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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, driven by 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 underground roots, started around a quarter-century back, according to recent research.

Trees naturally store carbon during growth and release it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is expected to increase with higher CO2 levels.

However, nearly 50 years of data gathered from tropical forests across northern Australia has revealed that this essential carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, 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 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 mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are required.

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 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 merely temporarily, but for two decades,” stated an expert in climate change science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under many climate models and policies.

But if similar shifts – from sink to source – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Continued Function

Although the equilibrium between growth and decline had shifted, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Research Approach

This study utilized a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the changes in soil and roots.

An additional expert emphasized the value 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 find that is incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”