The research, published in Geophysical Research Letters, a journal of the American Geophysical Union, links dry conditions in the Murray-Darling Basin—Australia’s largest river basin—to a southward shift of tropical rainfall in the Western Pacific.
Shifts in the Pacific monsoon drive changes in rainfall in Australia, but scientists have not been able to pinpoint how much of the rainfall variability in Australia can be attributed to natural climate variability and how much is due to human-caused climate change. The new study offers clues to understanding the relative contributions of these two factors.
“There’s been a drying trend in southeastern Australia over the past century, and there’s a lot of debate about whether that drying trend has a natural origin, or whether it is the result of human activities,” said Jess Adkins, a geochemist at the California Institute of Technology and lead author of the new study. “Our results suggest that natural variability can explain a large part of the drying over southeastern Australia—a pattern of change that just happens to coincide with human-driven climate change.”
During the Asian summer monsoon, warm ocean currents flowing toward the Asian continent carry warm moist air over the Indian Ocean, picking up moisture before reaching land. The summer monsoons often cause floods and landslides while nourishing crops and supporting hundreds of millions of people. In turn, the monsoons can cause changes in rainfall in areas farther south like the Murray-Darling Basin, which is home to more than a third of Australia’s population and produces 40% of its food.
However, the behavior of the Asian monsoon has been changing over the past century. The monsoon rains are shifting southward at a rate of 100 kilometers (60 miles) per decade and arriving later and later in the summer season. The exact cause or causes of the shifting monsoon remain unknown, but the changes have been occurring in tandem with a steady decline in rainfall in the Murray-Darling Basin.
To assess the connection between the shifting monsoon and rainfall in Australia, Adkins and her colleagues collected eight coral cores near the Great Barrier Reef, off the coast of Queensland in Australia. The corals recorded the history of both water temperature and seawater composition, which reflects whether monsoon rains fall into seawater over the coral colonies.
The researchers found that during periods when the monsoon rains shifted farther south and arrived later in the season, the Murray-Darling Basin experienced drier conditions. Rainfall was highest during the season leading up to the arrival of the monsoon rains.
“We observed that, as the monsoon rains fall a little bit farther south, the dry season gets a little bit longer and the wet season gets shorter,” Adkins said.
The researchers found that a southward shift in the monsoon of about 100 kilometers (60 miles) and a two-week delay in arrival time caused a decrease in annual rainfall of about 200 millimeters (7.9 inches) over the Murray-Darling Basin.
“This work suggests that natural variability in the monsoon system drives a substantial part of the rainfall variability in southeastern Australia, and that human activities may have only a secondary effect on the basin’s rainfall decline during the 20th century,” said co-author Michael Griffiths of the Australian National University.
“Our results suggest that there may be limits to how much rainfall can be expected in the Murray-Darling Basin, even in the absence of human emissions,” Adkins added.
