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Australian tropical rainforest trees have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, driven by increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This crucial shift, which impacts the trunks and branches of the trees but excludes the underground roots, began approximately 25 years ago, as per new studies.

Forests typically absorb carbon during growth and release it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to grow with higher CO2 levels.

However, nearly 50 years of data collected from tropical forests across northern Australia has shown that this essential carbon sink may be at risk.

Study Insights

Roughly 25 years ago, tree stems and limbs 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 display this sign of change,” 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 coming example for what tropical forests will experience in other parts of the world.”

Worldwide Consequences

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

But should that be the case, the results could have significant implications for global climate models, carbon budgets, and environmental regulations.

“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 identified clearly – not merely temporarily, but for two decades,” stated an expert in climate change 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 assumed to continue under many climate models and policies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate projections may understate heating trends in the future. “Which is bad news,” it was noted.

Ongoing Role

Although the balance between growth and decline had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Data and Methodology

The analysis drew on a unique set of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but excluded the changes in soil and roots.

Another researcher emphasized the value 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 long term empirical datasets, we discover that is incorrect – it allows us to compare models with actual data and improve comprehension of how these systems work.”