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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, driven by rising heat extremes and arid environments.

Critical Change Identified

This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, started around 25 years ago, as per recent research.

Forests typically absorb carbon during growth 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 expected to grow with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across northern Australia has revealed that this vital carbon sink may be at risk.

Research Findings

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

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

“We know that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a future analog for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are needed.

But if so, the findings could have major consequences for global climate models, carbon budgets, and climate policies.

“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 definitively spotted – not just for one year, but for two decades,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was assumed to continue under many climate models 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,” 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 take in additional CO2 would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Research Approach

This study utilized a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the changes in soil and roots.

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

“We thought 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 confront the theory with reality and better understand how these ecosystems work.”