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Trees in Australia's tropical rainforests have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and arid environments.

The Tipping Point Identified

This significant change, which impacts the trunks and branches of the trees but does not include the root systems, began approximately a quarter-century back, as per recent research.

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

However, nearly 50 years of data collected from tropical forests across Queensland has revealed 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 insufficient new growth, as the study indicates.

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

“We know that the moist tropics in Australia exist in 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 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 worldwide, and further research are required.

But should that be the case, the results could have major consequences for international climate projections, carbon budgets, and environmental regulations.

“This research is the initial instance 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,” stated an authority on climate science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was assumed to continue under many climate models and policies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate forecasts may underestimate global warming in the coming years. “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 absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and require an even more rapid shift from carbon-based energy.

Research Approach

The analysis drew on a unique set of forest data starting from 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the changes in soil and roots.

Another researcher emphasized the importance of gathering and preserving long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we find that is not the case – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”