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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and drier conditions.

Critical Change Identified

This significant change, which impacts the stems and limbs of the trees but excludes the root systems, began approximately a quarter-century back, according to new studies.

Trees naturally store carbon as they develop and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is expected to increase with higher CO2 levels.

However, nearly 50 years of data gathered from tropical forests across northern Australia has revealed that this vital 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 show this symptom of transformation,” commented the lead author.

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will experience in global regions.”

Global Implications

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

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

“This research is the initial instance that this tipping point 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,” remarked 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 assumed to continue under numerous projections and strategies.

But should comparable changes – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” he added.

Ongoing Role

Although the equilibrium between growth and decline had shifted, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an accelerated transition away from fossil fuels.

Data and Methodology

This study utilized a distinct collection of forest data starting from 1971, including records tracking 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 importance of collecting and maintaining long term data.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But looking at these decades of recorded information, we discover that is incorrect – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”