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Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and arid environments.

The Tipping Point Discovered

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

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

However, nearly 50 years of data collected from tropical forests across Queensland has shown 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 increased tree mortality and insufficient new growth, according to the research.

“It’s the first tropical forest of its kind to show this symptom of transformation,” commented the principal researcher.

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Global Implications

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

But should that be the case, the results could have significant implications for international climate projections, carbon budgets, and climate policies.

“This paper is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for two decades,” remarked an expert in climate change science.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from sink to source – were observed in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” it was noted.

Ongoing Role

Although the equilibrium between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Research Approach

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

An additional expert highlighted the value 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 decades of recorded information, we discover that is incorrect – it allows us to confront the theory with reality and improve comprehension of how these systems work.”