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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, driven by rising heat extremes and arid environments.

Critical Change 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 recent research.

Forests typically absorb carbon during growth and release it upon decay and death. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is assumed to increase with higher CO2 levels.

However, nearly 50 years of data collected from tropical forests across northern Australia has shown that this essential carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, 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 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 might serve as a future analog for what tropical forests will encounter in global regions.”

Global Implications

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 if so, the results could have significant implications for international climate projections, carbon budgets, and climate policies.

“This research is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for two decades,” remarked an expert in climate change science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under numerous projections and strategies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” it was noted.

Ongoing Role

Although the equilibrium between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Data and Methodology

The analysis utilized a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but excluded the changes below ground.

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

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But looking at these long term empirical datasets, we find that is incorrect – it enables researchers to compare models with actual data and improve comprehension of how these systems work.”