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Australian tropical rainforest trees have achieved a global first by transitioning from serving as a CO2 absorber to turning into a carbon emitter, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Discovered

This crucial shift, which impacts the trunks and branches of the trees but excludes the root systems, started around 25 years ago, according to recent research.

Trees naturally store carbon during growth and emit it when they decompose. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this uptake is expected to increase with higher CO2 levels.

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

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and inadequate regeneration, as the study indicates.

“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 somewhat hotter, arid environment than tropical forests on other continents, 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 needed.

But should that be the case, the findings could have significant implications for international climate projections, CO2 accounting, and environmental regulations.

“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 definitively spotted – not just for one year, but for 20 years,” stated an authority on climate 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 absorber to emitter – were observed in other rainforests, climate forecasts may underestimate global warming in the future. “Which is bad news,” he added.

Ongoing Role

Although the equilibrium between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and require an accelerated transition away from fossil fuels.

Research Approach

This study utilized a unique set of forest data dating back to 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the gains and losses in soil and roots.

An additional expert emphasized the value of collecting and maintaining extended datasets.

“We thought 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 compare models with actual data and better understand how these systems work.”