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

The Tipping Point Identified

This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, started around a quarter-century back, according to new studies.

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

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

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and insufficient new growth, as the study indicates.

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

“It is understood that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a future analog for what tropical forests will encounter in global regions.”

Global Implications

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

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

“This paper is the initial instance that this tipping point of a transition 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.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under many climate models and policies.

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

Ongoing Role

Although the equilibrium between gains and losses had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

The analysis drew on a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the changes in soil and roots.

An additional expert 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 looking at 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.”