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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, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Discovered

This crucial shift, which affects the stems and limbs of the trees but excludes the root systems, began approximately a quarter-century back, as per recent research.

Forests typically absorb carbon as they develop and release it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to increase with higher CO2 levels.

However, nearly 50 years 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 stems and limbs in these forests became a net emitter, with more trees dying and inadequate regeneration, according to the research.

“This marks the initial rainforest of its kind to show this symptom of transformation,” commented the lead author.

“It is understood that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will encounter in other parts of the world.”

Global Implications

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are required.

But if so, the results could have significant implications for global climate models, carbon budgets, and climate policies.

“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.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under numerous projections and policies.

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

Continued Function

Although the balance between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and require an accelerated shift from carbon-based energy.

Data and Methodology

This study utilized a distinct collection of forest data starting from 1971, including records tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the changes below ground.

An additional expert emphasized the value of gathering and preserving long term data.

“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 incorrect – it enables researchers to compare models with actual data and better understand how these ecosystems work.”