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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and arid environments.

The Tipping Point Discovered

This significant change, which affects the trunks and branches of the trees but excludes the root systems, started around a quarter-century back, as per recent research.

Trees naturally store carbon during growth and release it when they decompose. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this uptake is assumed to increase with higher CO2 levels.

However, nearly 50 years of data collected from tropical forests across Queensland has revealed that this vital carbon sink may be at risk.

Research Findings

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, according to the research.

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

“We know that the humid tropical regions in Australia occupy 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 encounter in global regions.”

Worldwide Consequences

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are required.

But if so, the findings could have significant implications for global climate models, carbon budgets, and climate policies.

“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.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

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

Ongoing Role

Although the equilibrium between gains and losses had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and require an even more rapid shift from carbon-based energy.

Research Approach

This study utilized a distinct collection of forest data dating back to 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but not the changes below ground.

Another researcher highlighted the importance of gathering and preserving long term data.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But examining 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.”