Since the first inklings that increased carbon dioxide in the atmosphere may cause an increase in global temperatures and, consequently, climate changes, the role of Antarctica has been a topic of great importance and heated debate. While it is clear that the past and present climate of Antarctica is of great academic interest, why would we care if it snows tomorrow at the South Pole? The answer is that Antarctica plays a fundamental role in the entire Earth's climate and could have a profound effect on the extent to which sea level rises in the coming centuries in response to greenhouse warming.
There are four major reasons why the cold and remote continent and its surrounding oceans and ice shelves deserve special attention when addressing the possibilities and impacts of future human induced climate changes:
Potential Impact on Future Sea-Level
The response of the Antarctic Ice Sheets (AIS) is fundamental to the rate and magnitude of future long term sea-level rise in response to global warming. Large scientific uncertainties surround the estimates of whether or not Antarctica will contribute to sea-level rise in the future. In fact, the paucity of data does not yet allow a firm judgment even as to whether or not Antarctica is currently contributing to sea-level rise . Some observational analysis (Jacobs et al. 1996) and numerical models indicate that this may be the case (Warrick et al. 1996). Whilst the conventional wisdom reflected in the Intergovernmental Panel on Climate Change (IPCC) Science Assessment 1 is that Antarctica will accumulate more ice than it will discharge in a warming world, and thus lower sea level, this does not account for:
*The possibility of the collapse of the West Antarctic Ice Sheet (WAIS) (Warrick et al. 1996);
*The very large uncertainties in the scientific understanding of the contribution of Antarctica to sea-level rise in the past century;
*Recent findings that some glacial discharge rates and the melting of ice shelves are higher than previously estimated. These findings have significant implications for the stability of the Antarctic Ice Sheet, and in particular the West Antarctic Ice Sheet.
Warming Greater in Polar Regions
Climate models 2 indicate that the warming trend is expected to be more pronounced in the polar regions (Henderson-Sellers and Hansen 1995; Houghton et al.1990; 1996) making these regions particularly sensitive to global climate change. New calculations with climate models which include coupled atmosphere and ocean components, however, show that changes in world ocean circulation may counter the accelerated warming around Antarctica, resulting in a smaller warming trend (nearer the global average) in the region. But uncertainty about the sensitivity of southern high latitudes remains, as comparisons of ocean model results with the observed ocean circulation patterns around Antarctica suggest that the heat-absorbing ability of the Southern Ocean may have been overestimated (e.g. England 1995).
Key Role in Global Climate
The vast cold Antarctic Ice Sheet, the sea-ice and the circumpolar Southern Ocean all play key roles in the energy balance of the climate system. The Southern Ocean removes carbon from the atmosphere into the deep oceans, thus influencing the level of CO. in the atmosphere (Sarmiento and Le Quere 1996). The Antarctic region not only responds to, but also produces climate fluctuations on time scales from years to centuries. This is accomplished by various climate feedback mechanisms, many of which are not completely understood. The two most powerful of these mechanisms include:
* An identified positive ice-albedo feedback 3 (Houghton 1994; Dickinson et al. 1987; 1996) which acts to amplify the warming. As ice melts, the surface warms as it absorbs more solar energy which had previously been reflected back to space; and
* An ocean-circulation feedback involving interactions of the Antarctic Ice Sheet and surrounding sea-ice with the global deep ocean circulation (Niiler 1992; Trenberth 1992; Hay 1993; McPhee and Martinson 1994) which could directly alter regional temperatures.
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| Greenpeace - Orcas |
Antarctic Ecosystems Are Very Vulnerable
Life in a cold polar environment is very fragile and the existing habitat boundaries and food chains are easy to distort (Everson, 1987; Gell 1989; Di Prisco, 1993). This means that the Antarctic environment is extremely vulnerable to climate change.
In summary, there is little doubt now that human interference is causing climate change (e.g. Houghton et al. 1996)4. The relevance of Antarctica to such global change is obvious. This review aims to provide some insight into the question of current and future impacts of enhanced greenhouse climate change to the unique Antarctic environment. Evidence of changes in the Antarctic climate, cryosphere and ecology are examined and some possible implications for the Antarctic and, indeed, for the world's future are discussed.
THE ANTARCTIC ENVIRONMENT
In a nutshell, Antarctica is the coldest, iciest and highest continent on Earth.
Windy And Cold
The monthly mean temperatures of the south polar plateau range from -25oC to -70oC (Dudeney 1987). The lowest temperature ever recorded on Earth was Vostok station's -89.6oC. Along the coast the climate is milder: the mean temperature in winter is between -15oC and -30oC. During summer it seldom goes above 0oC, although +7oC or 8oC can sometimes be reached (Gell 1989). Constant winds, produced by katabatic air flow from the very cold elevated central areas to the coastal slopes, average around 80 km/h over the year, with gusts in excess of 320 km/h (Gell 1989).
The Ice-Bound Land
The vast continent--nearly 12 million square kilometres--is almost entirely covered by ice (Oerlemans 1993). The average thickness of the Antarctic Ice Sheet is around 2.3 kilometres, with the maximum thickness reaching nearly five kilometres (Oerlemans 1993). The high reflectivity of the Antarctic ice and snow cover results in only a small part of solar radiation being absorbed at the surface. Indeed, for most of the year the surface of the continent loses more energy than it gains (Gell 1989).
The Elevated Desert
Antarctica is the highest of all continents, with an average elevation of over 2000 metres above sea level (Gell 1989; Oerlemans 1993). A unique combination of altitude and latitude means that Antarctica is also the largest desert on Earth: the mean annual accumulation is equivalent to less than 50 millimetres (a little more than in the Sahara). The total amount of precipitation increases to the west because of the ocean influence, and the northern end of the Antarctic Peninsula receives around 900 mm annually (Gell 1989). There is also a significant contrast between the climatic regimes on the east and west coasts. The east coast is affected by the cold icy Weddell Sea and has annual temperatures about seven degrees below those in the regions at similar latitudes on the west coast (Reynolds 1981).
The Continent of Variable Margins
The continent is surrounded by ice shelves 5 which fringe almost half of the coastline (Doake 1987). Beyond the shelves, extensive winter sea-ice covers about 20 million square kilometres, shrinking five-fold in summer (Zwally et al. 1983; Paren et al. 1993).
A Unique and Fragile Ecosystem
Life in the Antarctic environment is truly unique. There are two important features making the Antarctic ecosystem so special (Walton 1987). First, species diversity in the Antarctic region is low. Of the penguins, only emperors and Adelies breed on the continental shores (another five species are found on the Antarctic Peninsula and sub-Antarctic islands). There are only eight species of other birds, six species of baleen whales, and six species of toothed whales and dolphins. Four different seals and a few species of fish, crabs, squid, octopus and other molluscs have been also identified around the Antarctic coastline.
Second, although the number of species is low, the populations of most Antarctic species are enormous--for instance, there are millions of crabeater seals (Gell 1989) and penguins. But despite these huge populations, the fact that there are so few different species in such a harsh environment means that the balance of nature is extremely sensitive and fragile.
The ecosystem is in many ways an unusual array of specialised environments characterised by conditions of high sunlight for part of the year and low temperatures all year round. Growth in the oceans appears to be limited by light rather than by nutrient supply, and the major productivity increase coincides with the return of light each spring. A crucial factor influencing virtually all living organisms is the oceanic Antarctic convergence zone where the cold waters surrounding the continent meet cool temperate water masses. Abundant nutrients are supplied by upwelling waters from the deep ocean. These nutrients supply food for phytoplankton (microscopic marine plants drifting in the sea) and other microorganisms, which in turn supply food for krill, the dominant zooplankton organism commonly found in huge swarms covering hundreds of square metres. There is no significant influx of sediments or nutrients from the land mass as in other parts of the world. In the spring and summer the zone of maximum productivity is at the sea-ice edge and migrates south following the retreat of the ice.
Oceanic krill is thought to be the most abundant animal in the world. In Antarctica it plays the most important role in the relatively simple food web (Nicol 1994). Many of the whales, fish, squid, octopus, seals and birds are dependent on krill for food. As a source of nutrients and a food supply for larval krill in winter, sea-ice algae is also thought to play an important role in the ecosystem. This reliance on virtually one food source implies that the whole Antarctic biocommunity could be severely affected by changes in krill productivity. Examples from the past have shown that the natural balance is extremely subtle and can easily be destroyed.
A BRIEF HISTORY OF ANTARCTIC RESEARCH
Although the sealers and whaling fleets came to Antarctica soon after the publication of James Cook's account of his circumnavigation in 1777, little was known about the seventh continent until the 20th century. A number of expeditions in the early decades of this century initiated the era of discovery (Walton 1987). However, Antarctica essentially remained terra incognita until 1958, when the International Council of Scientific Unions (ICSU) sponsored the International Geophysical Year (IGY). Antarctica was selected as an area of special attention, and from then on the continuous monitoring of the Antarctic climate and environment was established at permanent scientific sites. At present there are about fifty permanent stations in Antarctica, representing 26 different nations. However, many parts of Antarctica are still inaccessible for field observations.
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| Greenpeace - scientist with ice cores |
1. The IPCC represents the pre-eminent scientific opinion on climate change formed in 1988 to advise world leaders on current scientific understanding of climate change issues, its latest assessment involved over 2,000 scientists from government. Universities, industry and environment groups and was published in early 1996.]
2. This report references results from two types of climate model experiment. The first is an "equilibrium" climate experiment where CO. concentrations in the atmosphere are instantaneously doubled and the model is then allowed to reach a new climate stage, or equilibrium. These experiments allow for the assessment of the initial and final stages of climate change. The second type of assessment is a "transient" climate experiment where the model is gradually forced with increasing amounts of CO. - say a one per cent increase per year - allowing the modellers to evaluate the evolution of climate change over time.
3. The essence of this feedback is that the initial warming provokes retreat of the highly reflective snow cover or sea-ice cover, which in turn reduces the net amount of solar energy reflected, resulting in further warming. This is also known as the ice-albedo feedback mechanism.
4. The IPCC Second Assessment Report found that "the balance of evidence suggests that there is a discernible human influence on global climate".
5. Ice shelves are floating sheets of ice which may persist for many decades or centuries. There boundaries are defined by the grounding line, where the glacier or ice sheet begins to float, and the ice front, where the ice shelf disintegrates into icebergs.