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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

The Tipping Point Discovered

This crucial shift, which affects the trunks and branches of the trees but excludes the underground roots, started around 25 years ago, according to new studies.

Forests typically absorb carbon during growth and emit it upon decay and death. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – 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 shown that this vital carbon sink may be at risk.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and inadequate regeneration, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of transformation,” stated the principal researcher.

“It is understood 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 coming example for what tropical forests will experience in other parts of the world.”

Global Implications

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

But should that be the case, the results could have major consequences for international climate projections, CO2 accounting, and climate policies.

“This paper is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” stated an authority on climate science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Continued Function

Although the balance between gains and losses had changed, these forests were still playing an important role in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the changes below ground.

Another researcher highlighted the value of gathering and preserving extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we discover that is not the case – it enables researchers to compare models with actual data and better understand how these systems work.”