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

Critical Change Discovered

This crucial shift, which affects the trunks and branches of the trees but excludes the underground roots, started around a quarter-century back, according to new studies.

Forests typically absorb carbon as they develop and emit it upon decay and death. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this uptake is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland has revealed that this vital carbon sink may be at risk.

Research Findings

Roughly 25 years ago, tree stems and limbs 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,” commented the lead author.

“We know that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will experience in other parts of the world.”

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 additional studies are needed.

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

“This paper 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 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 numerous projections and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “Which is bad news,” he added.

Continued Function

Although the equilibrium between growth and decline had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Data and Methodology

This study utilized 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 above ground, but excluded the changes in soil and roots.

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

“It was believed the forest would be able to store more carbon because [CO2] is rising. But looking at these decades of recorded information, we discover that is incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”