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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but excludes the root systems, began approximately 25 years ago, according to new studies.

Trees naturally store carbon as they develop and release it when they decompose. Generally, tropical forests are considered 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 revealed 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 inadequate regeneration, as the study indicates.

“This marks the initial rainforest of its kind to display this sign of change,” stated the principal researcher.

“We know that the moist tropics in Australia exist in 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 experience in global regions.”

Worldwide Consequences

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

But if so, the results could have major consequences for global climate models, CO2 accounting, and environmental regulations.

“This paper is the initial instance that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for two decades,” remarked an authority on climate 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 assumed to continue under numerous projections and policies.

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

Continued Function

Although the equilibrium between growth and decline 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.

Data and Methodology

This study utilized a distinct collection of forest data dating back to 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but excluded the changes below ground.

Another researcher highlighted the value of collecting and maintaining long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these decades of recorded information, we discover that is not the case – it enables researchers to confront the theory with reality and better understand how these systems work.”