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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Discovered

This significant change, which impacts the trunks and branches of the trees but excludes the root systems, began approximately 25 years ago, according to recent research.

Trees naturally store carbon as they develop and release it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this absorption is expected to grow with rising atmospheric concentrations.

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

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying 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.

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a future analog for what tropical forests will experience in other parts of the world.”

Global Implications

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 required.

But if so, the findings could have major consequences for global climate models, carbon budgets, 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 identified clearly – not just for one year, but for two decades,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable 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 projections may underestimate global warming in the future. “Which is bad news,” he added.

Continued Function

Although the equilibrium between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon 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 roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

An additional expert emphasized the value of gathering and preserving long term data.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we discover that is not the case – it allows us to confront the theory with reality and better understand how these ecosystems work.”