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Australian tropical rainforest trees have achieved a global first by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and arid environments.

The Tipping Point Identified

This significant change, which impacts the trunks and branches of the trees but excludes the underground roots, began approximately 25 years ago, according to new studies.

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

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

Study Insights

Approximately 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,” stated the principal researcher.

“It is understood that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, 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 is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

But if so, the findings could have major consequences for international climate projections, CO2 accounting, and climate policies.

“This research 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 two decades,” remarked an authority on climate science.

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

But should comparable changes – from sink to source – were observed in other rainforests, climate forecasts may underestimate global warming in the coming years. “This is concerning,” it was noted.

Ongoing Role

Although the balance between growth and decline had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Research Approach

This study drew on a distinct collection of forest data dating back to 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but excluded the changes in soil and roots.

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

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these decades of recorded information, we discover that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”