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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This crucial shift, which impacts the trunks and branches of the trees but excludes the root systems, began approximately a quarter-century back, according to new studies.

Trees naturally store carbon as they develop and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this absorption is expected to increase with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree stems and limbs 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 display this sign of change,” commented the principal researcher.

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

One co-author mentioned 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 major consequences 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 identified clearly – not just for one year, but for two decades,” stated 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 many climate models and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate forecasts may underestimate global warming in the future. “Which is bad news,” he added.

Ongoing Role

Even though the equilibrium between growth and decline had changed, these forests were still serving a vital function 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.

Research Approach

This study utilized a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the changes in soil and roots.

Another researcher emphasized the importance of collecting and maintaining long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these decades of recorded information, we discover that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”