When the first European settlers arrived in Australia in the 17th century, they observed a strange farming practice among the natives. The aborigines routinely burned grasslands and vegetation in many parts of northern Australia during the cool months of the dry season between winter monsoons. These controlled burns were intended to help stimulate regrowth during the upcoming rainy period. But they may have also inadvertently caused the end of that summer dry spell to be much warmer and drier than normal, a new study suggests.
Previously, researchers have linked such burning to the extinctions of some species of Australian megafauna, including several species of kangaroos, wombats, and other marsupials, thousands of years ago. But whether these burns affected the region's climate was unclear. The fact that previous climate simulations were limited to the core months of the rainy season may have limited their scope, says Michael Notaro, a climate scientist at the University of Wisconsin, Madison.
To strengthen the model, Notaro and his colleagues took a look at how aboriginal burning might have affected the entire pre-, mid-, and postmonsoon season, which stretches from November through March. In their simulations using a global climate model that incorporates interactions between the sea, land, atmosphere, and plants, they removed the vegetation from 20% of the landscape in northern Australia and then compared the results with those from a simulation where the grasslands and vegetation remained intact.
The team's results, published this month in Geophysical Research Letters, reveal substantial changes in climate during the premonsoon months of November and December. Total precipitation during these months declined by more than 3 centimeters. This may sound small, but those premonsoon rains are vital for the region's ecological recovery after the dry season.
"The dry season in this region is about 8 months long, so the first monsoon rains have quite an effect on biology," says Robert Wasson, a geomorphologist at Charles Darwin University (CDU) in Darwin, Australia. "The onset of the monsoon is a critically important time."
Stephen Garnett, a biologist at CDU, agrees. "Delaying those rains makes it tougher to survive the dry season and affects a lot of things, from survival of seed stocks in the soil to the size and scope of late-season wildfires," he says.
The delayed rainfall could have been due in part to the increased sparseness of vegetation cover at the end of the dry season, which reduced the amount of deep soil moisture pumped into the atmosphere over the region and resulted in lower humidity and decreased chances of precipitation. Also, the researchers note, declines in the moisture in surface layers of soil decreased the amount of evaporative cooling taking place, raising air temperatures in the region approximately 0.57°C during the premonsoon season.
The team's findings are a useful contribution, Wasson adds, especially because, unlike earlier studies, they look at the effect of burning on climate during the pre- and postmonsoon season. Nevertheless, he notes, the new analysis relies solely on modeling to weigh in on a debate that, at present, has little real-world evidence to bolster any particular notion of what the effect of aboriginal burning actually was. "Ecological signals of these shifts in climate would be subtle," he says. Also, he adds, there's almost no evidence about what the climate of the region was like 55,000 years ago, the date when aborigines are presumed to have arrived in Australia, denying researchers the opportunity to make a direct before-and-after comparison of climate. So for now, this dearth of data means that scientists must rely on climate models to estimate the effects of large-scale burning by aborigines.