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Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, driven by rising heat extremes and arid environments.

The Tipping Point Discovered

This crucial shift, which impacts the trunks and branches of the trees but does not include the underground roots, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon during growth and emit it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to grow with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across northern Australia has shown that this vital carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of change,” commented the lead author.

“We know that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will encounter in global regions.”

Worldwide Consequences

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are needed.

But should that be the case, the results could have significant implications for international climate projections, CO2 accounting, and environmental regulations.

“This research is the initial instance that this critical threshold 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,” stated an authority on climate 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 expected to persist under many climate models and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid shift from carbon-based energy.

Research Approach

This study utilized a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but excluded the changes in soil and roots.

An additional expert 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 examining these decades of recorded information, we discover that is not the case – it enables researchers to compare models with actual data and better understand how these ecosystems work.”