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

The Tipping Point Identified

This significant change, which impacts the trunks and branches of the trees but excludes the root systems, began approximately 25 years ago, as per new studies.

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

However, nearly 50 years of data gathered from tropical forests across northern Australia has revealed that this vital carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to show this symptom of change,” 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 might serve as a future analog for what tropical forests will encounter in other parts of the world.”

Global Implications

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

But if so, the results could have significant implications for global climate models, CO2 accounting, and climate policies.

“This paper is the initial instance 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 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 sink to source – were observed in other rainforests, climate forecasts may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Continued Function

Even though the balance between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but excluded the changes below ground.

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 increasing. But examining these decades of recorded information, we discover that is incorrect – it enables researchers to confront the theory with reality and better understand how these systems work.”