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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, due to rising heat extremes and arid environments.

The Tipping Point Discovered

This significant change, which impacts the stems and limbs of the trees but excludes the underground roots, started around 25 years ago, as per new studies.

Forests typically absorb carbon during growth and release it when they decompose. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across Queensland has revealed that this essential carbon sink could be under threat.

Study Insights

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 principal researcher.

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

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and additional studies are required.

But should that be the case, the results could have major consequences for global climate models, CO2 accounting, and climate policies.

“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 definitively spotted – not just for one year, but for two decades,” stated an expert in climate change science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under many climate models and strategies.

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,” it was noted.

Ongoing Role

Even though the balance between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Research Approach

This study utilized a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but excluded the changes below ground.

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

“We thought the forest would be able to store more carbon because [CO2] is rising. But looking at these long term empirical datasets, we discover that is not the case – it enables researchers to confront the theory with reality and improve comprehension of how these systems work.”