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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and arid environments.

Critical Change Discovered

This significant change, which affects the trunks and branches of the trees but excludes the underground roots, began approximately 25 years ago, according to recent research.

Trees naturally store carbon during growth and emit it when they decompose. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is assumed to increase with rising atmospheric concentrations.

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

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to display this sign of transformation,” stated the principal researcher.

“We know that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will encounter in global regions.”

Global Implications

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are required.

But should that be the case, the findings could have significant implications for international climate projections, carbon budgets, and climate policies.

“This paper is the initial instance that this critical threshold of a transition 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 authority on climate science.

On a global scale, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the past few decades, 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 underestimate global warming in the coming years. “This is concerning,” it was noted.

Continued Function

Although the equilibrium between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

This study drew on a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but not the changes below ground.

Another researcher emphasized the value of collecting and maintaining extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these long term empirical datasets, we discover that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these systems work.”