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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and arid environments.

Critical Change Discovered

This crucial shift, which impacts the trunks and branches of the trees but does not include the underground roots, started around 25 years ago, according to new studies.

Trees naturally store carbon as they develop and emit it upon decay and death. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to increase with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, according to the research.

“This marks the initial rainforest of its kind to show this symptom of transformation,” stated 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 experience in global regions.”

Worldwide Consequences

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests globally, and further research are needed.

But should that be the case, the findings could have significant implications for global climate models, CO2 accounting, and climate policies.

“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 merely temporarily, but for two decades,” remarked an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and policies.

But if similar shifts – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” he added.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

The analysis drew on a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but excluded the changes in soil and roots.

Another researcher 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 find that is not the case – it enables researchers to confront the theory with reality and better understand how these ecosystems work.”