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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, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This crucial shift, which affects the trunks and branches of the trees but does not include the underground roots, began approximately a quarter-century back, according to new studies.

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

However, nearly 50 years 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 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 display this sign of change,” commented the principal researcher.

“It is understood that the moist tropics in Australia exist in a somewhat hotter, arid environment 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 mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and additional studies are needed.

But should that be the case, the findings could have significant implications for international climate projections, CO2 accounting, and climate policies.

“This research is the initial instance that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for two decades,” stated an authority on climate science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was assumed to continue under many climate models and policies.

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

Ongoing Role

Although the equilibrium between growth and decline 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 require an even more rapid transition away from fossil fuels.

Data and Methodology

The analysis drew on a distinct collection of forest data dating back to 1971, including records tracking roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the gains and losses in soil and roots.

An additional expert emphasized the value of collecting and maintaining long term data.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But looking at these long term empirical datasets, we find that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”