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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

The Tipping Point Identified

This significant change, which impacts the stems and limbs of the trees but excludes the underground roots, started around 25 years ago, according to recent research.

Trees naturally store carbon as they develop and emit it upon decay and death. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this absorption is expected to increase with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland has shown that this essential carbon sink may be at risk.

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, according to the research.

“This marks the initial rainforest of its kind to show this symptom of transformation,” stated the lead author.

“We know that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will encounter in other parts of the world.”

Global Implications

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are required.

But if so, the results could have major consequences for global climate models, carbon budgets, and environmental regulations.

“This research is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for two decades,” stated an expert in climate change science.

On a global scale, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was assumed to continue under numerous projections and policies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Even though the balance between growth and decline had changed, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Data and Methodology

The analysis utilized a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the gains and losses in soil and roots.

An additional expert emphasized the importance of gathering and preserving extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these long term empirical datasets, we find that is incorrect – it enables researchers to confront the theory with reality and improve comprehension of how these systems work.”