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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to turning into a carbon emitter, driven by rising heat extremes and arid environments.

The Tipping Point Identified

This crucial shift, which affects the stems and limbs of the trees but excludes the underground roots, began approximately 25 years ago, according to new studies.

Forests typically absorb carbon during growth and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this absorption is expected to grow with rising atmospheric concentrations.

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

Research Findings

Roughly 25 years ago, tree stems and limbs 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 display this sign of change,” 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 future analog for what tropical forests will experience in other parts of the world.”

Global Implications

One co-author noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and additional studies are required.

But if so, the findings could have significant implications for international climate projections, CO2 accounting, and climate policies.

“This research is the first time 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 authority on climate science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under numerous projections and policies.

But if similar shifts – from sink to source – were observed in other rainforests, climate forecasts may underestimate global warming in the future. “This is concerning,” it was noted.

Continued Function

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

Research Approach

The analysis drew on a distinct collection of forest data starting from 1971, including records tracking approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but not the gains and losses below ground.

An additional expert highlighted the importance of gathering and preserving long term data.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we discover that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”