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Australian tropical rainforest trees have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and arid environments.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but excludes the root systems, began approximately 25 years ago, according to recent research.

Trees naturally store carbon during growth and emit it when they decompose. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is assumed to grow with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland has shown that this vital carbon sink could be under threat.

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to show this symptom 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 other continents, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Global Implications

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 findings could have major consequences for international climate projections, CO2 accounting, and environmental regulations.

“This research is the first time that this critical threshold 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,” remarked an expert in climate change science.

Worldwide, the share of carbon dioxide absorbed 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 detected in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” it was noted.

Ongoing Role

Although the balance between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Data and Methodology

The analysis utilized 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 above ground, but not the gains and losses in soil and roots.

Another researcher highlighted the importance of collecting and maintaining long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these decades of recorded information, we discover that is not the case – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”