All things ice in Antarctica
The first recorded sighting on an iceberg in Antarctica was on 1 February 1700 when Edmond Halley was on an expedition to measure the Earth’s magnetic field. He wrote in his diary that ‘he fell in with great Islands of Ice, of Soe Incredible a hight and Magnitude that I scare dare to write my thoughts on it’. Today scientists have classified Antarctic ice as two types: Land and sea ice. Land ice originates from snow that has flowed out to sea as glacier ice and has eventually broken away from ice shelves to form icebergs while sea ice is formed directly from seawater freezing over.
The formation of ice shelves from land ice
The ice shelves you can see during an Antarctica cruise form when glaciers flow off the land and begin to float without melting or break up into icebergs. Ice shelves are common along Antarctica’s coastline. The largest ice shelf is the Ross Ice Shelf which has an area size larger than Spain and the thickest is the Ronne-Filchner Ice Shelf, which is more than 3 km thick in parts. The only part of Antarctica which does not have ice shelves is the northwest portion of the Antarctic Peninsula with scientists reasoning that it could be due to a ‘climatic limit of viability’ as air temperatures are on average a warm -9°C.
Retreating ice shelves
Over the past several decades at least nine ice shelves on the Antarctic Peninsula have shown evidence of retreating due to warming temperatures linked to climate change. The first recorded retreat was the Wordie Ice Shelf in the early 1990s. To measure the scale of the retreat scientists were able to use satellite images from as early as the 1970s to map the retreat. In 1995 there were several ice shelf retreats on the east coast of the Antarctic Peninsula, most famously the Larsen A Ice Shelf that broke off into ship-sized icebergs that floated out to the Weddell Sea. At the same time ice in the Prince Gustav Channel broke up to open a passage between the James Ross Island and the main Antarctic Peninsula for the first time in human history. Even more dramatic was the collapse of Larsen B Ice Shelf in 2002. In just 40 days 3,200 square kilometres of ice shattered to form small icebergs.
Ice shelves sensitive to rising temperatures
Since then several other ice shelves along with Antarctic Peninsula have retreated including the Wilkins Ice Shelf in 2009. Many researchers speculate that Larsen C Ice Shelf will retreat significantly. Some scientific insights have come out from these ice shelf collapses. For instance, by studying the retreat of ice shelves scientists can then identify the climatic limit of viability as well as determine how much of an ice shelf is required to ensure the stability of glaciers that feed it. However, over the next decades with rising temperatures Antarctic ice shelves are likely to be at risk as ice shelves are sensitive to changes in the number of days when temperatures rise above zero degrees Celsius. Even a minor rise in the number of warm summer days could have a significant impact on the survival of glaciers on the Antarctic Peninsula. It could even be that future glaciers have migrated inland and become land-terminating with ice-free ground separating them from the sea.
An iceberg larger than a tropical island
Icebergs originate from the glaciers and ice shelves that surround Antarctica from a process called calving. The glacier front continuously moves downhill into the sea with ice fracturing off to form small icebergs. This process can be fast: In the Antarctic Peninsula glaciers can flow up to 1 metre per day. When large icebergs form – greater than 10 nautical miles across – an identification letter and number are given by the United States National Ice Center and is tracked on satellite imagery until they shrink below the limit required for monitoring. The letter defines the quadrant in which the iceberg originated from with icebergs forming in the Bellingshausen and Weddell Sea sector between 0° and 90°W given the letter ‘A’ while other icebergs formed in other quadrants are assigned letters anticlockwise by ‘B’, ‘C’ and ‘D’. Up to now around 140 icebergs have been numbered. Some icebergs monitored have been larger than countries: One iceberg, B-15, which broke off the Ross Ice Shelf in March 2000 was 295 km long and 37 km wide with an area of around 11,000 square kilometres, which is about the same size as Jamaica.
Icebergs out to sea
Icebergs can ‘survive’ for long periods of time at sea, up to years at a time, and cover large distances around the continent by being blown around by the winds and currents that run along the Antarctic coast in generally an anticlockwise direction. For instance icebergs found off the Antarctic Peninsula have either come from the southern part of the Larsen or Ronne-Filchner ice shelves or even from other ice shelves on the other side of the continent and drifted right around. The Weddell Sea, which has a current that rotates anticlockwise, typically pushes large icebergs originating from the Larsen of Ronne-Filchner ice shelves into a narrow region 100 km across between the South Shetland Islands and the South Orkney Islands. The icebergs that don’t get grounded here drift off into the South Atlantic up past South America. In 1890, a ship recorded an Antarctic iceberg at the same latitude as southern Brazil!
Icebergs not too much of a hazard for shipping
With today’s technology icebergs are not a hazard in international shipping lanes as modern radars are easily detect them floating along. However, it is extremely risky for ships to get close to icebergs as they frequently have hidden underwater keels and can potentially turn or roll at any moment. The main hazard is the eroded parts of icebergs breaking off into small pieces that lie just on the water line and are hard to detect in a swell. Small fragments off icebergs that measure less than 1 metre can damage a ship as they often get grouped together by wind and currents.
The formation of sea ice
Meanwhile, sea ice in Antarctica forms when the temperature of saltwater drops below -1.8°C. At first the ice looks like an oily film on the sea surface and is often called ‘grease ice’. If there is little wind this ice clumps together to form ‘pancake ice’.
As more ice forms and individual ice floes get clumped together from the wind pack ice starts to form. When the pack becomes tightly compressed the individual ice floes form a continuous sheet called an ice field. As the ice thickens the salt is excreted and by the end of winter sea ice can be 1-2 metres thick. During the summer most of the sea ice melts however a parts survive and these become even thicker over the next winter, with its thickness reaching 3-4 metres. In the Antarctic Peninsula the pack ice is quite light but its extent varies significantly year-to-year. This is due to changing air temperatures with years of little pack ice having air temperatures around the same freezing temperature of seawater and vice versa during years with heavy pack ice.