rifts together with larger blocks.
That’s according to a new study in the Proceedings of the National Academy of Sciences, which found the circulation of ocean water beneath ice shelves and radiative warming from above gradually deteriorates the ice melange.
Counterintuitively, if the ice shelves themselves thin, rifts tend to heal, with average annual widening rates dropping from 79 to 22 meters, or 259 to 72 feet.
Additionally, if both the shelves and the melange thinned, this also slowed rift widening.
Only when the melange thinned separately to the ice shelf was rift widening found to increase, from an average annual rate of 76 to 112 meters, or 249 to 367 feet.
The reason for that is that the ice melange starts out much thinner than the ice shelf itself, so when it thins down to 10 or 15 meters thick, it becomes akin to water, allowing the ice shelf rifts to be released and start to crack.
The study’s lead author, Eric Larour, cited by SciTechDaily, says this idea explains why the A68 iceberg was able to break from the Larsen C ice shelf in the dead of the Antarctic winter, because even in winter, warmer ocean water can reach the melange from below.
Previously scientists had thought such large iceberg calving events in the Antarctic Peninsula were caused by hydrofracturing, according to Larour, whereby ‘melt pools on the surface allow water to seep down through cracks in the ice shelf, which expand when the water freezes again,’ but this would not be possible in the dead of winter, with no melt pools present.
The study, then, partially explains how ice shelves can start retreating and becoming unstable decades before hydrofracturing could act on them, and this, according to one of the study’s co-authors, means “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.”