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Trees in Australia's tropical rainforests have achieved a global first by transitioning from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

The Tipping Point Discovered

This crucial shift, which impacts the stems and limbs of the trees but excludes the underground roots, started around a quarter-century back, according to new studies.

Trees naturally store carbon during growth and emit it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this absorption is expected to grow with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across Queensland has shown that this vital carbon sink could be under threat.

Research Findings

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and inadequate regeneration, as the study indicates.

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

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Worldwide Consequences

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 required.

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

“This research 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 20 years,” remarked an authority on climate science.

Worldwide, the share 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 absorber to emitter – were observed in other rainforests, climate projections may understate heating trends 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 serving a vital function 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.

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.

An additional expert emphasized the importance 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 find that is not the case – it allows us to confront the theory with reality and better understand how these systems work.”