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Trees in Australia's tropical rainforests have achieved a global first by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, started around a quarter-century back, as per recent research.

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

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

Research Findings

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, 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 transformation,” commented the principal researcher.

“We know that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

One co-author noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are needed.

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

“This paper is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for two decades,” remarked an expert in climate change science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under many climate models and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” he added.

Ongoing Role

Even though the equilibrium between gains and losses had changed, these forests were still playing an important role in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an accelerated shift from carbon-based energy.

Research Approach

The analysis utilized a distinct collection of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the changes in soil and roots.

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

“We thought the forest would be able to store more carbon because [CO2] is rising. But looking at these long term empirical datasets, we find that is not the case – it allows us to compare models with actual data and improve comprehension of how these systems work.”