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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 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.

Trees naturally store carbon during growth and emit it when they decompose. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is expected to grow with higher CO2 levels.

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

Study Insights

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

“This marks the initial rainforest of its kind to display this sign of change,” stated the lead author.

“It is understood that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will encounter in global regions.”

Worldwide Consequences

One co-author noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are needed.

But if so, the findings could have significant implications for global climate models, 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 definitively spotted – not just for one year, but for 20 years,” remarked an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under many climate models and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the future. “Which is bad news,” it was noted.

Continued Function

Even though the balance 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 accelerated shift from carbon-based energy.

Data and Methodology

The analysis utilized a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but not the changes below ground.

An additional expert highlighted the importance of gathering and preserving long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we find that is not the case – it enables researchers to confront the theory with reality and better understand how these ecosystems work.”