The world's rainforests are typically thought of as crucial "carbon sinks" -- sucking huge quantities of planet-heating emissions from the atmosphere.

But new research has found rainforests in Australia's northern tropics have become net carbon emitters, "the first globally to show this response to climate change".

"Forests help to curb the worst effects of climate change by absorbing some of the carbon dioxide released by burning fossil fuels," said lead author Hannah Carle, from Western Sydney University.

"But our work shows this is under threat."

The researchers pored through records charting the growth of Queensland's rainforests over almost 50 years.

Extreme temperatures and severe droughts linked to climate change were making it harder for trees to grow, they found.

Trees store carbon dioxide in their trunks and branches as they grow but release the gas into the atmosphere when they die.

"Tropical rainforests are among the most carbon-rich ecosystems on the planet," said Carle.

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

"Regrettably, the associated increase in carbon losses to the atmosphere has not been offset by increased tree growth."

Co-author Adrienne Nicotra said it "remains to be seen whether Australian tropical forests are a harbinger for other tropical forests globally".

But the study, published in peer-reviewed journal Nature, suggested there was "the potential for a similar response to climate change" in other rainforests around the world.

"The rainforest sites at the heart of this research provide unusually long-term and high-resolution data on forest health through time," said Nicotra, from Australian National University.

"We need to pay attention to that data."

Despite its growing vulnerability to climate-linked natural disasters, Australia remains one of the world's biggest exporters of gas and thermal coal.

Australia's carbon dioxide emissions per person are among the highest in the world, World Bank figures show.

Global emissions have been rising but need to almost halve by the end of the decade to limit warming to safer levels agreed under the 2015 Paris climate agreement.