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

The Tipping Point Discovered

This significant change, which impacts the trunks and branches of the trees but does not include the underground roots, started around a quarter-century back, according to new studies.

Trees naturally store carbon during growth and release it upon decay and death. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to increase with higher CO2 levels.

However, close to five decades of data collected from tropical forests across Queensland has shown that this essential carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of change,” commented the principal researcher.

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Global Implications

A study contributor 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 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 transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, 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 quite stable over the last 20 to 30 years, which was expected to persist 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 future. “This is concerning,” it was noted.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Data and Methodology

The analysis drew on a distinct collection of forest data dating back to 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the changes below ground.

Another researcher highlighted the value of gathering and preserving long term data.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But examining these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”