“Huge Antarctic iceberg poised to break away”

One of the reasons I was motivated to make Ice Flows was to help people understand iceberg calving (breaking off) behaviour: sometimes iceberg calving is a normal part of the life cycle of a glacier which ends in the sea – snow feeds the glacier, which is turned into ice, which is then lost further down the glacier, as melting, or as icebergs breaking off the ice front.  On the other hand, sometimes an iceberg calving event is a sign that all is not well for the ice shelf.

As long as the amount of snow going in is the same as the amount lost through melting and calving, the glacier front stays in more or less the same place.  Sometimes icebergs are small, sometimes they are absolutely massive, like the one about to break off the Larsen C Ice Shelf, described in the story by the BBC:

http://www.bbc.co.uk/news/science-environment-38522954

What we need to assess is whether this iceberg is just part of the life cycle of this part of the Antarctic Ice Sheet – is it just that this part of the ice shelf has been hanging on for a while, so it is bigger than your usual iceberg, or is a sign of things to come for this ice shelf, leading to a potential collapse of the ice shelf.

Adrian Luckman, leading the project reported in the story, indicates that there is uncertainty over the implications of this iceberg for the future of the ice shelf.

To fully assess the implications, we need to use computer models that can represent the important aspects of the ice sheet/ice shelf/ocean system, and do some experiments with these models to see what might happen after this iceberg calves.

A study by Johannes Fürst published last year in Nature Climate Change1 assessed the areas of ice shelves which were important for “holding back” (buttressing) the grounded (not floating) parts of the ice sheet.  The study found that most of the ice in the part of the ice shelf about to break off was “passive” ice, i.e. that if it broke off, it wouldn’t lead to a speed up of the grounded ice behind it.  However, they noted that the ice shelf front would then take on a shape which was similar to that of other parts of the Larsen Ice Shelf before they collapsed.

Members of the Project MIDAS team published some modelling results in the Cryosphere in 20152 (open access) which suggested the rift presents a considerable risk to the stability of the Larsen C Ice Shelf.

If this iceberg calving event does lead to the collapse of the rest of the ice shelf – which is holding back the grounded ice behind it – then speed-up of the grounded ice would be likely to occur and this would then lead to sea level rise.  However, if ice shelf collapse does not occur, the ice will flow forward as normal and the ice front will most likely advance again until the next calving event occurs.

For more information on how ice sheets, ice shelves and icebergs behave, take a look at: http://www.iceflowsgame.com/scienceorfiction.html

Project MIDAS http://www.projectmidas.org/

1Fürst, J.J., Durand, G., Gillet-Chaulet, F., Tavard, L., Rankl, M., Braun, M. and Gagliardini, O., 2016. The safety band of Antarctic ice shelves. Nature Climate Change, 6(5), pp.479-482.

2Jansen, D., Luckman, A. J., Cook, A., Bevan, S., Kulessa, B., Hubbard, B., and Holland, P. R., 2015. Brief Communication: Newly developing rift in Larsen C Ice Shelf presents significant risk to stability, The Cryosphere, 9, 1223-1227, doi:10.5194/tc-9-1223-2015.

(Edited 09/01/17)

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