Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctic Climate Change and Environment - Deep-Time, the Geological Dimension

The climate system of the Antarctic is coupled closely to other parts of the global climate system and the Antarctic climate system varies on various time scales of orbital, millennial and sub-annual. Highlighted in this report is the profound effect the ozone hole has had on the environment in the Antarctic over the last 30 years, which has shielded the continent from much of the effects of global warming. The situation will not last however. It is expected that ozone concentrations above Antarctica will recover over the next century, and if the concentrations of atmospheric greenhouse gas continues to increase at the present rate temperatures across Antarctica will increase by several degrees and the amount of sea ice will decrease by about ⅓.

Deep time – the geological dimension

The context for an understanding of the climate and environmental changes of the present is provided by the study of the history of the climate and environment of Antarctica. It allows the determination of the processes leading to the development of the present, which is an interglacial period, and the defining of the ranges of natural climate and environmental variability on timescales from decades to millennia prevailing over the last million years. Researchers will be able to identify when change occurring at the present exceeds the natural range. It is shown by the palaeorecordssthat change is normal and the unexpected can happen, the problem is identifying when these natural ranges are exceeded.that change is normal and the unexpected can happen, the problem is identifying when these natural ranges are exceeded.

In the Early Cretaceous, 130 Ma, atmospheric concentrations of greenhouse gas, CO₂, ranged from about 3,000 ppm to about 1,000 ppm in the Late Cretaceous, about 70 Ma, and Early Cainozoic, about 45 Ma, resulting temperatures 6 or 7^oC warmer than the present. At this time there was little or no ice on land. The high atmospheric levels of CO₂ resulted from the biogeochemical cycles of the Earth.  

About 34 Ma the first continental scale ice sheets formed, most likely as a response to declining atmospheric carbon dioxide levels which was caused by lower levels of outgassing of CO₂ from mid-ocean ridges and volcanoes, as well as increased carbon burial. This resulted in a fall of global temperatures to about 4^oC higher than the present. These early ice sheets reached the edge of the Antarctic continent at their maximum, though they were probably warmer and thinner than at the present. At about 14 Ma there was a further sharp cooling that is suggested to have been accelerated by the growing physical and thermal isolation of Antarctica as other continents drifted away from it, which allowed the Antarctic Circumpolar Current (ACC) to develop, rather than any changes in the levels of atmospheric CO₂. At that time the ice sheet thickened to approximately its present configuration. 5-3 Ma, during the Pliocene, mean global temperatures were 2-3^oC higher than pre-industrial levels, with atmospheric CO₂ levels possibly reaching to 400 ppm, and sea levels were 15-20 higher than the present levels.

About 35 Ma is believed to be the time when the earliest cold climate marine fauna arose. A barrier to the migration between Antarctic waters and the lower latitudes of shallow and open water marine organisms was formed by the development of the Polar Front that separated the warmer water to the north from the colder water to the south. Adaptive evolution to cold temperatures and extreme seasonality was promoted by this isolation of the cold environments in Antarctic waters from warmer environments to the north of the Polar Front, which resulted in the Antarctic marine biota of the present, which in terms of diversity and biomass is second only to coral reefs. A single highly endemic taxonomic group – the suborder Notothenioidei dominates the Antarctic fish fauna, which is a situation unlike that of any other fish faunas elsewhere. A particularly advanced adaptation to the environment is the evolution of antifreeze proteins and the loss of haemoglobin is some members if this order. All trophic levels of the open ocean ecosystem were shaped by the development of the sea-ice which made possible the success of krill. The adjacent deep sea areas to the north shared similar environmental conditions with the cold Antarctic waters and this allowed the invertebrates of these deep sea areas to experience considerable exchange with invertebrates inhabiting the Antarctic waters to the south.

Circumpolar atmospheric circulation patterns also isolated terrestrial habitats of Antarctica from potential sources of colonisation from lower latitudes. Unlike the situation of the marine environment, the combination of the formation and advance of ice sheets on a continental scale, and the extreme environmental conditions, resulted in a large-scale, though incomplete, extinction of pre-existing biota, and among the remaining survivors, evolutionary radiation. The change of species that are associated with the arid subtropical climates of Gondwana to cool temperate rainforests and eventually to cold tundra that occurred with the opening of Drake Passage and the separation of the Tasman Rise, is shown by the fossil evidence of these species. Though most of these species are now extinct on the Antarctic continent it is suggested by recent phylogenetic and fossil evidence that adaptation to the changed environmental conditions occurred in some species groups, such as chironomids, mites, copepods, springtails, nematodes and cyanobacteria, with many having endemic representatives on the continent.

Concluding remarks

In high latitude areas the climate is more variable than that in tropical or mid-latitude regions and over the last few million years has experienced a huge range of conditions. In the long history of the Antarctic continent that snapshot gained in the instrumental period is tiny, and the separation of influences on climate variability from natural and anthropogenic factors is difficult. It is already evident the effects of increased greenhouse gasses and stratospheric ozone concentration decreases are present. If the effects of concentrations of greenhouse gasses increase at the current rate as expected over the next century, it will be remarkable because of their speed. The problem will be exacerbated as the removal of the cooling effect of the ozone hole as it reduces in extent. Reasonably broad estimates can be made of how quantities such as temperature, precipitation and the extent of sea ice might change, and consider the possible impact on marine and terrestrial biota. To date it cannot be said with confidence how the large Antarctic ice sheets will respond, though cause for concern is given by rapid recent changes, especially for the stability of West Antarctica.

More information is required from ecophysiological field studies and lab experiments concerning the sensitivity of ecological key species, as well as more information on the geography of the hot- and cold-spots of Antarctic biodiversity and the functioning of their ecosystems, and identification of the main biological and physical driving forces. According to Turner et al. the basis should be provided by these challenges for further development of numerical simulations that are spatially explicit of the state-of-the-art of the Antarctic ecosystem, and extrapolations from this, with support of, or combined with results from physical models, into the future, by the use of various climate scenarios.

Sources & Further reading

Turner, j., Bindschadler, R., Convey, P., di Prisco, G., Fahrbach, E., Gutt, J., Hodgson, D., Mayewski, P., Summerhayes, C., (Eds.), 2009, Antarctic Climate Change and the Environment, Scientific Committee on Antarctic Research Scott Polar Research Institute, Cambridge