Around 77 per cent of the fresh water on the Earth's surface is held in permanent ice sheets, 90 per cent of that, so 69.3 percent of all freshwater on the planet, is in the Antarctic ice sheet. If all the ice on Antarctica melted, the world’s oceans would rise by a staggering 70 m (229 ft). The Antarctic ice sheet is the largest single mass of ice on Earth. It covers an area of almost 14 million km2 and contains 30 million km3 of ice. In East Antarctica the ice sheet rests on a major land mass, but in West Antarctica the rock is in places more than 2500 m below sea level. It would be seabed if the ice sheet were not there.
Glacial ice appears solid but under the pressures it experiences in the ice sheet, it will flow like a thick liquid. So the ice sheet does not just get thicker as extra snow falls, but, under the action of gravity, flows over and around the underlying topography and down to the sea.
Although the surface of Antarctica is cold, the base of the ice sheet is generally warmer, in places it melts and the melt water lubricates the ice sheet so that it flows quicker. This process produces faster flowing rivers of ice in the ice sheet - these are ice streams.
As their name suggests, Ice streams are streams of flowing ice within an ice cap or ice sheet. In Antarctica up to 10% of the ice that leaves will travel down an ice stream and they can be massive. Up to 2 Km thick, 50km wide and hundreds of km long. The edges of ice streams are often defined by sheared or crevassed ice. They can flow at up to 1km per year.
Another feature of the Antarctic ice sheet is the number of ice shelves all around the coasts. Because ice is less dense than water, as the ice that is flowing to the sea extends from the bedrock base, it floats on the seawater, and experiencing no friction, it can move at speeds of up to 3km per year, far faster than the ice in an ice stream. Antarctica has several large Ice shelves, the Ross Ice shelf has an area of 487,000 square km, slightly smaller than Spain. The seaward front is 600km long and the height of the ice above water varies from 15 to 50m, thick and 90 % of the ice in the ice shelf is below water. The Ross Ice shelf pushes out into the sea at a rate of between 1.5m and 3m (5-10ft) a day, The largest known Iceberg to break off the Ross ice shelf was B-15 in 2000, it was 31,000 km² (12,000 square miles), and slightly larger than Belgium. There are other substantial Ice shelves in Antarctica, the Ronne-Filchner ice shelf is about the same size as Iraq at 430,000sq km and sits in the Weddell Sea. The Amery Ice shelf is on the coast of East Antarctica and for twenty years scientists have been watching a rift that has become known as the Loose-Tooth, these cracks threaten to break off a 1,000 square kilometre chunk of ice. The crack grows at about 3 to 5m /12 ft a day and during the summer, movement is monitored by scientists from the Scripps Institute and from the University of Tasmania. There hasn’t been a major calving like this from the Amery Ice Shelf since the early 1960s and scientists are curious as to whether this is caused by Global Warming or is just part of a 50 to 60 year cycle. Because the ice is already floating, it won’t raise sea levels when it does break off.
Ice shelves appear to work as a brake on the glaciers that flow into them, as has been seen on the Antarctic Peninsula, once Larsen B Ice shelf broke away, the glaciers behind it flowed much faster into the sea. In 2006 New Zealand Scientists working on the sea floor samples near Scott Base announced that they had evidence that the Ross Ice shelf had undergone a collapse in the past, probably suddenly, and that this could trigger catastrophic sea level rises of at least 5m and possibly up to 17m, as the glaciers behind it dumped their ice into the sea.
SUB GLACIAL LAKES
Although Antarctica is bitterly cold, deep below its surface, there are lakes containing liquid water. These show up in photographs taken by projects like Radarsat and the ice over the lakes is flat. One of the largest and best known is Lake Vostok close to the Russian Research station of Vostok. Discovered but not named in 1973 the lake is believed to be two basins divided by a ridge. The lakes are below 3km of the ice sheet and the water is believed to be very old and to be at about -3̊C but remains liquid because of the intense pressure from the icecap and the water at the bottom could be warmed by geothermal heat from the Earth’s interior. In April 2005, German, Russian, and Japanese researchers found that the lake has tides. Depending on the position of the Sun and the Moon, the surface of the lake rises between 1 and 2 cm.
It is suspected that the Antarctic Subglacial lakes may be connected by a network of subterranean rivers. More than 145 subglacial lakes have been discovered in Antarctica. Because of the darkness, low nutrient level, high pressure and isolation from the atmosphere, these lakes represent unique biological habitats. Lake Ellsworth in West antarctica has been chosen to test many of the processes that can be used to drill down to and into the lake without contaminating it. The next expeditions there are planned for 2012-13
ANTARCTIC SEA ICE
The sea ice around Antarctica forms in open, if cold oceans, where there is a constant flow of currents and winds in a west to east direction, this tends to exclude warmer winds and water. Antarctic sea ice also doesn’t have the pressure ridges found in Arctic sea ice where the plates are constrained by land masses and moved against each other by currents and winds The winter extent of Antarctic sea ice is roughly in a circle from the South Pole and reaches up to 18 million square kilometres (6.9 million square miles) It is showing a slight increase over the past decade. By the time the summer is over this has thawed until only 3 million square kilometers (1.1 million square miles) of sea ice remain.
Antarctic sea ice is usually 1 to 2 metres thick and thaws out almost totally during the summer months.
Text © B & C Alexander