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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

Critical Change Discovered

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

Trees naturally store carbon as they develop and release it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to increase with rising atmospheric concentrations.

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.

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

“It is understood that the moist tropics 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 encounter in global regions.”

Global Implications

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

But should that be the case, the results could have major consequences for global climate models, CO2 accounting, and climate policies.

“This paper is the first time that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” stated an expert in climate change science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may underestimate global warming in the future. “This is concerning,” it was noted.

Continued Function

Although the balance between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated shift from carbon-based energy.

Research Approach

This study drew on a distinct collection of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots.

An additional expert highlighted the value of gathering and preserving extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is rising. But examining these long term empirical datasets, we find that is not the case – it allows us to compare models with actual data and better understand how these systems work.”