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Trees in Australia's tropical rainforests have achieved a global first by shifting from serving as a CO2 absorber to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

The Tipping Point Discovered

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

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

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

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to show this symptom of transformation,” commented the lead author.

“We know that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are needed.

But if so, the results could have major consequences for international climate projections, CO2 accounting, and environmental regulations.

“This research is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for two decades,” stated an authority on climate science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and policies.

But should comparable changes – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” it was noted.

Continued Function

Even though the equilibrium between gains and losses had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Data and Methodology

This study utilized a unique set of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the changes below ground.

An additional expert emphasized the value of gathering and preserving extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we discover that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these systems work.”