Australia's role in climatic history
12 March 2002
If Australia hadn't broken away from Antarctica, worldwide climate probably would have stayed warm.
Earth has experienced huge climatic changes over its four billion-year history — from ice ages to global warming. The number of climatic fluctuations, their frequency and influence on life forms, and predictions of Earth's climate this millenium capture worldwide attention.
The break up of the once-huge continent of Gondwana to form Australia and Antarctica, and the resulting changes in oceans and currents, had a huge impact on global climate.
Grappling with climate change requires detailed scientific study of Earth's records in sediment cores — the layers of mud, debris, and microscopic plants and animals that have built up on the ocean floor over eons.Antarctic Circumpolar Current
Some sites studied by Geoscience Australia as part of the Ocean Drilling Program were at polar latitudes when Australia was joined to Antarctica.
From the end of the Cretaceous (the age of dinosaurs) until 35 million years ago, Antarctica was relatively warm with little ice, and supported temperate rainforests with southern beeches and ferns. The sea was also relatively warm and shallow. Temperatures peaked about 55 million years ago, when the planet was much warmer than today.
It was forty-five million years ago that Australia started to move northward away from Antarctica, at roughly five centimetres per year. A cool-temperate rainforest covered Antarctica at this time. The vegetation was similar to that growing nowadays in the Tasmanian highlands, New Zealand and Patagonia.
By about 33 million years ago, the Tasmanian land bridge had separated from Antarctica.*** Cold currents — the Antarctic Circumpolar Current — began to circulate around Antarctica, cutting it off from the warm currents flowing south from the tropics and creating perfect conditions for ice sheets to form.
At one site studied by Geoscience Australia, spores and pollen indicate that there once was stunted, 'rainforest scrub' growing in Antarctica around this time, featuring extinct relatives of the Wollemi Pine — a rare tree discovered recently near Sydney.
As well, the formerly warm shallow seas around Tasmania were cooling and deepening, much of the land bridge was subsiding beneath the ocean, and less sediment came from the land. There were large temperature changes over relatively short time periods.
By 15 million years ago, most of Antarctica was a frozen continent. Vegetation vanished, unable to survive the dramatic change.
And by about five million years ago, Australia's movement northward into mid-latitudes, along with global climate change, led to massive aridity.
Global cooling eventually led to the ice ages of the last two million years. Currently the planet is experiencing a warm spell between glacial periods. But what would Earth be like if the Antarctic Circumpolar Current had never begun?
Determining the role of Antarctica and the Southern Ocean in global climate is a focus of study by Geoscience Australia. This work is part of the Ocean Drilling Program, which is an international partnership of scientific institutions and governments, exploring the Earth's history and evolution. Australia is a partner in the Australia-Canada-Taiwan-Korea Consortium for Ocean Drilling.
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