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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and arid environments.

The Tipping Point Discovered

This significant change, which affects the trunks and branches of the trees but excludes the underground roots, began approximately 25 years ago, as per new studies.

Trees naturally store carbon as they develop and emit it upon decay and death. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to increase with rising atmospheric concentrations.

However, nearly 50 years of data collected from tropical forests across northern Australia has shown that this vital 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 insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to show this symptom of transformation,” stated the principal researcher.

“We know that the humid tropical regions in Australia occupy 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 experience in global regions.”

Global Implications

A study contributor noted 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 results could have major consequences for international climate projections, CO2 accounting, and climate policies.

“This paper is the first time that this critical threshold 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,” remarked an expert in climate change science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, 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. “Which is bad news,” he added.

Ongoing Role

Although 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 even more rapid shift from carbon-based energy.

Research Approach

The analysis drew on a distinct collection of forest data dating back to 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but not the gains and losses in soil and roots.

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

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and improve comprehension of how these systems work.”