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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and arid environments.

Critical Change Identified

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

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 emit – and this absorption is expected to grow with higher CO2 levels.

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

Research Findings

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to show this symptom of change,” commented 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 encounter in global regions.”

Global Implications

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

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

“This research is the initial instance 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 expert in climate change science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under many climate models and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” he added.

Ongoing Role

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

Data and Methodology

This study drew on a unique set of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the changes in soil and roots.

Another researcher highlighted the importance of gathering and preserving extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these decades of recorded information, we find that is incorrect – it enables researchers to confront the theory with reality and improve comprehension of how these systems work.”