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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.

The Tipping Point Discovered

This crucial shift, which impacts the stems and limbs of the trees but does not include the root systems, began approximately 25 years ago, according to new studies.

Trees naturally store carbon as they develop and release it when they decompose. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to grow with rising atmospheric concentrations.

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

Research Findings

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and insufficient new growth, according to the research.

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

“It is understood that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will encounter in global regions.”

Global Implications

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are required.

But if so, the results could have significant implications for global climate models, carbon budgets, and environmental regulations.

“This research 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.

On a global scale, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable 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 projections may understate heating trends in the future. “Which is bad news,” he added.

Ongoing Role

Even though the equilibrium between gains and losses had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated shift from carbon-based energy.

Research Approach

This study drew on a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the changes below ground.

Another researcher emphasized the importance 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 long term empirical datasets, we discover that is not the case – it allows us to confront the theory with reality and better understand how these ecosystems work.”