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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and drier conditions.

The Tipping Point Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the root systems, started around 25 years ago, according to recent research.

Forests typically absorb carbon during growth and emit it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this absorption is expected to grow with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across northern Australia has shown that this vital carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and insufficient new growth, according to the research.

“It’s the first tropical forest of its kind to show this symptom of transformation,” stated the lead author.

“We know that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Worldwide Consequences

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

But should that be the case, the results could have significant implications for global climate models, carbon budgets, and climate policies.

“This research is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” remarked an expert in climate change science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the future. “Which is bad news,” he added.

Ongoing Role

Although the balance between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Research Approach

The analysis utilized a unique set of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but not the changes below ground.

Another researcher highlighted the value of gathering and preserving long term data.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But looking at these long term empirical datasets, we find that is incorrect – it enables researchers to confront the theory with reality and better understand how these systems work.”