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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and drier conditions.

The Tipping Point Discovered

This significant change, which impacts the trunks and branches of the trees but excludes the root systems, began approximately 25 years ago, according to new studies.

Trees naturally store carbon as they develop and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this uptake is expected to grow with rising atmospheric concentrations.

However, nearly 50 years of data collected 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 more trees dying and insufficient new growth, as the study indicates.

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

“We know that the moist tropics in Australia exist in a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will experience in other parts of the world.”

Global Implications

One co-author mentioned that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are needed.

But if so, the results could have significant implications for global climate models, CO2 accounting, and climate policies.

“This research 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 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 underestimate global warming in the future. “This is concerning,” it was noted.

Continued Function

Even though the balance between gains and losses had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Data and Methodology

This study drew on a unique set of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the gains and losses below ground.

An additional expert emphasized 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 decades of recorded information, we find that is not the case – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”