The international team behind the study, published in Nature, found that the region's woody biomass - tree trunks and branches - has shifted from storing carbon to emitting it, making Australia's wet tropics the first globally to show this change under a warming climate.
Researchers attribute the reversal to rising temperatures, droughts, and increasing atmospheric dryness caused by human-driven climate change. These stressors are killing more trees than new growth can replace, undermining the forests' long-held role as a carbon sink.
"Tropical forests are among the most carbon-rich ecosystems on the planet. We rely on them more than most people realise," said lead author Dr Hannah Carle from Western Sydney University, who conducted the research as part of her PhD at ANU. "Forests help curb the worst effects of climate change by absorbing some of the carbon dioxide released from burning fossil fuels. But our work shows this is under threat."
Dr Carle explained that while rising CO2 levels can theoretically stimulate plant growth, this effect has been outweighed by climate-induced stress and tree mortality. "Regrettably, the associated increase in carbon losses to the atmosphere has not been offset by increased tree growth," she said.
The study also found that cyclones significantly reduce the carbon storage potential of tropical forests, a concern given projections for stronger and more southerly storms in coming decades.
"Current models may overestimate the capacity of tropical forests to help offset fossil fuel emissions," Dr Carle warned. Co-author Professor Adrienne Nicotra of ANU added that the long-term monitoring data from Australia's rainforest sites offer critical insights into forest health and climate resilience that must not be ignored.
Research Report:Aboveground biomass in Australian tropical forests now a net carbon source
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