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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and arid environments.

The Tipping Point Discovered

This crucial shift, which impacts the trunks and branches of the trees but does not include the underground roots, began approximately a quarter-century back, according to new studies.

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

However, close to five decades of data collected from tropical forests across northern Australia has revealed that this essential carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

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

“We know that the moist tropics in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a coming example for what tropical forests will experience in other parts of the world.”

Global Implications

One co-author mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are required.

But if so, the results could have significant implications for global climate models, CO2 accounting, and climate policies.

“This paper is the initial instance that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for 20 years,” stated an authority on climate science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under many climate models and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” it was noted.

Ongoing Role

Even though the balance between growth and decline had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

This study utilized a unique set of forest data starting from 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the gains and losses 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 increasing. But looking at these decades of recorded information, we discover that is not the case – it enables researchers to compare models with actual data and better understand how these ecosystems work.”