Prehistoric polluters

Did early man turn the outback into a barren desert? Kate Ravilous meets the scientists scouring Australia for clues about climate change

Once upon a time, Australia had a lush, green interior where grazing animals roamed, shrubs grew and the rain fell. Then, about 55,000 years ago, man arrived and started hunting the animals and burning the vegetation; ultimately, he drove the rain away and turned Australia's interior into the harsh, red, desert landscape that we see today.

Once upon a time, Australia had a lush, green interior where grazing animals roamed, shrubs grew and the rain fell. Then, about 55,000 years ago, man arrived and started hunting the animals and burning the vegetation; ultimately, he drove the rain away and turned Australia's interior into the harsh, red, desert landscape that we see today.

There is no doubt that Australia's environment and climate has changed dramatically, but was man responsible? Gifford Miller, from the University of Colorado at Boulder, thinks so, and he and his colleagues have discovered convincing evidence to back up their theory.

The story starts at Lake Eyre, a huge salt flat covering one-sixth of Australia's landmass. Miller has been working with John Magee, of the Australian National University in Canberra, to drill down through the layers of mud, sand and salt at the site to uncover Lake Eyre's past. Going back 125,000 years, they have found that it used to be a vast freshwater lake, covering 35,000 square kilometres - an area the size of Taiwan. Rains used to swell the lake every year, following the patterns of the Australian monsoon. Then, about 14,000 years ago, "the monsoon stopped penetrating into the interior of Australia and Lake Eyre started to turn into a salt lake, like it is today," says Miller.

While he was looking for reasons as to why the monsoon could have failed, Miller discovered that the Australian environment had suffered other dramatic changes in the past. "About 50,000 years ago, some 60 different species of animal went extinct," he says. Miller's work focused on the demise of an ostrich-sized flightless bird called Genyornis newtoni. Measuring amino acids in the fossil eggshells of these birds and using radiocarbon dating, Miller and his colleagues found that Genyornis suddenly disappeared about 50,000 years ago. Meanwhile, other scientists have recorded that a host of other creatures, including a giant horned tortoise (the size of a small car) and a hippopotamus-sized relative of the wombat, were also snuffed out.

And animals were not the only ones to suffer. Pollen records suggest that many species of tree also vanished at this point. "Some of the most fire-sensitive plants, such as rainforest gymnosperms [plants whose seeds are not encased and thus protected from fire], disappear and never come back," says Miller. The evidence was circumstantial, but Miller became suspicious that all these sudden environmental changes were linked. He believes that early man may have pushed the natural balance too far by burning large areas of vegetation on a regular basis. The burning was probably used to hunt animals, promote new plant growth and exchange signals, but eventually it changed the environment irreparably. As well as burning plants and forcing animals to extinction, Miller thinks that man may be indirectly responsible for the monsoon failure - by removing the vegetation that sucked the rain into Australia's interior.

Miller and his colleagues have been looking at the link between Australia's vegetation and its climate, and using climate models to better understand the pattern of Australia's monsoons. Matching up pollen records with the Lake Eyre data has indicated that vegetation and climate used to be strongly linked. "Prior to 50,000 years ago, the vegetation beat to the same rhythm as the monsoon," says Miller. The lake data shows that Australia's monsoons followed precessional cycles, related to the tilting of the Earth as it spins on its axis. Over a 22,000-year period, the Australian monsoon swung from being a deluge to a drizzle and back again. Similarly, the vegetation swung from being dominated by lush rainforest to being made up of higher proportions of shrubby plants and grass, and back again.

"The earliest humans arrived in Australia about 55,000 years ago, at the tail end of one of the stronger monsoon periods. There would have been lots of animals and plenty of green plants," says Miller. But that didn't last. By 45,000 years ago, Lake Eyre sediments show that the monsoon entered its weaker phase and became more of a dribble. At the same time, the Earth entered an ice age, making the planet cold and dry. It wasn't until about 14,000 years ago that the ice retreated and the monsoon rains started again. But, unlike on previous occasions, the strong monsoon rains never returned to the Australian interior. "We would have expected the climate to stay quite dry until about 14,000 years ago, but then the heavy monsoon should have reappeared," says Miller. Instead, the Lake Eyre sediments show that the interior of Australia continued to remain dry.

Using general circulation models (GCMs - climate simulators), Miller and his colleagues have been testing how sensitive the Australian monsoon is to changes in vegetation. They have found that plants appear to be the key to holding on to monsoon rainfall. When the model is run with vegetation covering the Australian interior, it gets twice the rainfall compared with a model run with no vegetation. "The GCM suggests that rainfall in the interior would be about 600mm per year when trees and plants cover the ground, compared with about 300mm per year when the ground is bare," Miller says.

Vegetation is likely to be important because it helps to recycle the rain via evaporation and transpiration. "Plants collect moisture and hold onto it. Without any vegetation the rain either evaporates, or sinks into the ground and disappears," explains Miller. Trees also add "surface roughness" to a landscape, which is thought to promote convection and to encourage rain-cloud formation. If Australia's earliest human inhabitants burnt enough vegetation, Miller believes that this could have tipped the balance and prevented the monsoon rains from reaching the interior.

Today, northern Australia still receives an annual monsoon, dousing cities like Darwin with more than 1,600mm of rain a year. The GCM models have shown that Australia's monsoon is connected to the northern-hemisphere climate and the Asian monsoon. "Early man didn't have enough influence to affect the global monsoon pattern, but it appears that localised burning was enough to produce a continental-scale change in the water balance and climate," says Miller.

It is unlikely that we can turn the clock back for Australia. In principle, trees could be planted to entice the moisture back, but thousands of years of desert weathering has left Australian soil very low in nutrients, making it almost impossible for trees to get a grasp again. "Physics is working against us right now. Perhaps we could try planting in another 11,000 years, when we are in a strong monsoon period again," says Miller.

If Miller and his colleagues are right about Australia's past, it provides a sobering lesson. Rainforest is being felled all over the world at an unprecedented rate and ecosystems pushed way out of kilter. What kind of effect will this have on the world's climate? Are we leaving a legacy to future generations of desert landscapes and unpredictable rainfall?