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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, due to rising heat extremes and arid environments.

Critical Change Discovered

This crucial shift, which impacts the stems and limbs of the trees but excludes the root systems, began approximately 25 years ago, according to recent research.

Forests typically absorb carbon as they develop and release it upon decay and death. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this absorption is expected to increase with rising atmospheric concentrations.

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

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, 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.

“We know that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will encounter in global regions.”

Worldwide Consequences

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

But if so, the results could have significant implications for international climate projections, carbon budgets, 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 identified clearly – not merely temporarily, but for two decades,” remarked an authority on climate science.

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

But if similar shifts – from sink to source – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “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 absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Data and Methodology

The analysis utilized a unique set of forest data starting from 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

An additional expert highlighted the value of collecting and maintaining extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is rising. But looking at these long term empirical datasets, we discover that is not the case – it enables researchers to confront the theory with reality and better understand how these systems work.”