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

The Tipping Point Discovered

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

Forests typically absorb carbon during growth and emit it upon decay and death. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this absorption is expected to grow with higher CO2 levels.

However, close to five decades of data collected from tropical forests across northern Australia has shown that this essential carbon sink may be at risk.

Study Insights

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

“It’s the first tropical forest of its kind to show this symptom of transformation,” commented the principal researcher.

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are needed.

But should that be the case, the findings could have significant implications for global climate models, CO2 accounting, and climate policies.

“This research 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 two decades,” remarked an expert in climate change science.

Worldwide, the portion of carbon dioxide taken in 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 should comparable changes – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Even though the equilibrium between growth and decline had changed, these forests were still playing an important role in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Data and Methodology

The analysis drew on a distinct collection of forest data dating back to 1971, including records tracking approximately 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but not the changes in soil and roots.

An additional expert highlighted the importance of collecting and maintaining extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is rising. But examining these long term empirical datasets, we discover that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”