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Trees in Australia's tropical rainforests 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 crucial shift, which affects the trunks and branches of the trees but excludes the root systems, started around a quarter-century back, as per new studies.

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

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

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, as the study indicates.

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

“It is understood that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Global Implications

One co-author noted that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are needed.

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

“This paper 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 just for one year, but for 20 years,” remarked an authority on climate science.

On a global scale, the share 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 numerous projections and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” it was noted.

Ongoing Role

Even though 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.

Data and Methodology

This study utilized 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 above ground, but not the gains and losses in soil and roots.

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

“It was believed the forest would be able to store more carbon because [CO2] is rising. But examining these long term empirical datasets, we discover that is not the case – it enables researchers to confront the theory with reality and improve comprehension of how these ecosystems work.”