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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, started around 25 years ago, according to recent research.

Forests typically absorb carbon as they develop and emit it upon decay and death. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to grow with higher CO2 levels.

However, nearly 50 years of data collected from tropical forests across northern Australia has revealed that this essential carbon sink could be under threat.

Study Insights

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

“This marks the initial rainforest of its kind to display this sign of transformation,” stated the principal researcher.

“It is understood that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will experience in global regions.”

Global Implications

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 significant implications for global climate models, carbon budgets, and environmental regulations.

“This research is the first time that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for 20 years,” stated an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” he added.

Continued Function

Although the balance between growth and decline had changed, these forests were still playing an important role in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

The analysis utilized a distinct collection of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the changes below ground.

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

“We thought 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.”