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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 increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This significant change, which impacts the trunks and branches of the trees but excludes the underground roots, began approximately 25 years ago, 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 uptake is assumed to increase with higher CO2 levels.

However, nearly 50 years of data collected from tropical forests across northern Australia has revealed that this vital carbon sink could be under threat.

Research Findings

Approximately 25 years ago, tree stems and limbs 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 change,” commented the principal researcher.

“It is understood that the humid tropical regions in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example 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 further research are required.

But should that be the case, the findings could have significant implications for global climate models, CO2 accounting, and climate policies.

“This paper is the first time that this critical threshold 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,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide absorbed 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 absorber to emitter – were detected in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” it was noted.

Continued Function

Although the equilibrium between growth and decline had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Data and Methodology

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

Another researcher emphasized the importance of gathering and preserving extended datasets.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we find that is not the case – it allows us to compare models with actual data and better understand how these ecosystems work.”