The phenomenon of ice shelf collapse is becoming increasingly worrying, with climatic warming on the Antarctic Peninsula leading to the disintegration of a number of ice shelves in the region. The disappearance of ice shelves is associated with the acceleration of glaciers draining inland ice sheets, leading to an overall decrease in the ice sheet mass balance and contributing to global sea level rise.
What is an Ice Shelf?
In polar regions much of the land is covered by thick masses of ice known as ‘ice sheets’. Ice sheets can measure 100s of metres to several kilometres in thickness and cover huge areas of land (e.g. most of Antarctica in the south and Greenland in the north). Ice flows slowly from the ice sheets down to the sea via glaciers and ice streams (a current of ice that flows faster than the surrounding ice).
When the ice reaches the sea it does not melt immediately as the polar oceans are very cold, so it can extend out across the sea surface, potentially for hundreds of kilometres. These ice sheet ‘extensions’ are called ice shelves.
Ice shelves generally occupy bays or regions between islands. Most ice shelves are found in the Antarctic, although a few small shelves exist along the north coast of Ellesmere Island in the Arctic. The largest Antarctic ice shelves are the Ross shelf (south of the Ross Sea), which covers almost 473,000 square km and the Filchner-Ronne (or Ronne-Filchner) shelf (south of the Weddell Sea), which covers more than 422,000 square km.
The Ice Sheet - Ice Shelf System
The ice sheet - ice shelf system forms part of the ‘cryosphere’ – the portion of the Earth covered in ice. The flow of glaciers or ice streams into an ice shelf causes it to grow, while it loses mass at its seaward edge by melting and ‘calving’ icebergs. Snowfall on the ice sheet replenishes the ice drained by the glaciers and ice streams. If the system is in balance then the ice sheet replenishment rate will equal the draining rate.
Likewise, the growth of the ice shelf due to the addition of glacial ice is balanced, at least in part, by iceberg calving and melting. If the system is in equilibrium then there will be no overall change in the mass of ice over time and there will be no advance or retreat of ice shelf fronts. A system in balance can also expect no overall change in glacier or ice stream flow rates.
Recent Antarctic Peninsula Ice Shelf Collapse
In recent years a number of Antarctic Peninsula ice shelves have collapsed. The Antarctic Peninsula stretches far north from the Antarctic continent, and experiences a relatively mild climate, due in a large part to the proximity of the ocean. All ice shelves in this region experience some melting each summer but recent climatic warming has been associated with the collapse of a number of ice shelves.
British Antarctic Survey (BAS) first showed in 1991 that the Wordie Ice Shelf on the west Antarctic Peninsula was retreating. Since then that ice shelf has almost disappeared. BAS has found that the ice shelf retreat began in the 1950s, with at least six other shelves in addition to the Wordie having experienced significant retreat between 1950 and 2008.
Perhaps the best known cases of ice shelf collapse were those of the Larsen A and B shelves in 1995 and 2002, respectively. The most recent collapse to be observed closely was that of the Wilkins Ice Shelf in 2008. It has been shown that ice shelves are sensitive to ocean temperature so it is important to monitor changes in ocean climatology as well as atmospheric temperature changes.
Why Are Ice Shelves Important?
Ice shelves float on the ocean so we could reasonably expect their melting to have no effect on sea level, but the 2010 Geophysical Research Letters article “Recent loss of floating ice and the consequent sea level contribution” by Andrew Shepherd and colleagues showed that, because fresh water is less dense than seawater, when ice shelves and icebergs melt they do actually lead to a slight increase in sea level. However, this effect is very small.
More importantly, ice shelves play a crucial role in the mass balance of ice sheets. Ice shelves effectively buttress the glaciers that feed them, thereby slowing the rate at which the ice sheet is drained. The retreat of Antarctic Peninsula ice shelves has been associated with glacier acceleration and BAS reports that this, coupled with other processes, is leading to the northern Antarctic Peninsula contributing 0.16mm per year to global sea level rise.
This contribution seems small and little cause for concern. However, more worryingly, there is evidence for glacier retreat and an increase in drainage of the West Antarctic Ice Sheet (WAIS), which is much larger than the Antarctic Peninsula Ice Sheet and also inherently unstable. According to the International Polar Foundation interview “Robert Bindschadler on West Antarctica’s Pine Island Glacier”, collapse of the WAIS could lead to a significant increase in global sea level.
At this time, the effects of Antarctic ice shelf collapse and glacier retreat are so small as to be virtually undetectable. The real cause for concern lies in what present changes mean for the future of the vast ice sheets, whose collapse certainly would be noticed. Some of the ice shelves that are retreating existed for at least 10,000 years, suggesting that current climatic changes are leading to a huge upheaval of the Antarctic cryosphere.
Sources
British Antarctic Survey, "The Antarctic Peninsula’s retreating Ice Shelves", 2008 Science Briefing, accessed August 6 2010.
National Snow and Ice Data Center, "Quick Facts on Ice Sheets", accessed August 6 2010.
Margaret Wallace - Hello, my name is Mags Wallace. I did an MESci in geophysics and Geology from Liverpool University, followed by a PhD in Physical ...
