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Australia: The Land Where Time Began |
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Antarctic Climate Change and Environment -
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 7oC 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 4oC 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 2-3oC 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.
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
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |