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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 rising heat extremes and drier conditions.

Critical Change Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the root systems, started around 25 years ago, according to recent research.

Forests typically absorb carbon during growth and emit it upon decay and death. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is expected to increase with rising atmospheric concentrations.

However, nearly 50 years of data collected from tropical forests across northern Australia has revealed that this essential carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and insufficient new growth, according to the research.

“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 somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will encounter in global regions.”

Worldwide Consequences

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

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

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

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was assumed to continue under numerous projections and strategies.

But should comparable changes – from absorber to emitter – were detected in other rainforests, climate forecasts may underestimate global warming in the future. “Which is bad news,” he added.

Continued Function

Although the balance between gains and losses had changed, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

This study drew on a unique set of forest data dating back to 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but not the changes in soil and roots.

Another researcher highlighted the value of gathering and preserving extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But examining these decades of recorded information, we discover that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”