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The study, led by Western Sydney University’s Dr Hannah Carle, analysed nearly five decades of data from 20 forest sites across Queensland. It found that extreme heat, prolonged droughts, and more frequent cyclones have pushed tree mortality rates above growth levels. In effect, the forests’ trunks and branches, known as woody biomass, have flipped roles: from storing carbon to releasing it.

Mumbai: Australia’s tropical rainforests, long seen as a natural bulwark against global warming, have crossed an alarming threshold — they are now emitting more carbon than they absorb. The startling finding, published in Nature, marks the first recorded instance of any tropical forest on Earth switching from a carbon sink to a carbon source.

The study, led by Western Sydney University’s Dr Hannah Carle, analysed nearly five decades of data from 20 forest sites across Queensland. It found that extreme heat, prolonged droughts, and more frequent cyclones have pushed tree mortality rates above growth levels. In effect, the forests’ trunks and branches, known as woody biomass, have flipped roles: from storing carbon to releasing it. “Forests help curb the worst effects of climate change by absorbing carbon dioxide released from burning fossil fuels,” Dr Carle said.

“But our work shows this ability is under threat. The change in Australia’s moist tropical forests could be a canary in the coal mine for the rest of the world.” The research reveals the tipping point occurred roughly 25 years ago, when tree deaths began outpacing regeneration. With fewer new trees emerging to replace the old, the vast stores of carbon locked in decaying wood have begun leaking back into the atmosphere.

Dr Carle attributed the shift to rising temperatures and increasing atmospheric dryness — trends consistent with global climate change. “Our forests are experiencing hotter, drier conditions than ever before,” she said. “And those stressors are killing trees faster than they can recover.”

The implications are global. Tropical forests are key buffers in the planet’s climate system, collectively absorbing billions of tonnes of carbon dioxide each year. Models used by climate scientists and policymakers assume these forests will continue to serve as dependable carbon sinks. But the Australian findings suggest that assumption may now be dangerously optimistic.

“Current climate models may overestimate the capacity of tropical forests to offset fossil fuel emissions,” warned Dr Carle. “If similar trends appear elsewhere, it would mean our planet’s natural carbon balance is shifting far sooner than expected.” Senior researcher Professor Patrick Meir called the results “deeply concerning”. He told AFP that while the phenomenon was first observed in Australia’s tropical forests, “it’s likely that all tropical forests will respond fairly similarly” as global warming accelerates.

Other scientists agree the findings are a red flag. Professor David Karoly, a veteran climate scientist at the University of Melbourne, said the research“marks the first clear evidence of a long-term tipping point where tropical rainforests have switched from absorbing carbon to releasing it — not just for one year, but for two decades.”

If replicated across other rainforests, the consequences could be severe. Global carbon budgets — which determine how much greenhouse gas humanity can emit to stay within safe temperature limits — may be drastically underestimated. “If forests worldwide begin losing their capacity to absorb carbon, our emissions targets will have to be far more aggressive,” Karoly said.

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