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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, due to increasingly extreme temperatures and drier conditions.

Critical Change Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the root systems, started around a quarter-century back, according to recent research.

Forests typically absorb carbon during growth and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this absorption is assumed to increase with higher CO2 levels.

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

Research Findings

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

“It’s the first tropical forest of its kind to display this sign of change,” stated 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 might serve as a future analog for what tropical forests will encounter in global regions.”

Global Implications

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are needed.

But should that be the case, the findings could have significant implications for international climate projections, CO2 accounting, and climate policies.

“This paper 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 merely temporarily, 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 relatively constant over the past few decades, which was assumed to continue under many climate models and policies.

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

Continued Function

Although the balance between growth and decline had changed, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Data and Methodology

The analysis utilized a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but excluded the gains and losses below ground.

An additional expert emphasized the importance of gathering and preserving extended datasets.

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