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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 arid environments.

The Tipping Point Identified

This significant change, which affects the trunks and branches of the trees but excludes the underground roots, began approximately a quarter-century back, according to new studies.

Forests typically absorb carbon during growth and emit it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to grow with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland has shown that this essential carbon sink may be at risk.

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to display this sign of transformation,” commented the principal researcher.

“It is understood that the moist tropics 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 encounter in other parts of the world.”

Global Implications

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

But if so, the findings could have significant implications for international climate projections, CO2 accounting, and climate policies.

“This paper is the first time that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for 20 years,” remarked an authority on climate science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” he added.

Ongoing Role

Even though the equilibrium between gains and losses had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an even more rapid transition away from fossil fuels.

Data and Methodology

This study 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.

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 rising. But examining these long term empirical datasets, we find that is not the case – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”