Several groups of researchers are reporting that a large rift (Figure 1) is stretching across the West Antarctic Ice Sheet’s Larsen C Ice Shelf (e.g. MIDAS and IceBridge). This crack will eventually separate an iceberg the size of a small country from the main ice sheet. And the loss of this large ice mass might do a lot more than just give a few hundred penguins something to peep about.
What are ice shelves?
Ice shelves are floating bodies of ice that extend offshore from terrestrial ice sheets and glaciers. They extend offshore from the outward flow of the terrestrially-grounded ice mass. Their growth is kept in check by melting at their base and calving at their margin.
Where is this ice shelf and why do we care?
The Larsen C ice shelf is located on the western Antarctic Peninsula. This peninsula is largely covered and surrounded by the West Antarctic Ice Sheet. Also, here’s a random fact: along much of its coast, this peninsula smells of oily gobs of penguin poo.
West Antarctica is an important place. The West Antarctic Ice Sheet is considered to be relatively unstable and responsive to climate and sea level change (Mercer, 1978). In other words, if the climate keeps being so annoying, it may just take its ball and go home. (No one is positive about this; we’ve never seen it get so mad before. That’s why studying it is important.) If the West Antarctic Ice Sheet does break up and/or melt, it would single-handedly cause over ten feet (3.3 metres) of eustatic sea level rise (Bamber et al., 2009). (Eustatic sea level change means that the change is caused by adding or removing water. This means that the added effects of isostatic flexure or the thermal expansion of water are not considered.)
What’s with this crack and upcoming iceberg?
As of January 19, 2017, the Larsen C Ice Shelf crack was 175 km long (Figure 1). This leaves only about 20 km of ice connecting the massive soon-to-be-iceberg to the ice shelf. If—no, actually, when—this piece of ice detaches, it will be one of the biggest icebergs we’ve ever seen. How big? It will be approximately 5,000 km2. Just for perspective, that’s bigger than Rhode Island and a bit smaller than Delaware.
Discussion (three things you should know):
One: things are uncertain
First off, these things happen. This ice shelf rift may not have anything to do with changes in climate or sea level. And, the loss of this huge chunk of ice may not destabilize the neighboring ice sheet.
However, just the opposite may be true. Previous ice shelf collapse events have likely been caused by warming and/or sea level change (Figure 2). Furthermore, these events have caused drastic ice mass accelerations (Rott et al., 2002). Sometimes these accelerations lead to ice streaming (De Angelis and Skvarca, 2003).
What is likely is that previous ice sheet retreat has been affected by the loss of large ice shelves. For instance, some of my own research has shown that the last British-Irish Ice Sheet had a large ice shelf offshore of western Ireland (Peters et al., 2015). When this ice shelf deteriorated, the overall rate of retreat of the ice sheet increased by about 65% (Peters et al., 2016). These events were some of the initial causes of ice sheet retreat along the marine margin west of Ireland.
No one is certain what the loss of this 5,000 km2 block of ice that currently clings to the shore of Antarctica will induce. This is why continued research is a good idea.
Two: potential changes in ice flow mechanics
Ice shelves don’t just float around and look pretty. They also buttress the outward flow of the entire ice sheet (Gudmundsson,
2013). So when a large iceberg separates from an ice shelf, the remaining ice is free to flow relatively faster. Imagine how the stone walls of an old gothic cathedral would be free to crumble without flying buttresses (Figure 3). The consequence of this is that after ice shelves are lost or deteriorated, terrestrial ice experiences increased mass loss. This has been documented after the break-up of the Larsen B ice shelf (Rignot et al., 2004; Figure 2).
This increased outward ice flow thins the remaining ice sheet and adds water to the oceans. Furthermore, the thinned ice sheet is left more vulnerable to continued climate change.
Three: changes in bottom water formation
Ice shelves melt from underneath, where they are in contact with ocean water. This generates large volumes of cold (obviously) and fresh water. Cold, fresh water is also supplied below ice shelves from subglacial meltwater. This glacial water induces stratification of the water column and helps produce Antarctic Bottom Water (Foldvik and Gammelsrød, 1988). Antarctic Bottom Water is the densest water mass in the world’s oceans. This means it plays a big role in driving thermohaline circulation.
Thermohaline (Thermo = temperature, haline = salt) circulation is a density-driven conveyer belt that constantly circulates the world’s oceans. Thanks to this circulation, the oceans aren’t frozen all the time near the poles and they aren’t close to boiling near the equator. If the Earth’s equatorial waters were substantially warmer, we would all suffer from much worse storms. In fact, without the constant thermohaline circulation, the world would experience abrupt and drastic climate change.
The upcoming birth of a mega-iceberg from Antarctica may not be particularly significant. However, it might be very significant. Regardless, of its significance, it’s cool. Also, it’s an excellent excuse to discuss some of the concerns that scientists have for the future of Earth’s ice sheets, oceans and climate.
Bamber, J. L., Riva, R. E., Vermeersen, B. L., & LeBrocq, A. M. (2009). Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet. Science, 324(5929), 901-903. http://science.sciencemag.org/content/324/5929/901
De Angelis, H., & Skvarca, P. (2003). Glacier surge after ice shelf collapse. Science, 299(5612), 1560-1562. http://science.sciencemag.org/content/299/5612/1560
Foldvik, A., & Gammelsrød, T. (1988). Notes on Southern Ocean hydrography, sea-ice and bottom water formation. Palaeogeography, Palaeoclimatology, Palaeoecology, 67(1-2), 3-17. http://www.sciencedirect.com/science/article/pii/0031018288901198
Gudmundsson, G. H. (2013). Ice-shelf buttressing and the stability of marine ice sheets. The Cryosphere, 7(2), 647. http://search.proquest.com/openview/8b91cc025b7a2e17fb2f435b0eed02aa/1?pq-origsite=gscholar&cbl=105732
Mercer, J. H. (1978). West Antarctic ice sheet and CO 2 greenhouse effect- A threat of disaster. Nature, 271(5643), 321-325. http://www.nature.com/nature/journal/v271/n5643/abs/271321a0.html
Peters, J. L., Benetti, S., Dunlop, P., & Cofaigh, C. Ó. (2015). Maximum extent and dynamic behaviour of the last British–Irish Ice Sheet west of Ireland. Quaternary Science Reviews, 128, 48-68. http://www.sciencedirect.com/science/article/pii/S027737911530113X
Peters, J. L., Benetti, S., Dunlop, P., Cofaigh, C. Ó., Moreton, S. G., Wheeler, A. J., & Clark, C. D. (2016). Sedimentology and chronology of the advance and retreat of the last British-Irish Ice Sheet on the continental shelf west of Ireland. Quaternary Science Reviews, 140, 101-124. http://www.sciencedirect.com/science/article/pii/S0277379116300737
Rignot, E., Casassa, G., Gogineni, P., Krabill, W., Rivera, A. U., & Thomas, R. (2004). Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf. Geophysical Research Letters, 31(18). http://onlinelibrary.wiley.com/doi/10.1029/2004GL020697/full
Rott, H., Rack, W., Skvarca, P., & De Angelis, H. (2002). Northern Larsen ice shelf, Antarctica: further retreat after collapse. Annals of Glaciology, 34(1), 277-282. http://www.ingentaconnect.com/content/igsoc/agl/2002/00000034/00000001/art00041
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