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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, driven by increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the underground roots, started around a quarter-century back, as per new studies.

Forests typically absorb carbon during growth and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to increase with higher CO2 levels.

However, close to five decades of data collected from tropical forests across northern Australia 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 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 principal researcher.

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will experience in other parts of the world.”

Global Implications

One co-author noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

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

“This research 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 20 years,” stated an expert in climate change science.

On a global scale, 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 policies.

But should comparable changes – from sink to source – were observed in other rainforests, climate forecasts may underestimate global warming in the future. “This is concerning,” he added.

Continued Function

Although the equilibrium between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid shift from carbon-based energy.

Data and Methodology

The analysis utilized a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the gains and losses below ground.

An additional expert emphasized the importance of gathering and preserving extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is rising. But examining these decades of recorded information, we find that is incorrect – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”