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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and arid environments.

The Tipping Point Identified

This crucial shift, which impacts the trunks and branches of the trees but excludes the root systems, started around 25 years ago, according to new studies.

Forests typically absorb carbon as they develop and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is assumed to increase with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across Queensland has revealed that this vital carbon sink could be under threat.

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and inadequate regeneration, according to the research.

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

“It is understood that the humid tropical regions in Australia occupy a slightly warmer, drier climate 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.”

Global Implications

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are required.

But if so, the findings could have significant implications 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 identified clearly – not merely temporarily, but for two decades,” stated an authority on climate science.

On a global scale, the share of carbon dioxide taken in 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 sink to source – were observed in other rainforests, climate forecasts may underestimate global warming in the future. “Which is bad news,” it was noted.

Continued Function

Even though the balance between growth and decline 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 “a lot harder”, and require an even more rapid shift from carbon-based energy.

Research Approach

The analysis utilized a distinct collection of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the changes 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 rising. But looking at these long term empirical datasets, we discover that is incorrect – it allows us to confront the theory with reality and better understand how these ecosystems work.”