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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

Critical Change Identified

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, began approximately a quarter-century back, as per recent research.

Trees naturally store carbon as they develop and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is expected to grow with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across northern Australia has revealed that this vital carbon sink may be at risk.

Research Findings

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 lead author.

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

A study contributor mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are required.

But if so, the results could have major consequences for global climate models, CO2 accounting, and environmental regulations.

“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 definitively spotted – not just for one year, but for two decades,” remarked an authority on climate science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and policies.

But if similar shifts – from sink to source – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “Which is bad news,” he added.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and require an even more rapid shift from carbon-based energy.

Research Approach

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 above ground, but not the gains and losses below ground.

Another researcher emphasized the value of collecting and maintaining long term data.

“It was believed the forest would be able to store more carbon because [CO2] is rising. But examining these decades of recorded information, we find that is not the case – it enables researchers to compare models with actual data and better understand how these systems work.”