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Early fire-farmers switched off monsoon


Fire could have led to a rapid change in Australia's interior landscape (Image: Gifford Miller, University of Colorado)

Early humans' use of fire in Australia could have switched off the monsoon in the continent's interior causing deserts to form, scientists say.

US researcher Professor Gifford Miller of the University of Colorado and colleagues publish their theory in the January issue of the journal Geology.

Their theory also supports the idea that early humans were indirectly responsible for extinction of the Australian megafauna.

Previous research by the team at Lake Eyre in South Australia found that something stopped monsoonal rains from reaching Australia's interior regions 12,000 years ago.

At this warming interglacial time the planet's monsoonal system was being switched on after the long, cold and dry glacial period.

While the inland areas of Africa were getting a watering, the monsoon rains were not getting past the coastal areas of north and western Australia.

They were not penetrating, as they had in the past, inland parts of the continent, including the headwaters that feed Lake Eyre.

Once a permanent lake

Co-author Dr John Magee of the Australian National University in Canberra, says in previous interglacials Lake Eyre was a permanent lake up to 25 metres deep. But now it is mainly a dry salt lake that is only occasionally flooded with no more than 6 metres of water.

"The obvious thing that's happened in between those two interglacials is that the continent was first colonised by humans," says Magee, who also worked on the previous study.

The researchers theorised that the failure of monsoon rains to penetrate the continent could have been a delayed reaction to changes in vegetation brought about by the use of fire by humans, who arrived in the area about 50,000 years ago.

By suddenly interrupting the natural fire regime, humans would have likely changed the dominant forest to grassland, says Magee. This type of vegetation has a lower ability to recycle water into the atmosphere and thus could have reduced rainfall.

"Our argument would be that if you have a system that's dominated by savannah and spinifex grasslands, a lot of that rainfall runs off back to the coast and doesn't penetrate into the continent," he says.

Estimating rainfall

To see if a change in vegetation could have switched off the monsoon in interior Australia, the researchers used a climate model to give rainfall output for different vegetation types.

They found that although a change from a forest to grassland reduced rainfall by a few millimetres a day, this would have been enough to stop monsoon rains from reaching the interior.

"We also believe the same change was responsible for the extinction of the Australian megafauna," says Magee.

Rather than directly killing megafauna by hunting, Magee thinks humans may have been indirectly responsible for the extinction through transforming the landscape.

To support this theory Magee is analysing isotopes in giant bird fossils that give clues to the type of vegetation they relied on for food.

Not convinced

Dr Stephen Wroe of the University of Sydney agrees with the conclusion that the monsoon has retreated from inland areas of Australia.

But he is not convinced that humans were responsible for such widespread effects on the ecosystem.

"Australia's been getting drier for at least 300,000 years. Long before humans arrived," says Wroe. "They cannot discount this trend as the cause of the change."

He says scientists also don't know whether the earliest Australians used fire in the same way as Aboriginal people did when the Europeans arrived.

"There is absolutely no strong evidence that the first human Australians deployed firestick farming," says Wroe.

He says even if fire was involved, it is highly unlikely that it alone was responsible.

Wroe says theories like this may be based on the erroneous assumption that early humans had as much power as current day humans to destroy the environment and is concerned about the political implications of such ideas.

"If it's a solid argument, fine," he says. "That's science, those are the results, put it on the table. As it is, it's a circumstantial case that can be seriously misused by special interest groups in society from environmentalists to people opposed to Aboriginal control of land."

"I'm not saying that they might not be right in the end, but why draw this long bow between results and explanation at this point?"
