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Trees in Australia's tropical rainforests have achieved a global first by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, began approximately a quarter-century back, as per new studies.

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

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

Research Findings

Approximately 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.

“It’s the first tropical forest of its kind to display this sign of change,” stated the lead author.

“We know that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will experience in other parts of the world.”

Global Implications

One co-author noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, 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 first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” remarked 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 forecasts may understate heating trends in the coming years. “Which is bad news,” he added.

Ongoing Role

Although the equilibrium between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

The analysis utilized a distinct collection of forest data dating back to 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 below ground.

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

“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 not the case – it enables researchers to compare models with actual data and better understand how these ecosystems work.”