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Trees in Australia's tropical rainforests have achieved a global first by transitioning from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and arid environments.

The Tipping Point Identified

This significant change, which affects the stems and limbs of the trees but does not include the underground roots, started around 25 years ago, as per recent research.

Trees naturally store carbon as they develop and emit it when they decompose. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to increase with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across northern Australia has shown that this essential carbon sink may be at risk.

Research Findings

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

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

“It is understood that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

Global Implications

A study contributor mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are required.

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 first time that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – 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 quite stable over the last 20 to 30 years, which was assumed to continue under numerous projections and policies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Ongoing Role

Even though the balance between growth and decline 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 “more challenging”, and necessitate an even more rapid shift from carbon-based energy.

Data and Methodology

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

Another researcher emphasized the importance of gathering and preserving extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is rising. 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 systems work.”