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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to becoming a source of emissions, due to increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This significant change, which impacts the trunks and branches of the trees but excludes the root systems, began approximately a quarter-century back, according to recent research.

Forests typically absorb carbon as they develop and emit it upon decay and death. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this absorption is expected to increase with higher CO2 levels.

However, nearly 50 years of data gathered from tropical forests across Queensland has shown that this essential carbon sink may be at risk.

Research Findings

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of transformation,” stated the lead author.

“It is understood that the moist tropics in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will experience in global regions.”

Worldwide Consequences

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

But if so, the results could have significant implications for global climate models, carbon budgets, and climate policies.

“This research is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for two decades,” remarked an expert in climate change science.

On a global scale, the portion 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 absorber to emitter – were detected in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” he added.

Ongoing Role

Although the balance between gains and losses 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 even more rapid shift from carbon-based energy.

Data and Methodology

The analysis utilized a distinct collection of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the gains and losses below ground.

Another researcher highlighted the value of gathering and preserving extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But examining these decades of recorded information, we discover that is not the case – it enables researchers to compare models with actual data and better understand how these ecosystems work.”