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Australian tropical rainforest trees have become the first worldwide by transitioning 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 affects the trunks and branches of the trees but does not include the root systems, started around a quarter-century back, according to new studies.

Trees naturally store carbon during growth and release it when they decompose. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this uptake is expected to grow with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, according to the research.

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

“It is understood that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Global Implications

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and additional studies are needed.

But if so, the findings could have major consequences for global climate models, CO2 accounting, and climate policies.

“This research is the initial instance that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” stated an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under numerous projections and strategies.

But should comparable changes – from sink to source – were observed in other rainforests, climate forecasts may understate heating trends in the future. “This is concerning,” it was noted.

Continued Function

Although the equilibrium between gains and losses had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an even more rapid transition away from fossil fuels.

Research Approach

This study utilized a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but excluded the gains and losses below ground.

Another researcher emphasized the importance of gathering and preserving long term data.

“It was believed the forest would be able to store more carbon because [CO2] is rising. But examining these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and better understand how these ecosystems work.”