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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to turning into a carbon emitter, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Discovered

This crucial shift, which affects the trunks and branches of the trees but excludes the root systems, began approximately 25 years ago, according to recent research.

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

However, nearly 50 years 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 trunks and branches in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, according to the research.

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

“It is understood that the moist tropics 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.”

Worldwide Consequences

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

But if so, the findings could have major consequences for international climate projections, carbon budgets, and environmental regulations.

“This paper is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – 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 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 underestimate global warming in the future. “Which is bad news,” he added.

Continued Function

Even though the equilibrium between growth and decline had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Data and Methodology

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

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

“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 discover that is not the case – it allows us to compare models with actual data and improve comprehension of how these systems work.”