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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and arid environments.

The Tipping Point Discovered

This significant change, which impacts the trunks and branches of the trees but excludes the root systems, started around 25 years ago, as per new studies.

Trees naturally store carbon during growth and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this absorption is expected to grow with rising atmospheric concentrations.

However, nearly 50 years of data collected from tropical forests across Queensland has revealed that this vital carbon sink could be under threat.

Study Insights

Approximately 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.

“It’s the first tropical forest of its kind to show this symptom of transformation,” commented the lead author.

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate 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

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are required.

But if so, the results could have major consequences for global climate models, 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 definitively spotted – not just for one year, but for 20 years,” stated 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 if similar shifts – from sink to source – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “This is concerning,” it was noted.

Ongoing Role

Even though the equilibrium between growth and decline had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

This study drew on a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the changes in soil and roots.

Another researcher highlighted the value 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 long term empirical datasets, we find that is incorrect – it allows us to confront the theory with reality and better understand how these systems work.”