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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, due to rising heat extremes and drier conditions.

The Tipping Point Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon during growth and release it when they decompose. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this absorption is assumed to increase with rising atmospheric concentrations.

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

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and inadequate regeneration, as the study indicates.

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

“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 future analog 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 globally, and additional studies are needed.

But if so, the findings could have major consequences 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,” remarked an authority on climate science.

On a global scale, the portion 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 many climate models and strategies.

But should comparable changes – from sink to source – were detected in other rainforests, climate projections may underestimate global warming in the coming years. “Which is bad news,” he added.

Ongoing Role

Although the equilibrium between gains and losses had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability 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 drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but excluded the changes in soil and roots.

Another researcher emphasized the importance of collecting and maintaining long term data.

“It was believed the forest would be able to store more carbon because [CO2] is rising. But looking at these decades of recorded information, we find that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”