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Trees in Australia's tropical rainforests have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and arid environments.

The Tipping Point Identified

This crucial shift, which affects the stems and limbs of the trees but does not include the underground roots, began approximately 25 years ago, as per recent research.

Trees naturally store carbon during growth and emit it upon decay and death. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to increase with rising atmospheric concentrations.

However, nearly 50 years of data collected from tropical forests across northern Australia has shown that this vital carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and insufficient new growth, according to the research.

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

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a coming example for what tropical forests will encounter 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 should that be the case, the findings could have major consequences for international climate projections, CO2 accounting, and climate policies.

“This research is the first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for two decades,” remarked 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 last 20 to 30 years, which was expected to persist 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 coming years. “This is concerning,” he added.

Ongoing Role

Although the equilibrium between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

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 excluded the changes in soil and roots.

Another researcher emphasized the importance of gathering and preserving long term data.

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