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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and drier conditions.

The Tipping Point Identified

This crucial shift, which impacts the stems and limbs of the trees but excludes the underground roots, started around 25 years ago, according to recent research.

Forests typically absorb carbon as they develop and release it when they decompose. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this uptake is expected to grow with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to show this symptom of change,” commented the lead author.

“We know that the humid tropical regions in Australia occupy 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

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

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

“This paper is the initial instance 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 20 years,” remarked an authority on climate science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was assumed to continue under many climate models and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate projections may underestimate global warming in the future. “This is concerning,” it was noted.

Continued Function

Although the balance between gains and losses had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, 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 considered the carbon stored in trunks and branches, but not the gains and losses below ground.

An additional expert highlighted the importance of collecting and maintaining long term data.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But examining these decades of recorded information, we discover that is not the case – it enables researchers to compare models with actual data and better understand how these systems work.”