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Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This significant change, which affects the stems and limbs of the trees but does not include the root systems, began approximately a quarter-century back, 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 – absorbing more CO2 than they emit – and this uptake is expected to grow with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and inadequate regeneration, as the study indicates.

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

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will experience in global regions.”

Global Implications

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are needed.

But should that be the case, the results could have major consequences for international climate projections, carbon budgets, and climate policies.

“This research is the first time that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for 20 years,” stated an authority on climate science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under many climate models and policies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate projections may underestimate global warming in the coming years. “This is concerning,” he added.

Ongoing Role

Even though 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 take in additional CO2 would make emissions cuts “more challenging”, and necessitate an accelerated transition away from fossil fuels.

Data and Methodology

The analysis drew on 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 gains and losses in soil and roots.

An additional expert emphasized the importance of gathering and preserving long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we find that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these systems work.”