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Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

Critical Change Identified

This crucial shift, which impacts the trunks and branches of the trees but does not include the root systems, started around a quarter-century back, as per new studies.

Trees naturally store carbon during growth and emit it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this absorption is assumed to grow with higher CO2 levels.

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

Research Findings

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, according to the research.

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

“We know that the moist tropics in Australia exist in a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

One co-author mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are needed.

But should that be the case, the results could have significant implications for global climate models, CO2 accounting, and environmental regulations.

“This research 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 20 years,” remarked an expert in climate change science.

On a global scale, 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 policies.

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

Continued Function

Even though the balance between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Data and Methodology

This study drew on a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but excluded the gains and losses below ground.

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

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we discover that is not the case – it allows us to compare models with actual data and better understand how these systems work.”