Australia's tropical rainforests - once key carbon sinks - are releasing more carbon dioxide into the atmosphere than they absorb, scientists say, marking the first such recorded shift anywhere in the world.

A new study published in Nature notes that the trunks and branches of trees in the country's wet tropics, known as woody biomass, are a net source of carbon dioxide now due to rising temperatures, atmospheric dryness and more frequent droughts linked to climate crisis.

The research found that standing woody biomass in Australia's wet tropics was now losing about 0.9 tonnes of carbon per hectare per year compared with gains of about 0.6 tonnes per hectare before the early 2000s, marking a clear transition from sink to source.

"Tropical forests are among the most carbon-rich ecosystems on the planet," Dr Hannah Carle, the study lead author who conducted the work at the Australian National University, said.

"We rely on them more than most people realise. Forests help to 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."

The research, which examined data from rainforest sites across northern Queensland, found that rising tree deaths caused by heat stress and drought outpaced new growth.

"The change our study describes is largely due to increased tree mortality driven by climate change, including increasingly extreme temperatures, atmospheric dryness and drought," Dr Carle said.

"Regrettably, the associated increase in carbon losses to the atmosphere has not been offset by increased tree growth. This is surprising because higher carbon dioxide levels should make it easier for plants to scavenge carbon dioxide from the air, leading to more tree growth and greater carbon sink capacity."

The findings challenge the long-held assumption that tropical forests will continue to absorb more carbon as emissions rise. "Current models may overestimate the capacity of tropical forests to help offset fossil fuel emissions," Dr Carle said.

Researchers warn that if this trend continues, it will undermine global emissions reduction targets that rely on forests to offset a part of the carbon released from burning fossil fuels.

Although it was long feared that forests could go from being a carbon sink to source, the findings still came as a surprise.

"The switch from net carbon sink to carbon source in Australian tropical rainforests appears to have occurred sooner than in other tropical regions, where rainforests remain carbon sinks," Dr David Bauman and Professor Patrick Meir, the study's co-authors, told The Independent.

The study also found that tropical cyclones - already a major disturbance in northern Australia - were further weakening the ability of rainforests to absorb carbon.

"We found that cyclones suppress the carbon sink capacity of woody biomass in these forests," Dr Carle said.

"This is cause for concern with cyclones projected to become increasingly severe under climate change, and to impact areas further southwards, affecting additional stretches of forest to a potentially greater extent."

Professor Adrienne Nicotra, co-author of the study, said the research drew on some of the most comprehensive and long-term forest health data ever collected in Australia.

"The rainforest sites at the heart of this research provide unusually long-term and high-resolution data on forest health through time," she said. "We need to pay attention to that data."

The study's authors said the findings underscored the importance of protecting remaining tropical forests and restoring degraded areas to maintain their role in stabilising the climate.

"Our finding is clear evidence that the potential of climate change mitigation via the action of a large carbon sink from tropical forests is in danger, though these forests remain irreplaceable stores of carbon and biodiversity," the study's authors said.

Australia's rainforests start releasing more carbon than they absorb, scientists warn