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Australian tropical rainforest trees have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to increasingly extreme temperatures and arid environments.

Critical Change Identified

This crucial shift, which affects the trunks and branches of the trees but does not include the underground roots, started around a quarter-century back, according to new studies.

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

However, nearly 50 years of data collected from tropical forests across Queensland has revealed that this vital carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, according to the research.

“This marks the initial rainforest of its kind to show this symptom of transformation,” stated the lead author.

“It is understood that the moist tropics 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 experience in other parts of the world.”

Worldwide Consequences

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are needed.

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

“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 identified clearly – not just for one year, but for two decades,” remarked an expert in climate change science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under many climate models and policies.

But if similar shifts – from sink to source – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “Which is bad news,” he added.

Ongoing Role

Although the equilibrium between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an even more rapid transition away from fossil fuels.

Research Approach

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

Another researcher emphasized the value of collecting and maintaining extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these long term empirical datasets, we discover that is incorrect – it allows us to confront the theory with reality and better understand how these systems work.”