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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 excludes the root systems, started around a quarter-century back, as per new studies.

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

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

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and inadequate regeneration, according to the research.

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

“We know that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

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

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

“This paper is the first time that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for 20 years,” remarked an expert in climate change science.

Worldwide, 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 observed in other rainforests, climate forecasts may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Ongoing Role

Although the balance between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Research Approach

This study utilized 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 in trunks and branches, but excluded the changes below ground.

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

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we find that is incorrect – it enables researchers to confront the theory with reality and improve comprehension of how these ecosystems work.”