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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This significant change, which affects the trunks and branches of the trees but excludes the underground roots, began approximately a quarter-century back, according to recent research.

Forests typically absorb carbon as they develop and emit it when they decompose. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this absorption is assumed to grow with higher CO2 levels.

However, nearly 50 years of data gathered from tropical forests across northern Australia has revealed that this essential carbon sink could be under threat.

Research Findings

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

“It’s the first tropical forest of its kind to display this sign of transformation,” commented the lead author.

“We know that the humid tropical regions in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will experience in other parts of the world.”

Global Implications

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

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

“This research is the initial instance that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for 20 years,” remarked an expert in climate change science.

Worldwide, the portion 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 if similar shifts – from sink to source – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “This is concerning,” it was noted.

Continued Function

Although the equilibrium between gains and losses had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an even more rapid shift from carbon-based energy.

Data and Methodology

The analysis drew on a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the gains and losses below ground.

An additional expert highlighted the importance of collecting and maintaining long term data.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we find that is not the case – it enables researchers to compare models with actual data and better understand how these systems work.”