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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, started around 25 years ago, according to new studies.

Forests typically absorb carbon during growth and emit it upon decay and death. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is expected to increase with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across northern Australia has shown that this vital carbon sink may be at risk.

Research Findings

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, 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 change,” commented the principal researcher.

“It is understood 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 experience in other parts of the world.”

Worldwide Consequences

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

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

“This paper 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 just for one year, but for 20 years,” stated an expert in climate change science.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Continued Function

Although the balance between gains and losses had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

This study drew on a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots.

An additional expert emphasized the value of collecting and maintaining 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 incorrect – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”