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Australian tropical rainforest trees have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, due to rising heat extremes and drier conditions.

The Tipping Point Identified

This significant change, which affects the stems and limbs of the trees but excludes the root systems, began approximately a quarter-century back, as per recent research.

Trees naturally store carbon as they develop and release it when they decompose. Overall, tropical forests are regarded as 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 gathered from tropical forests across Queensland has revealed that this essential 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 more trees dying and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to display this sign of transformation,” commented the lead author.

“It is understood that the humid tropical regions in Australia exist in a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will experience in global regions.”

Global Implications

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

But should that be the case, the findings could have major consequences for global climate models, carbon budgets, and environmental regulations.

“This research is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for 20 years,” stated an expert in climate change science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under many climate models and strategies.

But should comparable changes – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” he added.

Ongoing Role

Although the equilibrium between growth and decline 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 “a lot harder”, and require an accelerated shift from carbon-based energy.

Research Approach

This study utilized a unique set of forest data starting from 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the changes below ground.

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

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these long term empirical datasets, we find that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”