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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and arid environments.

Critical Change Discovered

This significant change, which affects the trunks and branches of the trees but excludes 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 release – and this absorption is assumed to grow with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across northern Australia has shown that this essential carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, according to the research.

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

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

Global Implications

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are required.

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

“This research is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for two decades,” 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 last 20 to 30 years, which was assumed to continue under many climate models and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate projections may underestimate global warming in the coming years. “This is concerning,” he added.

Ongoing Role

Even though the balance between gains and losses had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid transition away from fossil fuels.

Research Approach

This study drew on a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the changes below ground.

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

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we find that is not the case – it enables researchers to confront the theory with reality and better understand how these systems work.”