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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and drier conditions.

Critical Change Identified

This crucial shift, which affects the stems and limbs of the trees but excludes the root systems, began approximately a quarter-century back, according to recent research.

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

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of transformation,” stated the lead author.

“We know that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in other parts of the world.”

Worldwide Consequences

A study contributor mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and additional studies are required.

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

“This paper is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for 20 years,” stated an authority on climate science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate forecasts may understate heating trends in the future. “This is concerning,” he added.

Ongoing Role

Even though the balance between gains and losses had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an accelerated shift from carbon-based energy.

Research Approach

The analysis utilized a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the changes below ground.

An additional expert highlighted the value of gathering and preserving long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these long term empirical datasets, we discover that is not the case – it enables researchers to confront the theory with reality and better understand how these systems work.”