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Australian tropical rainforest trees have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

Critical Change Identified

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

However, nearly 50 years of data collected from tropical forests across Queensland has revealed that this vital carbon sink may be at risk.

Research Findings

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to display this sign of change,” commented the principal researcher.

“It is understood that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a coming example for what tropical forests will encounter in global regions.”

Global Implications

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and additional studies are required.

But if so, the results could have significant implications for global climate models, carbon budgets, and climate policies.

“This paper is the initial instance that this critical threshold 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 share 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 observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Although the balance between growth and decline had changed, these forests were still playing an important role 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 drew on a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but excluded the changes below ground.

Another researcher highlighted the importance of collecting and maintaining long term data.

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