HAD Sydney existed 2500 years ago, its inhabitants would have been choking on outback dust 3-4 times more frequently than they do today.
Australia has been losing vast quantities of dust for at least 10,000 years, a fact that has been lost in the discussion of the two big dust storms that crossed the east coast last week.
Ice cores taken from New Zealand glaciers show that since the end of the last Ice Age, Australian dust has continuously blown across the Tasman.
Between 2000-3000 years ago, the quantity of outback dust reaching New Zealand was "significantly more" than is being recorded today, according to University of Queensland researcher Dr Sam Marx.
The New Zealand data hasn't yet quantified how hoof and plough impacted on Australian dustiness over the past couple of centuries, but there is plenty of anecdotal evidence to suggest wind erosion increased sharply through until WWII.
In the past couple of decades, Australians appear to have learned to better manage their outback environment.
Modern dust records began in the 1960s, a droughty and dusty time, but through the wet 1970s dust loss from the inland dropped dramatically.
There were spikes in the drought of the early 1990s, and again with the 2002 drought, which hasn't ended in parts of inland Australia.
Dustiness has risen since 2002, but not to levels recorded in the 1960s - at least until last week's storms.
The first storm, which extended north-south across most of the Australian continent, is thought to be substantially bigger than the October 2002 dust storm that reached the east coast.
However, no-one is prepared to say how much bigger until they have crunched the numbers. "Somewhere in the range of 10-20 million tonnes," Dr Ross Mitchell of CSIRO Marine and Atmospheric Research estimated.
Untangling what influences the severity of a dust storm isn't straightforward, dust researchers say, but weather is certainly the primary driver.
Dr Marx relates the increased dustiness of 2000-3000 years ago to coral records that indicate the El Nino Southern Oscillation Index (ENSO) underwent big swings during that period, with vast floods moving inland followed by prolonged drought.
On a lesser scale, this swing from wet to dry seems to have been one of the factors behind last week's storms.
John Leys of the NSW Department of Climate Change, and southern co-ordinator for the Dustwatch network, said eyeballing the satellite imagery suggested that major sources of dust included inland evaporation pans and floodplains like Lake Eyre, Lake Blanch, Lake Callabonna and the Bullo Overflow.
These pans collected huge amounts of sediment in the floods that swept down from the north last February.
Now dry, the fine sediment was whipped into the air by what Dr Mitchell called a "very strong anti-cyclone" that pushed a front and 100 km/h wind gusts inland, across lake pans, floodplains and dunefields made vulnerable by drought.
Moomba, on the Strzelecki Track of north-east South Australia, has recorded just 11mm of rain for the 2009 calendar year.
Agriculture's contribution to the dust appears to be relatively modest.
Contrary to media reports of dust blowing off "farmland", the dust came off the rangelands of the Lake Eyre Basin and even as far west as the Nullarbor, where there is no cropping.
"Globally, agriculture is thought to contribute about 10 per cent of the dust in the atmosphere," Dr Mitchell said.
"But it's clear that what we are looking at here is a natural phenomenon."
Dust researchers say it is generally impossible to separate how much rangeland dust is due to overgrazing of livestock, and what is due to constant unmanaged grazing pressure by feral camels, goats and rabbits.
Dr Leys believes that given the extreme weather conditions, the storms would have been considerably worse if agricultural systems hadn't improved over the past decade or so.
"Management certainly has a role in minimising erosion," he said. "Certain properties are a lot better off now because they destocked earlier in the drought."
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