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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, due to increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This crucial shift, which affects the trunks and branches of the trees but excludes the underground roots, began approximately a quarter-century back, as per new studies.

Forests typically absorb carbon as they develop and emit it when they decompose. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this absorption is expected to increase with rising atmospheric concentrations.

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

Study Insights

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

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

“It is understood that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will experience in other parts of the world.”

Worldwide Consequences

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and additional studies are required.

But should that be the case, the results could have major consequences for international climate projections, CO2 accounting, and environmental regulations.

“This paper is the initial instance 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 authority on climate science.

On a global scale, the share of carbon dioxide absorbed 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 absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the future. “Which is bad news,” he added.

Continued Function

Even though the balance between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Research Approach

The analysis drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

Another researcher emphasized the importance of collecting and maintaining long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we discover that is incorrect – it enables researchers to compare models with actual data and better understand how these ecosystems work.”