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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, driven by rising heat extremes and arid environments.

The Tipping Point Discovered

This significant change, which impacts the trunks and branches of the trees but excludes the underground roots, started around 25 years ago, as per recent research.

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

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to show this symptom of change,” commented 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 experience in other parts of the world.”

Worldwide Consequences

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

But should that be the case, the results could have major consequences 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 definitively spotted – not merely temporarily, but for 20 years,” stated an authority on climate science.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under numerous projections and strategies.

But should comparable changes – 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 equilibrium between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid shift from carbon-based energy.

Data and Methodology

This study utilized a unique set of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

Another researcher emphasized the value of collecting and maintaining extended datasets.

“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 not the case – it allows us to confront the theory with reality and better understand how these systems work.”