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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 increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately 25 years ago, according to recent research.

Trees naturally store carbon during growth and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is assumed to grow with rising atmospheric concentrations.

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

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to show this symptom of change,” commented the lead author.

“It is understood 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 coming example for what tropical forests will encounter in global regions.”

Global Implications

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

But if so, the findings could have major consequences for international climate projections, carbon budgets, and environmental regulations.

“This research is the initial instance that this critical threshold of a transition 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.

On a global scale, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “Which is bad news,” it was noted.

Continued Function

Even though the balance between gains and losses had changed, these forests were still playing an important role in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate 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 monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots.

An additional expert emphasized 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 find that is incorrect – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”