Tree rings reveal a puzzling trend in monsoon intensity

Tree rings confirm that in northern Australia, the past 40 years have experienced more rain than any similar length of time in the past 600 years.

December to March is monsoon season in northern Australia. Heavy rains make rivers and streams race, refilling the reservoirs that plants and animals need to survive the next year. Generally, flow in monsoon rivers has become more intense since officials began keeping records in the 1970s. But dry spells do occur; for example, in 2019 and 2020 the monsoon failed, leading to widespread water restrictions. With limited historical measurements and model disagreements, scientists have been left guessing at how today’s monsoon seasons differ from those of centuries past—and what might happen in the future.

In a new study, Higgins et al. constructed a 592-year history of the monsoon in northern Australia’s Daly River catchment. They used tree rings as a proxy for streamflow to determine whether the upward trend in rainfall is unusual. The researchers used publicly available measurements from two species of cypress pines, most of which were from monsoon Asia, with some from Australia and New Zealand. Using these data, the team reconstructed streamflow by matching stream gauge measurements between 1959 and 2005. Historical floods were reflected in the reconstructions, and the team’s results paralleled previous work, giving the researchers confidence in their methods.

Although streamflow has increased since the 1800s, the tree rings showed that the past 40 years have experienced more rain than any similar length of time in the past 600 years. Scientists don’t know yet what’s caused this change. In fact, many thought the recent increase in El Niño events would lead to fewer wet years, but that’s the opposite of what’s been seen. Some researchers have speculated that changes in the average temperatures of the ocean and the land may combine to increase monsoon activity.

As Australians contemplate increasing agriculture in Australia’s north, the authors write that officials should be aware of monsoon history. Current trends don’t tell the complete story of these weather systems, and the future may bring still more surprises, they say.