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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This crucial shift, which affects the trunks and branches of the trees but excludes the underground roots, started around a quarter-century back, according to recent research.

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

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

Study Insights

Roughly 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 transformation,” stated the principal researcher.

“We know 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 future analog for what tropical forests will experience in other parts of the world.”

Global Implications

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and additional studies are needed.

But should that be the case, the findings could have major consequences for international climate projections, CO2 accounting, and climate policies.

“This paper is the initial instance that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for 20 years,” 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 many climate models and strategies.

But should comparable changes – from sink to source – were detected in other rainforests, climate forecasts may understate heating trends in the future. “Which is bad news,” it was noted.

Ongoing Role

Even though the equilibrium between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated transition away from fossil fuels.

Data and Methodology

The analysis drew on a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but not the gains and losses below ground.

Another researcher highlighted the importance of collecting and maintaining long term data.

“It was believed 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 not the case – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”