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Trees in Australia's tropical rainforests have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Discovered

This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, started around 25 years ago, as per new studies.

Trees naturally store carbon during growth and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed 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 could be under threat.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and inadequate regeneration, according to the research.

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

“It is understood that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are needed.

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

“This research 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,” remarked an authority on climate 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 absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the future. “Which is bad news,” it was noted.

Ongoing Role

Although the equilibrium between gains and losses had shifted, 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 “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Data and Methodology

This study utilized a unique set of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the gains and losses below ground.

Another researcher highlighted the importance of collecting and maintaining long term data.

“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 incorrect – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”