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Trees in Australia's tropical rainforests have achieved a global first by shifting from serving as a CO2 absorber to turning into a carbon emitter, driven by increasingly extreme temperatures and arid environments.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but does not include the root systems, started around 25 years ago, as per recent research.

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

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

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and inadequate regeneration, as the study indicates.

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

“It is understood that the moist tropics in Australia exist in a somewhat hotter, arid environment 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.”

Worldwide Consequences

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

But if so, the findings could have significant implications for global climate models, CO2 accounting, and environmental regulations.

“This paper is the initial instance that this critical threshold 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 authority on climate science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under many climate models and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate forecasts may underestimate global warming in the coming years. “Which is bad news,” he added.

Ongoing Role

Even though the balance between gains and losses had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Data and Methodology

This study utilized a distinct collection of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but not the changes below ground.

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

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these long term empirical datasets, we discover that is incorrect – it enables researchers to compare models with actual data and better understand how these ecosystems work.”