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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to turning into a carbon emitter, driven by increasingly extreme temperatures and arid environments.

Critical Change Discovered

This significant change, which impacts the stems and limbs of the trees but excludes the root systems, began approximately a quarter-century back, according to new studies.

Trees naturally store carbon during growth and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this uptake is expected to grow with rising atmospheric concentrations.

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

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and insufficient new growth, according to the research.

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

“We know that the moist tropics in Australia exist in 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 experience in global regions.”

Global Implications

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 should that be the case, the findings could have major consequences for global climate models, CO2 accounting, and climate policies.

“This paper is the first time that this critical threshold 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,” remarked an authority on climate science.

On a global scale, the share of carbon dioxide absorbed 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 absorber to emitter – were observed in other rainforests, climate forecasts may understate heating trends in the future. “Which is bad news,” he added.

Ongoing Role

Although the balance between growth and decline had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Data and Methodology

This study utilized a unique set of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

An additional expert highlighted the value of gathering and preserving 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 enables researchers to confront the theory with reality and better understand how these systems work.”