The climate outlook? Increasingly dry conditions over the next few hundred millennia, barring upsets due to the greenhouse effect. Thats the long-range forecast from a team of researchers at ANSTO and from universities around Australia.
The scientists, led by ANSTO environmental scientist Dr Henk Heijnis, have used nuclear techniques to analyse samples from ancient lakes. Their results suggest that Australia has become more and more arid over the past few hundred thousand years.
They say that rainfall is the main variable affecting the Australian climate. The scientists have been investigating past climates at sites from the equator to Antarctica. The work is aimed at getting a glimpse of the greenhouse future by reconstructing climates of the deep past.
The researchers "read" climate records written in fossil pollen, plant remains algae and microscopic animals lodged in sediments in the lakes. They have been dating the sediments using a sophisticated technique exploiting the radioactive decay of a naturally-occurring isotope of uranium. That isotope - uranium-234 - decays into thorium 230 by emitting an alpha particle (helium nucleus). Since the rate of decay is known, the relative amounts of the isotopes indicate the age of the sample. The team has measured those amounts at ANSTO's alpha spectrometry facility.
Results from some of the sites are now in. These sites include a lake on Queenslands Fraser Island, which was investigated by scientists at ANSTO, the University of Wollongong and the University of Adelaide. A lake in Queenslands Atherton Tablelands and a third site - Lake Wangoom on the Victorian south coast - were studied by an ANSTO/Monash University group.
The scientists found evidence for drying in the last three interglacial periods the big thaws between the ice ages. Ice ages have occurred about once every 100,000 years, due to changes in the Earths orbit around the sun and wobbles in the planets axis of rotation, Dr Heijnis says.
During the big chills, when vast tracts of the continents of the northern hemisphere were shrouded in ice, the sea level dropped and the ocean cooled, reducing evaporation, and therefore rainfall, in Australia.
Ice ages were followed by warmer, more humid periods. "But the present interglacial and the two before it appear to have become progressively drier," Dr Heijnis said.
"The cycle does not appear to be dominated by temperature. Climate variability in Australia is affected primarily by effective precipitation, or the difference between the rainfall level and the evaporation rate."
The finding, he said, was crucial to modeling future climates in a greenhouse world.
"Our data can help in working out greenhouse scenarios - not just for the continent as a whole but for specific regions within Australia," he said.
The finding, he said, was crucial to modeling future climates in a greenhouse world.
"Our data can help in working out greenhouse scenarios - not just for the continent as a whole but for specific regions within Australia," he said.
The team is now doing similar studies on sites on King Island off Tasmania, and in Indonesia and Antarctica.
Published: 09/11/1998