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Trees in Australia's tropical rainforests have achieved a global first by shifting 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 affects the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, according to new studies.

Forests typically absorb carbon as they develop and release it when they decompose. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this absorption is expected to grow with higher CO2 levels.

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

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, as the study indicates.

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

“It is understood that the humid tropical regions in Australia occupy 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

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are required.

But if so, the results could have major consequences for global climate models, CO2 accounting, and environmental regulations.

“This paper is the first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for 20 years,” stated 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 last 20 to 30 years, which was expected to persist under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Continued Function

Even though the equilibrium 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 “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Research Approach

This study utilized a distinct collection of forest data dating back to 1971, including records tracking approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes below ground.

Another researcher highlighted the value of gathering and preserving long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and improve comprehension of how these systems work.”