Increased global temperatures are taking their toll on the Antarctic ice shelves. Here are eight points of concerns regarding the west Antarctic Ice Sheet.
By Gino Casassa
1. Antarctica accounts for 91 percent of the total mass of ice on the Earth, contained in a vast ice sheet up to 4.6 kilometres thick. If the Antarctic ice were to melt entirely, it would raise global sea level by 55 metres, a truly catastrophic scenario. Thankfully, this full-scale melt-down of the Antarctic ice is unlikely to occur over the East Antarctic Ice Sheet (EAIS), which accounts for 50 metres of global sea level and is considered stable because its bed lies well over sea level.
2. Some believe that the interior of the Antarctic continent is too cold to be affected by potential melting produced by surface warming of a few degrees expected to occur from over the next century. On the contrary, parts of inland Antarctica, such as the South Pole, are growing because of enhanced snow precipitation in a warmer atmosphere, which can retain higher humidity.
3. However global warming is already having a discernable effect on the fringes of the continent, as evidenced by the dramatic break-up of the Larsen A ice shelf and other smaller floating ice shelves on the Antarctic Peninsula, where summer temperatures are frequently above zero.
4. There is evidence that these peripheral changes are having a strong effect on glaciers of the inland Antarctic ice due to ice dynamics. The glaciers that used to feed Larsen A ice shelf have accelerated threefold after the collapse of the ice shelf, suggesting that the ice shelves have a critical role in restraining the flow of the inland ice. A similar behaviour might be occurring on the Amundsen Sea sector of the west Antarctic Ice Sheet (WAIS), where Pine Island and Thwaites glaciers have lost significant portions of their fringing ice shelves, and show signs of recent acceleration. This ice sheet is a fraction of the size of the dominating east Antarctic ice sheet, but its mass is still great enough to raise global sea-level by five meters.
5. The east Antarctic ice sheet is known as a continental ice sheet since it is supported by land above sea level. Unlike its eastern sister, the western ice sheet is a marine ice sheet which is grounded on bedrock well below sea level. In addition, the underlying bedrock in many areas has a downward slope away from the coastline. This circumstance could result in a run-away effect leading to total collapse should the edges of the ice sheet start to retreat due to an initial trigger such as atmospheric and/or oceanic warming. The western ice sheet would therefore largely disappear if the ice would melt
6. The west ice sheet has been the subject of considerable research over the last 25 years.
Theoretical: Modelling has been developed to understand the nature of potential instability of West ice sheet, and the roles that ice shelves play in stabilizing the ice sheets. Differing views exist on the importance of the buttressing effect of ice shelves. Early models predicted an important stabilizing role of ice shelves, but later more sophisticated models suggested an insignificant role of ice shelves. There is now compelling evidence that shows the relevant stabilizing role of ice shelves, and in a few cases this new information is already being incorporated into the models.
Experimental: Of the major glaciers – ice streams – in the Antarctic Peninsula and in western Antarctica studies are being conducted to detect whether they are speeding up or slowing down. Results in the west ice sheet are contradictory. Ice streams in the Ross Sea sector are slowing down while the glaciers in the Weddell Sea are in near steady state.
Overall conclusion reached by the scientific community by the mid-1990s was that the west ice sheet appeared to be relatively stable, and any retreat of the ice sheet would occur relatively slowly over hundreds or thousands of years, despite loss of ice shelves.
7. Over the last few years, new evidence that changes may indeed be happening faster than previously thought has emerged:
Recent satellite image analyses show that the ice streams in the Amundsen region are clearly retreating, thinning and accelerating, particularly Thwaites and Pine Island glaciers;
New theoretical models suggest that ice shelves are important for the stability of ice sheets;
Small glaciers on the Antarctic Peninsula have speeded up greatly following the collapse of the Larsen A ice shelf.
8. A new urgency has been injected into the study of the stability of West ice sheet. A team from the Center for Scientific Studies of Santiago (CECS), NASA and the German Geological Survey (BGR) plans a new series of investigations, concentrating on the Amundsen Sea region in 2004/2005.
GINO CASASSA is Head of the Glaciology and Climate Change Laboratory at Centro de Estudios Científicos in Valdivia, Chile. He is the glaciology representative of Chile’s Scientific Committee for Antarctic Research (SCAR), and member of the International Council of Scientific Unions (ICSU) Planning Group for the International Polar Year 2007. He is currently also the head of the Andean Working Group on Glacier Mass Balance sponsored by the International Commission on Snow and Ice/International Association of Hydrological Sciences.