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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which affects the stems and limbs of the trees but excludes the underground roots, began approximately 25 years ago, according to new studies.

Forests typically absorb carbon during growth and emit it upon decay and death. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this absorption is assumed to grow with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland has shown that this essential carbon sink could be under threat.

Research Findings

Roughly 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 transformation,” commented the principal researcher.

“It is understood that the moist tropics 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.”

Global Implications

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

But if so, the findings could have major consequences 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 two decades,” stated an authority on climate science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist 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,” he added.

Continued Function

Even though the equilibrium between growth and decline had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated transition away from fossil fuels.

Research Approach

The analysis utilized a unique set of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the changes in soil and roots.

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

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