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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 increasingly extreme temperatures and arid environments.

The Tipping Point Discovered

This crucial shift, which affects the stems and limbs of the trees but excludes the root systems, started around a quarter-century back, according to new studies.

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

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

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and inadequate regeneration, as the study indicates.

“It’s the first tropical forest of its kind to show this symptom of change,” stated the lead author.

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in other parts of the world.”

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and additional studies are required.

But should that be the case, the findings could have significant implications for international climate projections, CO2 accounting, and environmental regulations.

“This paper 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 expert in climate change science.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under many climate models and strategies.

But should comparable changes – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Ongoing Role

Even though the equilibrium between growth and decline had changed, these forests were still playing an important role in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an accelerated shift from carbon-based energy.

Data and Methodology

The analysis utilized a unique set of forest data dating back to 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the gains and losses below ground.

Another researcher highlighted the value of collecting and maintaining extended datasets.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these long term empirical datasets, we find that is incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”