🔗 Share this article

Australian tropical rainforest trees have become the first worldwide by transitioning from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

The Tipping Point Identified

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, began approximately 25 years ago, according to recent research.

Trees naturally store carbon during growth and emit it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is expected to increase with higher CO2 levels.

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

Research Findings

Approximately 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 display this sign of change,” stated the lead author.

“It is understood that the humid tropical regions in Australia occupy 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 global regions.”

Global Implications

One co-author noted that it is yet unclear whether Australia’s tropical forests are a precursor 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 paper 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 just for one year, but for 20 years,” remarked an authority on climate science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under numerous projections and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and require an accelerated shift from carbon-based energy.

Data and Methodology

This study utilized a distinct collection 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 excluded the gains and losses in soil and roots.

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

“It was believed 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 allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”