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Trees in Australia's tropical rainforests have become the first worldwide by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by rising heat extremes and arid environments.

The Tipping Point Identified

This significant change, which affects the stems and limbs of the trees but excludes the underground roots, started around 25 years ago, as per new studies.

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

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

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to show this symptom of change,” commented the principal researcher.

“We know that the humid tropical regions in Australia exist in 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.”

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 further research are needed.

But should that be the case, the results could have major consequences for global climate models, carbon budgets, and climate policies.

“This paper is the first time that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for 20 years,” stated an expert in climate change science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and policies.

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

Ongoing Role

Even though the balance between gains and losses had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Research Approach

The analysis utilized a distinct collection of forest data dating back to 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but not the gains and losses below ground.

Another researcher emphasized the value of collecting and maintaining extended datasets.

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