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Australian tropical rainforest trees have become the first worldwide by transitioning from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

The Tipping Point Discovered

This crucial shift, which impacts the trunks and branches of the trees but does not include the underground roots, started around a quarter-century back, as per new studies.

Forests typically absorb carbon during growth and emit it upon decay and death. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this absorption is assumed to increase with higher CO2 levels.

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

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and inadequate regeneration, as the study indicates.

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

“It is understood that the moist tropics 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 encounter in other parts of the world.”

Worldwide Consequences

One co-author 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 should that be the case, the findings could have significant implications for international climate projections, CO2 accounting, and environmental regulations.

“This paper is the first time that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for two decades,” stated an expert in climate change science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

But if similar shifts – from sink to source – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Although the balance between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Research Approach

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 focused on the carbon stored in trunks and branches, but not the gains and losses below ground.

An additional expert 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 long term empirical datasets, we discover that is incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”