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Australian tropical rainforest trees have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

Critical Change Identified

This significant change, which impacts the stems and limbs of the trees but excludes the root systems, began approximately 25 years ago, as per recent research.

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

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

Study Insights

Roughly 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 display this sign of change,” stated the principal researcher.

“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 could act as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and further research are needed.

But if so, the results could have significant implications for international climate projections, carbon budgets, and environmental regulations.

“This research is the initial instance that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for 20 years,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the past few decades, 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 projections may understate heating trends in the coming years. “Which is bad news,” it was noted.

Ongoing Role

Even though the equilibrium between growth and decline had shifted, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid shift from carbon-based energy.

Research Approach

This study drew on a distinct collection of forest data starting from 1971, including records tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

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

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”