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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, due to rising heat extremes and drier conditions.

Critical Change Identified

This crucial shift, which affects the trunks and branches of the trees but excludes the underground roots, began approximately a quarter-century back, as per new studies.

Trees naturally store carbon as they develop and emit it when they decompose. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is assumed to grow with rising atmospheric concentrations.

However, nearly 50 years of data collected from tropical forests across northern Australia has shown 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,” stated the lead author.

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

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

But should that be the case, the findings could have significant implications for global climate models, carbon budgets, and environmental regulations.

“This paper is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for two decades,” stated an authority on climate 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 many climate models and strategies.

But if similar shifts – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the future. “Which is bad news,” it was noted.

Ongoing Role

Although the balance between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the changes in soil and roots.

An additional expert emphasized the value of collecting and maintaining extended datasets.

“It was believed 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 confront the theory with reality and improve comprehension of how these systems work.”