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Australian tropical rainforest trees have achieved a global first by transitioning from serving as a CO2 absorber to turning into a carbon emitter, driven by increasingly extreme temperatures and drier conditions.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, according to recent research.

Forests typically absorb carbon during growth and release it when they decompose. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is expected to grow with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across Queensland has revealed 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 increased tree mortality and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of transformation,” stated the principal researcher.

“It is understood that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a future analog 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 precursor for other tropical forests globally, and further research are required.

But should that be the case, the results could have major consequences for global climate models, carbon budgets, and environmental regulations.

“This research is the first time that this tipping point of a switch 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 authority on climate science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under numerous projections and policies.

But should comparable changes – from absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the future. “This is concerning,” he added.

Ongoing Role

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

Research Approach

This study utilized a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but not the changes in soil and roots.

Another researcher emphasized the value of gathering and preserving extended datasets.

“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 discover that is incorrect – it allows us to confront the theory with reality and better understand how these ecosystems work.”