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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

Critical Change Discovered

This significant change, which affects the trunks and branches of the trees but does not include the underground roots, began approximately a quarter-century back, according to recent research.

Forests typically absorb carbon during growth and release it when they decompose. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is expected to increase with rising atmospheric concentrations.

However, close to five decades of data gathered 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 became a net emitter, with increased tree mortality and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to display this sign of change,” stated the principal researcher.

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

Global Implications

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and additional studies are required.

But if so, the findings could have significant implications for international climate projections, CO2 accounting, 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 20 years,” stated an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under numerous projections and strategies.

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

Ongoing Role

Although the balance 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 require an accelerated shift from carbon-based energy.

Research Approach

This study drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes below ground.

Another researcher highlighted the importance 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 decades of recorded information, we discover that is not the case – it allows us to compare models with actual data and improve comprehension of how these systems work.”