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Australian tropical rainforest trees have become the first worldwide by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and arid environments.

The Tipping Point Discovered

This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon as they develop and release it upon decay and death. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this absorption is expected to grow with higher CO2 levels.

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and inadequate regeneration, according to the research.

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

“It is understood that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in other parts of the world.”

Global Implications

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

But if so, the findings could have significant implications for global climate models, CO2 accounting, and environmental regulations.

“This research is the first time that this critical threshold of a switch 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 expert in climate change 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 if similar shifts – from sink to source – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Continued Function

Although the equilibrium between growth and decline had shifted, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Research Approach

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

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

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we find that is not the case – it allows us to compare models with actual data and better understand how these systems work.”