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Trees in Australia's tropical rainforests have achieved a global first by transitioning from acting as a carbon sink to turning into a carbon emitter, due to increasingly extreme temperatures and drier conditions.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but excludes the underground roots, started around 25 years ago, as per 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 absorption is expected to grow with higher CO2 levels.

However, nearly 50 years of data collected from tropical forests across northern Australia has shown 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 inadequate regeneration, according to the research.

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

“It is understood that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a future analog for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

A study contributor mentioned that it is yet unclear 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 significant implications for international climate projections, carbon budgets, and climate policies.

“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 merely temporarily, but for two decades,” remarked an expert in climate change 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 expected to persist under many climate models and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the future. “Which is bad news,” it was noted.

Ongoing Role

Even though the equilibrium between gains and losses had shifted, 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 require an even more rapid transition away from fossil fuels.

Data and Methodology

The analysis utilized a distinct collection of forest data dating back to 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but not the gains and losses in soil and roots.

Another researcher highlighted the value of gathering and preserving extended datasets.

“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 allows us to confront the theory with reality and better understand how these ecosystems work.”