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Australian tropical rainforest trees have become the first worldwide by shifting from acting as a carbon sink to turning into a carbon emitter, due to rising heat extremes and arid environments.

Critical Change Discovered

This crucial shift, which affects the stems and limbs of the trees but excludes the root systems, started around a quarter-century back, according to recent research.

Trees naturally store carbon during growth and release it upon decay and death. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this uptake is expected to increase with rising atmospheric concentrations.

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

Study Insights

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

“This marks the initial rainforest of its kind to display this sign of change,” commented the principal researcher.

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in global regions.”

Global Implications

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are needed.

But should that be the case, the findings could have significant implications for global climate models, CO2 accounting, and climate policies.

“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.

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 policies.

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

Continued Function

Although the equilibrium between growth and decline had changed, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

This study utilized a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the changes below ground.

An additional expert highlighted the value of gathering and preserving extended datasets.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these decades of recorded information, we find that is incorrect – it enables researchers to compare models with actual data and improve comprehension of how these systems work.”