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Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the underground roots, started around a quarter-century back, as per recent research.

Forests typically absorb carbon during growth and release it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is expected to grow with higher CO2 levels.

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and insufficient new growth, according to the research.

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

“We know that the moist tropics in Australia occupy 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 encounter in other parts of the world.”

Global Implications

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

But if so, the findings could have major consequences for global climate models, CO2 accounting, and environmental regulations.

“This research is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for 20 years,” stated an expert in climate change science.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under numerous projections and policies.

But if similar shifts – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the future. “Which is bad news,” it was noted.

Continued Function

Although the balance between growth and decline had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and require an even more rapid shift from carbon-based energy.

Research Approach

The analysis drew on a unique set of forest data starting from 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the gains and losses below ground.

Another researcher highlighted the value of collecting and maintaining extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these long term empirical datasets, we find that is incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”