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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and drier conditions.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but does not include the root systems, started around 25 years ago, according to new studies.

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

However, close to five decades of data gathered 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 turned into a carbon source, with more trees dying and insufficient new growth, according to the research.

“It’s the first tropical forest of its kind to show this symptom of change,” commented the principal researcher.

“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 could act as a future analog for what tropical forests will encounter in other parts of the world.”

Global Implications

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

But should that be the case, the findings could have significant implications for international climate projections, 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 definitively spotted – not merely temporarily, but for two decades,” stated an authority on climate science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under many climate models and policies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate forecasts may underestimate global warming in the future. “This is concerning,” it was noted.

Ongoing Role

Even though the balance between growth and decline had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity 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 starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but excluded the changes below ground.

Another researcher highlighted the value of gathering and preserving extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But looking at these long term empirical datasets, we find that is not the case – it allows us to confront the theory with reality and better understand how these systems work.”