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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, driven by increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This crucial shift, which affects the trunks and branches of the trees but does not include the underground roots, began approximately a quarter-century back, according to new studies.

Forests typically absorb carbon as they develop and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this uptake is assumed to grow with rising atmospheric concentrations.

However, nearly 50 years of data collected from tropical forests across Queensland has revealed that this vital carbon sink could be under threat.

Study Insights

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

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

“It is understood that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Global Implications

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and further research are needed.

But if so, the findings could have significant implications for international climate projections, carbon budgets, and environmental regulations.

“This research is the first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for 20 years,” remarked an expert in climate change science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate projections may underestimate global warming in the coming years. “Which is bad news,” he added.

Continued Function

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

Data and Methodology

The analysis drew on a unique set of forest data dating back to 1971, including records tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the changes in soil and roots.

Another researcher highlighted the value of gathering and preserving extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and improve comprehension of how these systems work.”