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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and drier conditions.

The Tipping Point Discovered

This significant change, which affects the trunks and branches of the trees but does not include the root systems, started around 25 years ago, as per new studies.

Forests typically absorb carbon as they develop and release it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this absorption is assumed to grow with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, as the study indicates.

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

“It is understood that the moist tropics in Australia occupy 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

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and further research are needed.

But should that be the case, the results could have significant implications for international climate projections, carbon budgets, and climate policies.

“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 definitively spotted – not just for one year, but for 20 years,” stated an authority on climate science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under numerous projections and policies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate projections may underestimate global warming in the coming years. “This is concerning,” it was noted.

Ongoing Role

Although the balance between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Research Approach

This study drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the gains and losses below ground.

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

“We thought 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 confront the theory with reality and better understand how these ecosystems work.”