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Australian tropical rainforest trees 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 Identified

This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, started around a quarter-century back, according to recent research.

Forests typically absorb carbon as they develop and emit it when they decompose. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to increase 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 may be at risk.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

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

“It is understood that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will encounter in other parts of the world.”

Global Implications

One co-author noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are required.

But should that be the case, the findings could have significant implications for global climate models, carbon budgets, and environmental regulations.

“This paper is the initial instance that this tipping point 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 20 years,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was assumed to continue under many climate models and policies.

But should comparable changes – from sink to source – were observed in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” he added.

Ongoing Role

Although the balance between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid transition away from fossil fuels.

Research Approach

This study drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the changes below ground.

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

“We thought the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we discover that is incorrect – it enables researchers to compare models with actual data and better understand how these systems work.”