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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, driven by increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This crucial shift, which impacts the stems and limbs of the trees but excludes the root systems, started around 25 years ago, according to recent research.

Forests typically absorb carbon as they develop and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is expected to grow with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across northern Australia has shown 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, according to the research.

“This marks the initial rainforest of its kind to show this symptom of transformation,” stated 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 if so, the results could have major consequences for global climate models, CO2 accounting, and environmental regulations.

“This research is the initial instance that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for 20 years,” stated an expert in climate change science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under numerous projections and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate forecasts may understate heating trends in the future. “Which is bad news,” he added.

Ongoing Role

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

Research Approach

This study 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 in trunks and branches, but not the changes below ground.

Another researcher 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 rising. But looking at these long term empirical datasets, we find that is not the case – it allows us to confront the theory with reality and better understand how these systems work.”