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Trees in Australia's tropical rainforests have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and drier conditions.

Critical Change Identified

This significant change, which affects the stems and limbs of the trees but does not include the root systems, began approximately 25 years ago, as per new studies.

Trees naturally store carbon as they develop and release it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this absorption is expected to grow with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across northern Australia has shown that this vital carbon sink may be at risk.

Research Findings

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and inadequate regeneration, according to the research.

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

“It is understood that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Worldwide Consequences

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

But if so, the results could have significant implications for global climate models, CO2 accounting, and environmental regulations.

“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 identified clearly – not just for one year, but for 20 years,” remarked an authority on climate science.

Worldwide, the portion of carbon dioxide taken in 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 should comparable changes – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Ongoing Role

Although the balance between gains and losses had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

The analysis utilized a distinct collection of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but excluded the changes below ground.

Another researcher highlighted the value of collecting and maintaining long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these long term empirical datasets, we discover that is not the case – it enables researchers to compare models with actual data and better understand how these ecosystems work.”