Introduction

The most impressive feature of the Antarctic environment is undoubtedly the huge amount of ice in both the continent and Southern Ocean (in winter). Antarctic ice sheets lock up about 70-80 % of the Earth's freshwater, enough to raise the global sea level by some 60-70 m. Global climate models predict that, in the short term, changes in air temperature, clouds and precipitation will probably increase the accumulation of snow in Antarctica. This accumulation, together with possible changes in Southern Ocean temperatures and circulation, will have long-term effects on ice flow and ice ablation, with enhanced return of water to the ocean, through icebergs and meltwater runoff.

This chapter gives a brief account of the flow and mass balance of Antarctic ice sheets, and emphasises the role of Antarctic ice cores in research on global processes. As successive snow layers build up, those beneath are gradually compressed into solid ice, which preserves a unique and undisturbed record of past and recent changes in the composition and state of the atmosphere. Studies on deep ice cores drilled in Antarctica provide some of the best temporal accounts of the close correspondence between air temperature and greenhouse gas concentrations, and allow the reconstruction of climate changes over the last 500,000 years. In contrast to well-mixed greenhouse gases, the incorporation into snow layers of persistent atmospheric pollutants deposited through snowflakes or aerosols is governed by largely unknown processes. However, as will be discussed in Chapter 4, significant changes in pollutant concentrations in snow and ice on extended temporal scales can be used to infer similar changes in the composition of the atmosphere. These changes can be used to detect the impact of anthropogenic activities in Antarctica and elsewhere in the world.

For more than 25 Ma, Antarctica has had no terrestrial connection with any of the continental landmasses in the Southern Hemisphere and, during this time, the continent has been almost completely covered with permanent snow and ice. Even in the present warm interglacial period, less than 3 % (about 331,700 km2) of the continental area (including the islands) is permanently or seasonally free of ice and snow. Most of these ice-free areas are characterised by low temperatures and precipitation (cold deserts) and, like deserts in warmer regions, they show dry kettles, ventifacts and surface salt encrustations, with scarce biota. Antarctic terrestrial and freshwater ecosystems play a minor role in global water and carbon cycles; nevertheless, they are very important for research on the environmental and ecological effects of global climate changes. The remoteness of these ecosystems from human civilisation and the extreme environmental conditions make Antarctica a unique laboratory for studying cold adaptation and colonisation processes by organisms which are often located very far from sources of propagules. By virtue of the reduced number of species and interactions among organisms, Antarctic ecosystems allow the identification of critical factors operating in the environment, assessment of the flux of nutrients and pollutants between abiotic and biotic components, easy identification of accidentally introduced alien organisms, and of colonisation processes in old or newly exposed substrata. This knowledge is very useful for better understanding of more complex ecosystems elsewhere and predicting their possible response to climate and environmental changes. Besides contributing to ecological science as a whole, pristine Antarctic ecosystems offer a unique opportunity to detect changes in the amount and composition of global atmospheric pollutants. Knowledge of pollutant concentrations in the Antarctic environment allows the establishment of global baselines and proper management of polar ecosystems in view of the progressive expansion of field research, tourism and human activity in scientific stations.

In the previous chapter, we saw that areas of the Antarctic Peninsula are experiencing one of the largest warming trends in the Southern Hemisphere and probably, of the world. Simplified biotic communities in cold desert ecosystems are strongly affected by climate forcing, and the responses of terrestrial and freshwater ecosystems are already tangible in the Antarctic Peninsula. Antarctic ecosystems are expected to provide a better indication of the effects of climate change than ecosystems at lower latitudes, where the response of biotic communities to external forcing is buffered by more complex biological interactions and feedback processes.

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