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Trees in Australia's tropical rainforests have achieved a global first by transitioning from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and drier conditions.

The Tipping Point Discovered

This crucial shift, which affects the trunks and branches of the trees but does not include the underground roots, started around 25 years ago, according to recent research.

Trees naturally store carbon as they develop and emit it upon decay and death. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this absorption is expected to grow with rising atmospheric concentrations.

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

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and inadequate regeneration, as the study indicates.

“It’s the first tropical forest of its kind to show this symptom of change,” stated the lead author.

“It is understood that the humid tropical regions in Australia occupy a somewhat hotter, arid environment 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

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

But if so, the results could have major consequences for international climate projections, CO2 accounting, and environmental regulations.

“This research is the initial instance that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” stated an expert in climate change science.

Worldwide, the portion of carbon dioxide taken in 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 forecasts may underestimate global warming in the coming years. “This is concerning,” it was noted.

Ongoing Role

Although the balance between gains and losses had changed, 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 require an accelerated shift from carbon-based energy.

Research Approach

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 excluded the gains and losses below ground.

An additional expert emphasized the value of gathering and preserving extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is rising. But looking at these long term empirical datasets, we discover that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”