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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 increasingly extreme temperatures and drier conditions.

The Tipping Point Discovered

This crucial shift, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately 25 years ago, as per new studies.

Forests typically absorb carbon during growth and release it upon decay and death. Overall, tropical forests are regarded as 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 could be under threat.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and insufficient new growth, according to the research.

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

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a coming example for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

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

But if so, the results could have major consequences for international climate projections, CO2 accounting, and climate policies.

“This research is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for 20 years,” stated an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under many climate models and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate forecasts may understate heating trends in the future. “Which is bad news,” he added.

Continued Function

Even though the equilibrium between growth and decline had changed, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and require an even more rapid shift from carbon-based energy.

Research Approach

This study utilized a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the changes in soil and roots.

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

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we find that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”