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Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which impacts the trunks and branches of the trees but excludes the root systems, began approximately 25 years ago, as per new studies.

Forests typically absorb carbon during growth and release it upon decay and death. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to grow with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across Queensland has revealed that this essential 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, according to the research.

“It’s the first tropical forest of its kind to show this symptom of change,” stated the lead author.

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Global Implications

One co-author 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 results could have significant implications for global climate models, carbon budgets, 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 two decades,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under many climate models and strategies.

But should comparable changes – from sink to source – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “Which is bad news,” it was noted.

Continued Function

Even though the balance between growth and decline 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 require an even more rapid shift from carbon-based energy.

Data and Methodology

This study utilized a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but excluded the gains and losses below ground.

Another researcher highlighted the importance of collecting and maintaining extended datasets.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we discover that is not the case – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”