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Australian tropical rainforest trees have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This significant change, which impacts the trunks and branches of the trees but excludes the root systems, started around a quarter-century back, according to new studies.

Trees naturally store carbon during growth and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is assumed to increase with higher CO2 levels.

However, nearly 50 years of data gathered from tropical forests across Queensland has revealed that this vital carbon sink may be at risk.

Research Findings

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, according to the research.

“It’s the first tropical forest of its kind to show this symptom of transformation,” stated the lead author.

“We know that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will encounter in global regions.”

Global Implications

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

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

“This research is the initial instance 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,” stated an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under numerous projections and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate forecasts may underestimate global warming in the future. “This is concerning,” it was noted.

Ongoing Role

Although the equilibrium between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an even more rapid transition away from fossil fuels.

Data and Methodology

This study drew on a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but not the gains and losses in soil and roots.

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

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these long term empirical datasets, we discover that is incorrect – it enables researchers to confront the theory with reality and improve comprehension of how these ecosystems work.”