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Australian tropical rainforest trees have become the first worldwide by transitioning from acting as a carbon sink to becoming a source of emissions, due to rising heat extremes and drier conditions.

Critical Change Identified

This significant change, which impacts the trunks and branches of the trees but does not include the underground roots, began approximately 25 years ago, according to recent research.

Trees naturally store carbon as they develop and emit it upon decay and death. 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 Queensland has revealed that this essential carbon sink could be under threat.

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and inadequate regeneration, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of transformation,” stated the principal researcher.

“We know that the humid tropical regions in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will experience in other parts of the world.”

Global Implications

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 required.

But should that be the case, the results could have major consequences for international climate projections, CO2 accounting, and environmental regulations.

“This research is the first time 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,” stated an expert in climate change science.

Worldwide, 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 strategies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” it was noted.

Continued Function

Even though the balance between growth and decline had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid shift from carbon-based energy.

Research Approach

This study drew on a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the gains and losses below ground.

An additional expert emphasized the value of collecting and maintaining extended datasets.

“We thought 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 incorrect – it enables researchers to confront the theory with reality and better understand how these systems work.”