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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and arid environments.

Critical Change Identified

This crucial shift, 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 when they decompose. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this absorption is expected 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 could be under threat.

Research Findings

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

“This marks the initial rainforest of its kind to show this symptom of transformation,” commented the principal researcher.

“It is understood that the moist tropics in Australia exist in 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 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 should that be the case, the results could have significant implications for international climate projections, carbon budgets, and climate policies.

“This research is the first time that this tipping point 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.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate forecasts may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Ongoing Role

Although the equilibrium between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity 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 unique set of forest data dating back to 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered 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 absorb additional CO2 because [CO2] is rising. But looking at these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”