Ice with everything - Antarctica: Global Science from a Frozen Continent - page 73

ice albedo (its ability to re ect light and heat from the sun), its energy budget,

as well as the rate of its expansion and decay. The presence of sea ice also has a large
impact on the exchange of heat and water between the ocean and the atmosphere.
Due to the huge winter sea ice cover, the

and isolation of Antarctica
is strongly enhanced in the winter: the distance between the open ocean waters
and the Antarctic coast can increase by up to 3000 km!
Today, the extent of the Antarctic sea ice varies between approximately 2million
km² in local autumn (March) to 15 million km² in local spring (October) ( Figure 3.3 ).
A few reconstructions of past sea ice extent have been performed, based on
biological indicators measured in deep sea sediment cores drilled near the modern
sea ice margin. A synthesis of the data spanning the Last Glacial Maximum (about
21 000 years ago) suggests a doubling of the maximum sea ice extent, but limited
changes in summer sea ice extent. Major efforts are needed to better document
the history of the Antarctic sea ice extent, poorly known prior to satellite
measurements.
Large inter-annual to decadal variability has been observed since the
beginning of satellite measurements, in 1979. It has been observed that
El Niño events, by heating the surface waters of the Southern Ocean, strongly
affect inter-annual variations of Antarctic sea ice. Since 1979, the Southern
Ocean has been warming at a rate of about 0.17 C per decade, but Antarctic sea
ice shows a small increasing trend. Modelling studies suggest that two processes
may be involved. First, the depletion of the Antarctic ozone layer has caused a
cooling of the stratosphere, enhancing the westerly winds around Antarctica,
strengthening the formation of coastal polynyas and sea ice production.
Another explanation lies in changes in the mixing of surface ocean waters.
An increase in precipitation amounts at the surface of the Southern Ocean,
associated with warmer temperatures, can form a fresh, low density, surface
ocean layer. This can reduce the mixing between the cold surface waters and
the warm subsurface ocean waters, limiting the in

' continentality '

ow of heat, which can melt

sea ice.
This highlights the complexity of the mechanisms controlling the Antarctic
sea ice, which remains a major scienti c frontier. Intensive efforts have been

deployed during the International Polar Year to better document the physical
and mechanical processes associated with the sea ice formation, growth and decay.
A radar altimeter boarded on the CRYOSAT-2 satellite is mapping precisely the
thickness of sea ice. The improved knowledge from field and spatial observations

Figure 3.6 (opposite) In spring, sunlight and warmer

temperatures induce the start of sea ice collapse.
(Credit: IPEV/ D. Ruche)