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

Critical Change Identified

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

Forests typically absorb carbon during growth and emit it when they decompose. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is expected to grow with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across Queensland has revealed 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 increased tree mortality and inadequate regeneration, as the study indicates.

“This marks the initial rainforest of its kind to display this sign of change,” stated the principal researcher.

“It is understood that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

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 should that be the case, the findings could have major consequences for global climate models, CO2 accounting, and environmental regulations.

“This research is the first time that this critical threshold 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 share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Continued Function

Although the equilibrium between growth and decline had shifted, these forests were still playing an important role in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

The analysis drew on a unique set of forest data dating back to 1971, including records monitoring roughly 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 extended datasets.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these long term empirical datasets, we find that is incorrect – it allows us to confront the theory with reality and better understand how these ecosystems work.”