A University of Colorado researcher has proposed that the systematic burning of vegetation by Aboriginals beginning roughly 50,000 years ago may have changed the climate down under and triggered the desertification of Australia's interior.
Gifford Miller, a fellow at the Institute of Arctic and Alpine Research, said Australian climate records from lake sediments indicate annual summer monsoons driven by a flow of air originating on the Tibetan Plateau drenched the northern part of the continent on a regular basis from about 150,000 years ago to some 40,000 years ago. Although the intensification of African and Indian monsoons 10,000 years ago at the beginning of the Holocene Period suggests such storms were increasing on a global scale, the Australian monsoon inexplicably failed to strengthen at that time, according to the paleoclimate evidence.
"We know that monsoons are essentially driven by glacial activity and periodic changes in solar radiation," said Miller, who is also chair of CU-Boulder's geological sciences department. "Since the Earth was experiencing marked increases in monsoonal activity in the early Holocene, something regional must have modulated the Australian monsoon."
Miller said the only mechanism that appears to wield enough force to alter the Australian monsoon is vegetation, which has strong links with the atmosphere. In the Amazon rain forest, for example, one-half of the precipitation that falls is recycled during the wet season through plant transpiration and evaporation.
Archaeological evidence shows the first humans arrived in Australia from the southeast Asian region about 50,000 years ago. "Consistent burning by these people during dry periods may have altered the basic ecosystem, preventing the normal recovery of vegetation during the subsequent wet phase," he said.
Although the city of Darwin on the northern coast of Australia is annually drenched with about 80 inches of precipitation during the monsoon, rain reaching the continent's interior drops off to less than 12 inches annually, he said.
A paper on the subject by Miller, CU graduate student Jennifer Mangan, INSTAAR's Beverly Johnson, Australian National University's John Magee and National Center for Atmospheric Research scientists Starley Thompson, David Pollard and Ben Felzer was presented at the fall meeting of the American Geophysical Union held in San Francisco Dec. 8 to Dec. 12.
Unfortunately, alkaline soil conditions in Australia's interior have precluded the preservation of fossil vegetation and pollen, making it difficult to reconstruct past ecosystems, he said. Miller and his colleagues have been modeling the sensitivity of monsoon precipitation to vegetation 10,000 years ago at the beginning of the Holocene period using a general circulation model computer program.
One of the models they plugged in, a "vegetated Australia" simulation, produced an increase in precipitation of 3 inches of rain per month in the interior -- more than double the current amount -- during the monsoon season. "This suggests that during peak periods of the Australian monsoon the penetration of moisture is highly sensitive to vegetation type, and that the failure of the early Holocene summer monsoon may be a direct consequence of human activity," he said.
A related study spearheaded by Beverly Johnson, a former CU graduate student under Miller who worked at Australian National University on the biochemistry of fossil eggshells from flightless Australian birds, supports the theory that vegetation in the continent's interior began changing about 50,000 years ago.
Miller, who pioneered the dating of fossil eggshells of flightless birds like ostriches and emus through a process known as racemization, has been working with Johnson and Marilyn Fogel of the Carnegie Institution in Washington to analyze the carbon inside fossil eggshells. Collected from an arid region in South Australia where the summer and winter rainfall regions roughly meet, the shells harbor carbon isotopes that provide clues to what the birds were eating over time.
By analyzing the stable carbon isotopes in the fossil eggshells of an extinct, flightless bird known as Genyornis and emus -- which still roam the continent -- the team deduced that lush grasses that dominated from about 70,000 to 45,000 years ago during the moonsoons were being replaced by a more arid, mixed vegetation by 35,000 years ago due to the waning of the Australian monsoon. The changing vegetation eventually helped establish present rainfall patterns, they speculate.