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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and drier conditions.

Critical Change Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, as per recent research.

Trees naturally store carbon as they develop and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – 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 vital carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and insufficient new growth, according to the research.

“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 different landmasses, and therefore it might serve as a coming example for what tropical forests will encounter 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 worldwide, and further research are required.

But should that be the case, the findings could have major consequences for international climate projections, carbon budgets, 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 identified clearly – not merely temporarily, but for 20 years,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under numerous projections and policies.

But if similar shifts – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the future. “Which is bad news,” he added.

Ongoing Role

Even though the balance between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

This study utilized a unique set of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but not the changes below ground.

An additional expert highlighted the value of gathering and preserving long term data.

“We thought the forest would be able to store more carbon because [CO2] is rising. But examining these decades of recorded information, we find that is incorrect – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”