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Trees in Australia's tropical rainforests 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 drier conditions.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but does not include the underground roots, started around a quarter-century back, as per 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 expected to increase with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, 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 transformation,” commented the lead author.

“We know that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, 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 is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and further research are needed.

But if so, the findings could have major consequences for global climate models, CO2 accounting, and environmental regulations.

“This research is the initial instance that this tipping point 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 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 past few decades, which was expected to persist under numerous projections and strategies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Although the equilibrium between growth and decline had changed, these forests were still playing an important role 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.

Data and Methodology

The analysis drew on a distinct collection of forest data dating back to 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the changes below ground.

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

“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 find that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”