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Australian tropical rainforest trees have become the first worldwide by transitioning from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

Critical Change Discovered

This significant change, which affects the trunks and branches of the trees but excludes the root systems, started around 25 years ago, according to new studies.

Forests typically absorb carbon during growth and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to increase with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across Queensland has shown that this vital carbon sink may be at risk.

Research Findings

Roughly 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.

“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 somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will experience in global regions.”

Global Implications

One co-author mentioned 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 results could have significant implications for international climate projections, CO2 accounting, 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 identified clearly – not merely temporarily, but for two decades,” stated an expert in climate change 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 expected to persist under many climate models and policies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” he added.

Ongoing Role

Even though the balance between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an accelerated transition away from fossil fuels.

Research Approach

This study utilized 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 not the gains and losses in soil and roots.

An additional expert highlighted the value of gathering and preserving 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 discover that is incorrect – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”