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Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and drier conditions.

The Tipping Point Discovered

This crucial shift, which impacts the trunks and branches of the trees but does not include the underground roots, started around 25 years ago, as per new studies.

Forests typically absorb carbon as they develop and emit it upon decay and death. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to increase with higher CO2 levels.

However, close to five decades 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 stems and limbs in these forests turned into a carbon source, with more trees dying and inadequate regeneration, as the study indicates.

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

“We know 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 experience in other parts of the world.”

Worldwide Consequences

One co-author noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

But should that be the case, the findings could have major consequences for international climate projections, carbon budgets, and environmental regulations.

“This paper is the initial instance 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,” remarked an authority on climate 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 numerous projections and policies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” he added.

Ongoing Role

Although the balance between growth and decline had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, 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 tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the gains and losses below ground.

An additional expert emphasized the value of collecting and maintaining long term data.

“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 not the case – it allows us to compare models with actual data and better understand how these systems work.”