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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 rising heat extremes and arid environments.

The Tipping Point Identified

This crucial shift, which affects the trunks and branches of the trees but excludes the underground roots, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon during growth and emit it upon decay and death. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this absorption is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across northern Australia has revealed that this vital carbon sink could be under threat.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and insufficient new growth, as the study indicates.

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

“We know that the humid tropical regions in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, 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 is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and additional studies are needed.

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 switch 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 expert in climate change science.

Worldwide, the share of carbon dioxide absorbed 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 detected in other rainforests, climate projections may underestimate global warming in the future. “This is concerning,” it was noted.

Continued Function

Although the equilibrium between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an accelerated shift from carbon-based energy.

Research Approach

This study utilized a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the gains and losses below ground.

Another researcher highlighted the value of collecting and maintaining extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is increasing. 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.”