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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, driven by rising heat extremes and arid environments.

Critical Change Identified

This significant change, which impacts the stems and limbs of the trees but excludes the root systems, started around 25 years ago, as per recent research.

Trees naturally store carbon as they develop and emit it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this absorption is assumed to grow with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across northern Australia has shown 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, as the study indicates.

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

“It is understood that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Worldwide Consequences

A study contributor mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are needed.

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

“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 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 past few decades, which was assumed to continue under many climate models and policies.

But if similar shifts – 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 shift from carbon-based energy.

Data and Methodology

This study utilized a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots.

Another researcher highlighted the value of collecting and maintaining extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is rising. But looking at these decades of recorded information, we discover that is not the case – it allows us to confront the theory with reality and improve comprehension of how these systems work.”