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Trees in Australia's tropical rainforests have achieved a global first by transitioning from acting as a carbon sink to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This crucial shift, which impacts the stems and limbs of the trees but excludes the underground roots, began approximately a quarter-century back, as per recent research.

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

However, nearly 50 years of data collected from tropical forests across northern Australia has shown that this vital carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and inadequate regeneration, as the study indicates.

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

“It is understood that the moist tropics in Australia occupy 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 encounter in other parts of the world.”

Global Implications

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are needed.

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

“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 20 years,” stated an authority on climate science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was assumed to continue under numerous projections and strategies.

But if similar shifts – 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.

Ongoing Role

Even though the equilibrium between gains and losses had changed, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

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

Another researcher highlighted the importance of gathering and preserving long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we discover that is not the case – it allows us to compare models with actual data and better understand how these ecosystems work.”