Antarctica - Role in Global Environment

Australia: The Land Where Time Began

Antarctica is the only continent that experiences a truly polar climate, being separated from the other continents by the Southern Ocean, the section of all the oceans south of about 60^o S. The Antarctic Ice Sheet presently contains at least 80 % of its volume at the time of the last glacial maximum, (Denton & Hughes, 1981). Therefore, according to the author¹ a setting in the present for the study of Earth in the "Ice Age" is provided by Antarctica, with its long history of glaciation in the Cainozoic and its key role in regulating the climate of the world, as well as regulating oceanography, and eustasy for the greater part of the present geologic era.

According to the author¹ for much of the glacial history of Antarctica the continent's lithosphere, atmosphere, cryosphere, hydrosphere and biosphere have been linked, though it has not always been glaciated. Antarctica's coastal regions were covered by lush temperate forests that were inhabited by a highly diverse fauna, at a time when the continent was located at a similar latitude to South America. The climate of Antarctica cooled and the hours of daylight became distributed more seasonally as it drifted south and away from the other continents that comprised Gondwana. As the biota evolved to survive in the increasingly harsh conditions on the continent the author¹ suggests they made some of the most spectacular evolutionary changes that are palaeontologically poorly understood events in the geologic history of the Earth. Changes also occurred in the properties of the water mass and in circulation patterns of the Southern Ocean and organisms well away from the coast of Antarctica were influenced by these changes, and there is a suggestion that early humans may possibly have responded to it influence (Denton, Prentice and Burckle, 1991), but there is little doubt that the fragile ecosystems of the Southern Ocean have been seriously altered by the uncontrolled harvesting of seals, whales, finfish and krill.

The early evolution of the Antarctic Ice Sheet has been strongly influenced by tectonic activity, especially by the uplift of the Transantarctic Mountains (TAM). A physiographic boundary between the ice sheets of East and West Antarctica was eventually formed by the uplift of the TAM. These ice sheets evolved quite differently from each other, in their size, shape and dynamics, the differences being largely the result of their geologic settings.

The global systems are continuously influenced by the constant changes of the Antarctic atmosphere-cryosphere-lithosphere system. Antarctica acts as a short-term atmospheric heat sink that causes gradients of temperature that drive the circulation of the atmosphere in the Southern Hemisphere (Mullan & Hickman, 1990). Manufactured chemicals seriously altered the stable air mass of the upper atmosphere situated above the Antarctic continent with the result that the Antarctic Ozone Hole was formed (Farman, Gardner & Shanklin, 1985; Solomon et al., 1986). Also, evidence has been found in ice core records of a marked increase in atmospheric concentrations of carbon dioxide and methane, both greenhouse gases, over the past 200 years, the current levels of these gases being higher than at any time in the past 160,000 years (Oeschger & Siegenthaler, 1988; Lorius, Jouzel & Raynaud, 1993).

The climate and circulation of much of the Southern Hemisphere is influenced by the growth and decay of sea ice around Antarctica. One of the strongest temperature gradients is the one associated with the sea ice zone around Antarctica. The oscillating area of sea ice around Antarctica is associated intimately with the surface water masses and is the primary regulator of the vertical heat flux and stability of these water masses (Mullan & Hickman, 1990; Matrinson & Iannuzzi, 1998). It has been predicted that the area of Antarctic sea ice will be dramatically reduced (Budd, 1991). Heat loss from the ocean to the atmosphere would be increased by 2 orders of magnitude if the sea ice canopy is lost (Budd, 1991). In the Southern Ocean it has been documented that there is a correspondence between the location of the margin of the sea ice, oceanographic fronts, and primary production levels in the surface waters, though the prediction of the ultimate biologic perturbation resulting from a decrease in the area of sea ice surrounding Antarctica is problematic (Mortlock et al., 1991). A correlation has been established between fluctuations of the sea ice area around Antarctica, on annual and decadal time scales, and there is a correlation between this variability and changed atmospheric circulation, especially in the intensity of the wind. Some evidence has been found of production rates of Antarctic Bottom Waters, one of the main deep waters in the world ocean, being affected by these variations (Comiso & Gordon, 1998).

The melting of the Antarctic ice shelves is another result of global warming, which the author¹ suggests is already being observed in the region of the Antarctic Peninsula. During the 20th century a warming trend coincided with the significant reduction in the size of the Larsen [almost the entire Larsen B Ice Shelf, 3,250 km², collapsed between January 21 and April 13, 2002] and George VI ice shelves (Potter & Paren, 1985; Rott, Skvarca, and Nagler, 1996) and the complete disappearance of the Wordie Ice Shelf, that was smaller, (Doake & Vaughan, 1991) and the Müller Ice Shelf (Domack et al., 1995) in the region of the Antarctic Peninsula. The author¹ asks whether this warming trend is the result of anthropogenic activity or simply part of a natural cycle, suggesting the answer will be found by studying ice cores as well as sediment cores from the region.

Outlet glaciers and ice streams sustain ice shelves by their flowing to the sea, so when the ice shelves retreat the reduction of ice shelf mass initiates an increased flow rate, as there is less ice shelf mass to slow down the flow to the sea, the ultimate result will be a smaller ice sheet. There is a strong feedback between ice shelves and oceanographic processes. When the water mass beneath the ice shelves is warmer an increase in the basal melting rate of the ice shelves of an order of magnitude is the result, just such an input being observed by 1985 throughout West Antarctica (Potter & Paren, 1985; Jenkins & Doake, 1991; Jacobs et al., 1992; Jacobs, Hellmer & Jenkins, 1996; Jenkins et al., 1996). In the formation of water masses on the continental shelf of Antarctica ice shelves have a key role.

The production and volume of very cold shelf water masses would be decreased by the loss of the ice shelves, and this water is a vital component of Antarctic Bottom Water (ABW). One of the most delicate features of the Antarctic environment is the feedback between the ice shelves and water masses, which would quickly respond to global warming (Budd, 1991). As deep and intermediate water masses, which are formed near Antarctica, have a integral role in the global circulation of the ocean, any changes in the production rate of water mass would have a global impact.

Evidence has been found by studies that have been carried out on large ice streams that flow into the Ross Sea that the West Antarctic Ice Sheet (WAIS) is unstable and it is believed possible it could undergo rapid retreat at some point in the next few thousand years (Hughes, 1973, 1987; Alley, 1990). This instability is in part the result of interaction between the ice sheet and its bed, which has been subjected to stress to the extent that the basal material is functioning as a deforming layer along which the ice sheet moves, greatly increasing its velocity of flow (Alley et al., 1989). The result of this is thinning of the ice sheet if this occurs with no corresponding accumulation of ice thickness. The ice sheet may become buoyant and be decoupled from the seafloor if there is sufficient thinning of the ice sheet as a result of tidal pumping, and the author¹ suggests this mechanism may operate independent of climate. The ice sheet could collapse, resulting in a rapid increase in sea-level rise if decoupling occurred (Thomas & Bentley, 1978; Hughes, 1987; Alley, 1990; Alley & Williams, 1991). It has been suggested that rapid sea level rise in the Holocene may have resulted from the collapses of marine ice sheets (Anderson & Thomas, 1991). Coastal evolution experienced a profound impact from these eustatic rises, even though they were of low magnitude (Thomas & Anderson, 1994).

The idea that the WAIS may undergo sporadic collapse that occurs very rapidly has been supported by model simulations (MacAyeal, 1992). It has also been argued that rapid retreat of the WAIS may have been set in motion, given the delayed response the ice sheet displays to changes in its bed (Alley, 1990). In regard to the East Antarctic Ice Sheet (EAIS), some evidence has been found that it may also undergo a size reduction, that is at least local (Nakawo, 1989), though it has been argued (Jacobs, 1992) that available data on the EAIS mass balance are inconclusive.

There has been a change in the linkages between Antarctica and other global systems over time, in particular Antarctica's ice sheet size fluctuations. The continent and continental shelf have been eroded deeply, with glacial erosion along the geologic boundaries, such as fault zones, being the most pronounced, with the result that glacial troughs have been formed on the shelf. Ice streams, which is ice discharging from the continent, have used these troughs as conduits. The ice sheet increasingly becomes unstable as a result of the lowering of the bed beneath it, and its response to rising and falling sea level becomes more radical. It is in this manner that the WAIS has become increasingly sensitive to changes of volume of the northern ice sheets. As the ice sheets of the Northern Hemisphere extend to lower latitudes they are especially sensitive to changes of climate, which occur at high frequencies. The higher frequency of ice sheet grounding events of the Ross Sea continental shelf provides evidence for the linkage between the ice sheets in the Northern and Southern Hemispheres (Alonso et al., 1992; Anderson & Bartek, 1992) and the continental shelf of the Antarctic Peninsula (Bart & Anderson, 1995) during the Pliocene-Pleistocene compared to the Miocene.

The author¹ suggests knowledge of this system should be increased as there are strong linkages between the lithosphere, atmosphere, cryosphere and biosphere in Antarctica, and the impact it has on the rest of the Earth.

Sources & Further reading

1. Anderson, John B., 1999, Antarctic Marine Geology, Cambridge University Press

2. Earth Observatory