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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and arid environments.

Critical Change Identified

This crucial shift, which impacts the trunks and branches of the trees but excludes the root systems, began approximately 25 years ago, as per new studies.

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

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

Research Findings

Roughly 25 years ago, tree trunks and branches 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 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 future analog for what tropical forests will encounter in global regions.”

Worldwide Consequences

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are required.

But if so, the findings could have significant implications for global climate models, carbon budgets, and climate policies.

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

On a global scale, 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 numerous projections and strategies.

But should comparable changes – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Ongoing Role

Even though the equilibrium between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

This study drew on a unique set of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

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

“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 incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”