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Trees in Australia's tropical rainforests have achieved a global first by shifting from acting as a carbon sink to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This crucial shift, which impacts the stems and limbs of the trees but excludes the underground roots, started around a quarter-century back, as per new studies.

Forests typically absorb carbon during growth and release it when they decompose. Generally, 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, close to five decades of data collected from tropical forests across northern Australia has revealed that this vital carbon sink may be at risk.

Research Findings

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

“It’s the first tropical forest of its kind to display this sign of change,” commented the lead author.

“We know that the moist tropics in Australia occupy 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 global regions.”

Global Implications

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

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

“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,” stated an authority on climate science.

Worldwide, the share of carbon dioxide taken in 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 strategies.

But should comparable changes – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the future. “Which is bad news,” he added.

Ongoing Role

Even though the balance between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. 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 unique set of forest data dating back to 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 below ground.

Another researcher emphasized the importance of gathering and preserving long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these long term empirical datasets, we discover that is not the case – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”