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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, due to increasingly extreme temperatures and arid environments.

The Tipping Point Identified

This crucial shift, which impacts the trunks and branches of the trees but excludes the root systems, started around 25 years ago, as per new studies.

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

However, close to five decades of data collected from tropical forests across northern Australia has shown that this vital carbon sink could be under threat.

Research Findings

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of change,” commented the principal researcher.

“It is understood that the moist tropics 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 global regions.”

Worldwide Consequences

A study contributor mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are required.

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

“This paper 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,” remarked an authority on climate science.

Worldwide, the portion of carbon dioxide taken in 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 if similar shifts – from absorber to emitter – were observed in other rainforests, climate forecasts may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Continued Function

Although the equilibrium between growth and decline had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and require an even more rapid shift from carbon-based energy.

Data and Methodology

The analysis utilized a unique set of forest data dating back to 1971, including records tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but not the gains and losses in soil and roots.

An additional expert highlighted the importance of collecting and maintaining long term data.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we find that is not the case – it allows us to compare models with actual data and better understand how these systems work.”