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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

The Tipping Point Discovered

This significant change, which impacts the trunks and branches of the trees but does not include the underground roots, began approximately a quarter-century back, according to new studies.

Trees naturally store carbon during growth and release it when they decompose. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this absorption is assumed to increase with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across Queensland has shown that this essential carbon sink may be at risk.

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and inadequate regeneration, according to the research.

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

“We know that the humid tropical regions in Australia exist in a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a future analog 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 needed.

But should that be the case, the results could have significant implications for international climate projections, CO2 accounting, and climate policies.

“This paper is the initial instance 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 two decades,” stated an expert in climate change science.

On a global scale, the share 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 policies.

But should comparable changes – from absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “Which is bad news,” he added.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an even more rapid shift from carbon-based energy.

Research Approach

This study drew on a distinct collection 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 gains and losses 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 long term empirical datasets, we discover that is not the case – it enables researchers to compare models with actual data and better understand how these systems work.”