The coast and adjacent ocean
What is the coastline of Antarctica like, and what physical processes occur along the coast and in the Southern Ocean?
Only about 5% of Antarctica’s coastline is what we would think of as typical for a coast – where either land (or rock) meets the sea. Instead, much of Antarctica’s coastline consists to the edges of ice shelves, or some places where the grounded ice directly meets the sea. The key difference between these two, is that the grounded ice is on land and resting on beck rock, and the ice shelves are extensions of ice on land which are floating on the sea.
What’s the difference between sea ice and ice shelves?
Sea ice is another icy feature which is present round Antarctica’s coastline. Sea ice and ice shelves are often confused, however there are some distinct differences between the two.
Ice shelves are the seaward extensions of glaciers, and are therefore made up of glacier ice. They can be hundreds of metres thick; and, because they are the floating part of larger glaciers, the ice within the ice shelves can flow at speeds of several hundred metres per year. Once glacier ice has reached the edge of an ice shelf, it eventually breaks off forming an iceberg. This process, termed ‘calving’, is the main form of glacier ablation in Antarctica.
There are a great variety of ice shelves of different sizes and thicknesses along Antarctica’s coast. As noted in Ice sheets and glaciation, the Ross Ice Shelf and the Ronne-Filchner Ice Shelf are the two largest. The size of an ice shelf is determined by the balance between accumulation and ablation. Although some snow may fall on the ice shelf, most of the accumulation is from the mass added to the ice shelf from the grounded glacier ice behind it. Ablation of the ice shelf occurs through melting due to contact with the sea (both along the edges of the ice shelf and beneath it) and by iceberg calving.
In recent years much research has been devoted to measuring the velocity of ice flow in ice shelves and mapping changes in their size and shape. This is because ice shelves are seen to be an important indicator of climate change in the South Polar Region. The ice shelves of the Antarctic Peninsula have been intensively studied because over the last 50 years the average air temperature in this region has increased by about 3°C, contributing to enhanced melting, retreat, and collapse of several of the ice shelves. The Larsen A Ice Shelf broke up dramatically over a matter of weeks in 1995, and Larsen B disintegrated in 2002.
The latest Larsen Ice Shelf to show signs of complete collapse was Larsen C. In the Climate of the Future page, we learnt how this large ice shelf in 2017 had an iceberg calve off which was the size of Luxembourg.
Since ice shelves float, they do not contribute directly to sea level rise as they melt and break up. However, they have an indirect effect because the removal of an ice shelf causes the grounded ice that was formerly buttressed behind it to flow with greater velocity towards the coastal ablation zone. As the grounded ice experiences increased ablation, this transfers more water from land to ocean, thereby causing sea level to rise.
The warming of the past 50 years has caused the limit of ice shelf viability to retreat southward along the Antarctic Peninsula; and if the warming trend continues, as seems likely due to increases in atmospheric greenhouse gases (see Climate change: past and future), more ice shelves will break up and increased ablation from land-based Antarctic ice will contribute more to the total eustatic rise in sea level.
In contrast with the Arctic, where the extent and average thickness of sea ice has declined dramatically in recent years, the trend for sea ice in the Southern Ocean is less clear. However, with continued global warming, it is likely that sea ice will become less extensive in the Southern Ocean. This in turn would have implications for the south polar climate: the presence of sea ice makes conditions much colder than they would otherwise be, because of the increased albedo (reflection of sunlight) and the reduced ocean-to-atmosphere heat exchange that results when the ocean becomes capped by sea ice. A declining trend in sea ice extent has been observed for the area around the Antarctic Peninsula. Through positive feedback, this could lead to further warming in the region which would then reduce sea ice further.