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Australian tropical rainforest trees have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and arid environments.

The Tipping Point Identified

This crucial shift, which affects the stems and limbs of the trees but excludes the underground roots, started around 25 years ago, as per new studies.

Trees naturally store carbon during growth and emit it upon decay and death. 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 collected from tropical forests across Queensland has revealed that this essential carbon sink may be at risk.

Study Insights

Roughly 25 years ago, tree stems and limbs 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 principal researcher.

“We know that the moist tropics in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in other parts of the world.”

Global Implications

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are required.

But if so, the results could have significant implications for global climate models, carbon budgets, and environmental regulations.

“This paper is the first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, 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 observed in other rainforests, climate forecasts may underestimate global warming in the coming years. “Which is bad news,” he added.

Continued Function

Even though the balance between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an accelerated transition away from fossil fuels.

Data and Methodology

This study utilized a distinct collection of forest data dating back to 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but excluded the gains and losses in soil and roots.

An additional expert highlighted the importance 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 long term empirical datasets, we discover that is not the case – it enables researchers to confront the theory with reality and improve comprehension of how these systems work.”