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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to turning into a carbon emitter, driven by rising heat extremes and arid environments.

The Tipping Point Discovered

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, started around 25 years ago, according to recent research.

Trees naturally store 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 absorption is expected to increase with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across Queensland has shown that this essential carbon sink may be at risk.

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and inadequate regeneration, according to the research.

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

“It is understood that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

A study contributor mentioned that it is yet unclear 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 results could have major consequences for international climate projections, CO2 accounting, and climate policies.

“This paper is the initial instance that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for two decades,” remarked an authority on climate science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and strategies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” it was noted.

Ongoing Role

Although the equilibrium 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 drew on a distinct collection of forest data dating back to 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the changes in soil and roots.

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

“We thought the forest would be able to store more carbon because [CO2] is increasing. But looking at these decades of recorded information, we find that is incorrect – it enables researchers to confront the theory with reality and better understand how these systems work.”