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Australian tropical rainforest trees have become the first worldwide by transitioning from acting as a carbon sink to turning into a carbon emitter, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This significant change, which affects the trunks and branches of the trees but excludes the root systems, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon during growth and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is expected 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

Roughly 25 years ago, tree stems and limbs 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 change,” stated the lead author.

“It is understood that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act 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 globally, and further research are required.

But if so, the results could have significant implications for international climate projections, carbon budgets, and climate policies.

“This paper is the initial instance that this tipping point of a switch 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 authority on climate science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was assumed to continue 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 coming years. “Which is bad news,” it was noted.

Continued Function

Although the balance between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Data and Methodology

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

An additional expert highlighted the value of gathering and preserving extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we find that is not the case – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”