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

The Tipping Point Discovered

This significant change, which impacts the stems and limbs of the trees but excludes the underground roots, started around a quarter-century back, as per recent research.

Trees naturally store carbon as they develop and emit it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across northern Australia has shown that this essential carbon sink may be at risk.

Study Insights

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

“It’s the first tropical forest of its kind to display this sign of transformation,” commented the lead author.

“It is understood that the moist tropics 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 experience in other parts of the world.”

Worldwide Consequences

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are needed.

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

“This paper is the first time that this tipping point of a transition 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.

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

But should comparable changes – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” it was noted.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, 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 monitoring roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but excluded the changes below ground.

An additional expert highlighted the value of collecting and maintaining 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 not the case – it allows us to compare models with actual data and better understand how these ecosystems work.”