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Trees in Australia's tropical rainforests have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and arid environments.

Critical Change Discovered

This significant change, which affects the stems and limbs of the trees but does not include the root systems, started around 25 years ago, according to new studies.

Trees naturally store carbon during growth and emit it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to increase with higher CO2 levels.

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and insufficient new growth, according to the research.

“This marks the initial rainforest of its kind to show this symptom of transformation,” stated the lead author.

“It is understood that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are needed.

But should that be the case, the results could have significant implications for international climate projections, carbon budgets, and environmental regulations.

“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 portion of carbon dioxide taken in by forests, trees, and plants has been relatively constant 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 detected in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Continued Function

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

Research Approach

The analysis utilized a unique set of forest data starting from 1971, including records tracking approximately 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 highlighted the importance of collecting and maintaining long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these decades of recorded information, we find that is not the case – it enables researchers to confront the theory with reality and better understand how these ecosystems work.”