Tree pollen reveals 150,000 years of monsoon history—and a warning for Australia’s northern rainfall

(Press-News.org) Northern Australia's annual monsoon season brings relief to drought-stricken lands and revitalises crops and livestock for farmers. But a study of 150,000 years of climate records shows that the monsoon is likely to intensify — triggering a higher risk of flooding while worsening the impact of droughts in East Asia.

Led by Professor Michael Bird, researchers at James Cook University and Flinders University have assessed sediments at Girraween Lagoon near Darwin, revealing a continuous record of monsoon rainfall patterns dating back beyond the last interglacial period.

This research published in the scientific journal Quaternary Science Reviews offers insight into how climate change could alter monsoon patterns across East Asia and Australia.

“This is the longest terrestrial record ever produced at the southern end of the Indo-Australian monsoon system, which delivers vital rainfall to millions across the Southern Hemisphere. The record also has implications for the Northern Hemisphere where tens of millions in Asia rely on monsoons for food and their livelihoods.

“Our study shows how the two monsoon systems are interrelated over thousands of years and reveals what causes them to change. Our analyses shows that that rainfall in northern Australia is closely tied to sea level changes, which shift the location of the northern coastline by up to 320 km.

These shifts strongly alter local rainfall, with wetter periods occurring when the coastline is closer to the Australian landmass and the oppose effect is prolonged drought in East Asia.”

“Intriguingly, the research also uncovered what we consider bursts of intense monsoon activity — some lasting less than 10,000 years. These bursts align with Heinrich events — abrupt pulses of freshwater into the North Atlantic from rapidly melting ice linked to the weakening of the Gulf Stream in the Atlantic Ocean,” said Professor Bird.

These findings carry a warning from scientists because the Gulf Stream is already weakening due to climate change, and the study suggests this could lead to increased rainfall in northern Australia while contributing to droughts in parts of East Asia.

“This isn’t just ancient history. It is a window into the rainfall patterns that are emerging today. Our data suggest that the weather trends we’re witnessing like the drying in China and wetting in northern Australia could accelerate if the Gulf Stream continues to weaken, so we need to be ready for that scenario,” says study co-author, Professor Corey Bradshaw, a Global Ecologist at Flinders University.

“It’s not surprising. Decreasing rainfall in parts of the east Asian summer monsoon region has been identified in rainfall records since the 1960s, while increasing rainfall has been evident in north-western Australia since the last century, accelerating since the 1950s. Our new data suggest that further weakening of the Gulf Stream could reinforce these trends even more in the future, with consequences for both regions.”

“We need to put this impact into context because this region extends from China through Southeast Asia, the maritime continent, and western Indo-Pacific warm pool on the Equator, to Australia. The region is home to almost a billion people and five terrestrial Biodiversity Hotspots.”

The research was supported by the Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage (CE170100015) and Centre of Excellence for Indigenous and Environmental Histories and Futures (CE230100009), and an Australian Research Council Laureate Fellowship (FL140100044).

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