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

Critical Change Discovered

This crucial shift, which affects the trunks and branches of the trees but does not include the underground roots, started around 25 years ago, according to recent research.

Forests typically absorb carbon during growth and release it when they decompose. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to grow with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree stems and limbs 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 display this sign of change,” stated the lead author.

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will encounter in other parts of the world.”

Global Implications

One co-author mentioned 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 should that be the case, the results could have major consequences for global climate models, carbon budgets, and climate policies.

“This paper is the first time that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for two decades,” remarked an expert in climate change 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 assumed to continue under many climate models and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the future. “This is concerning,” it was noted.

Ongoing Role

Even though the equilibrium between growth and decline had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated shift from carbon-based energy.

Data and Methodology

The analysis drew on a distinct collection of forest data starting from 1971, including records monitoring 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 highlighted the importance of gathering and preserving 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 not the case – it allows us to confront the theory with reality and improve comprehension of how these systems work.”