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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This crucial shift, which impacts the trunks and branches of the trees but excludes the root systems, started around a quarter-century back, according to recent research.

Forests typically absorb carbon as they develop and release it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is assumed to increase with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across Queensland has shown that this essential carbon sink may be at risk.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, 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 humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will experience in other parts of the world.”

Worldwide Consequences

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

But if so, the findings could have significant implications for international climate projections, carbon budgets, and climate policies.

“This paper is the first time 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 20 years,” stated an authority on climate science.

On a global scale, the portion 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 strategies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate projections may underestimate global warming 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 absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

The analysis utilized a distinct collection of forest data dating back to 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 in soil and roots.

Another researcher emphasized the importance of gathering and preserving long term data.

“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 incorrect – it enables researchers to confront the theory with reality and improve comprehension of how these ecosystems work.”