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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and drier conditions.

The Tipping Point Identified

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, started around a quarter-century back, as per new studies.

Forests typically absorb carbon during growth and release it when they decompose. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across northern Australia has revealed that this essential carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and inadequate regeneration, as the study indicates.

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

“We know that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Worldwide Consequences

One co-author noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are required.

But if so, the results could have significant implications for international climate projections, carbon budgets, and environmental regulations.

“This research is the first time that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for two decades,” stated an expert in climate change science.

On a global scale, the portion 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 should comparable changes – from absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the future. “This is concerning,” it was noted.

Continued Function

Even though the balance between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the gains and losses in soil and roots.

An additional expert emphasized the importance of collecting and maintaining long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these long term empirical datasets, we discover that is not the case – it enables researchers to confront the theory with reality and better understand how these systems work.”