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Trees in Australia's tropical rainforests have achieved a global first by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to increasingly extreme temperatures and arid environments.

Critical Change Identified

This crucial shift, which impacts the trunks and branches of the trees but excludes the underground roots, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon as they develop and emit it when they decompose. Overall, tropical forests are considered carbon sinks – taking in more carbon dioxide than they release – and this absorption is assumed to grow with rising atmospheric concentrations.

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

Research Findings

Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and inadequate regeneration, according to the research.

“This marks the initial rainforest of its kind to show this symptom of change,” commented the lead author.

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a coming example for what tropical forests will encounter in other parts of the world.”

Global Implications

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

But should that be the case, the results could have major consequences for global climate models, carbon budgets, and environmental regulations.

“This research is the first time that this critical threshold of a switch 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 expert in climate change science.

On a global scale, the share of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under numerous projections and policies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “This is concerning,” 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 diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a unique set of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored above ground, but excluded the gains and losses in soil and roots.

An additional expert emphasized the value of gathering and preserving long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at 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.”