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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This significant change, which affects the stems and limbs of the trees but does not include the root systems, started around 25 years ago, according to new studies.

Forests typically absorb carbon during growth and emit it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this absorption is expected to grow with higher CO2 levels.

However, nearly 50 years of data gathered from tropical forests across Queensland has shown that this essential carbon sink may be at risk.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to show this symptom of change,” stated the principal researcher.

“It is understood that the moist tropics in Australia occupy a slightly warmer, drier climate 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

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and additional studies are needed.

But should that be the case, the results could have significant implications for global climate models, carbon budgets, 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 identified clearly – not merely temporarily, but for 20 years,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate forecasts may underestimate global warming in the future. “Which is bad news,” it was noted.

Ongoing Role

Even though 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 “a lot harder”, and require an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the changes in soil and roots.

Another researcher emphasized the value of gathering and preserving extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But looking at these long term empirical datasets, we find that is incorrect – it enables researchers to compare models with actual data and better understand how these ecosystems work.”