SYDNEY - Australia's tropical rainforests are among the first in the world to start emitting more carbon dioxide than they absorb, scientists said, linking the "very concerning" trend to climate change.
The world's rainforests are typically thought of as crucial "carbon sinks" -- sucking huge quantities of planet-heating emissions from the atmosphere.
Some models predict that increased carbon dioxide in the atmosphere could boost forest growth by offering trees more of the basic fuel they need for photosynthesis.
But new research found extreme temperatures have instead caused more forest death than growth, with rainforests in Australia's northern tropics becoming net carbon emitters.
"This is the first analysis to show this pattern occurring for natural undisturbed forest and as a persistent pattern over many years," senior author of the research Patrick Meir told AFP, describing the results as "very concerning".
The researchers pored through records charting the growth of Queensland's rainforests over almost 50 years.
They found that around the year 2000 they began emitting more carbon dioxide from the decay of dead trees than was being taken in and stored by growing trunks and branches.
Modelling showed the main cause was extreme temperatures linked to climate change, and its related effects on the moisture in the atmosphere and drought. Cyclones, which are expected to increase in intensity with climate change, also had an impact.
The findings are in line with some research on the Amazon that showed growing tree deaths weakening the forest's carbon storage capacity, said David Bauman, a research scientist at the French National Research Institute for Sustainable Development and second author of the study.
"So in that sense our result is not surprising, but the timing... happened earlier than we expected and the effects of the likely climatic drivers (high temperature, drought) are stronger than we expected," he told AFP.