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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by rising heat extremes and drier conditions.

The Tipping Point Discovered

This crucial shift, which affects the trunks and branches of the trees but excludes the root systems, started around 25 years ago, as per new studies.

Trees naturally store carbon as they develop and release it upon decay and death. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this uptake is assumed to increase with higher CO2 levels.

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and insufficient new growth, according to the research.

“It’s the first tropical forest 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 other continents, and therefore it might serve as a coming example for what tropical forests will encounter in other parts of the world.”

Global Implications

A study contributor mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies 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 tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, 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 last 20 to 30 years, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate forecasts may underestimate global warming 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 soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Research Approach

The analysis utilized a unique set of forest data starting from 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but not the changes in soil and roots.

An additional expert highlighted the importance of collecting and maintaining long term data.

“We thought the forest would be able to store more carbon 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.”