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Australian tropical rainforest trees have achieved a global first by transitioning from serving as a CO2 absorber to turning into a carbon emitter, due to increasingly extreme temperatures and drier conditions.

Critical Change Identified

This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, as per recent research.

Trees naturally store carbon during growth and emit it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this absorption is assumed to increase with higher CO2 levels.

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and insufficient new growth, as the study indicates.

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

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will experience in other parts of the world.”

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are required.

But should that be the case, the results could have major consequences for global climate models, CO2 accounting, and climate policies.

“This paper 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 20 years,” 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 past few decades, which was assumed to continue under numerous projections and strategies.

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

Ongoing Role

Although the equilibrium between growth and decline had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Data and Methodology

This study drew on a unique set of forest data dating back to 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 in soil and roots.

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

“It was believed 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 allows us to compare models with actual data and better understand how these ecosystems work.”