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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, due to increasingly extreme temperatures and arid environments.

Critical Change Identified

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

Trees naturally store carbon as they develop and emit it when they decompose. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is assumed to increase with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across northern Australia has shown that this essential carbon sink could be under threat.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and inadequate regeneration, as the study indicates.

“This marks the initial rainforest of its kind to display this sign of transformation,” commented the principal researcher.

“We know that the humid tropical regions 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.”

Worldwide Consequences

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

But if so, the findings could have major consequences for global climate models, carbon budgets, and environmental regulations.

“This paper is the first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for two decades,” remarked an expert in climate change science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was expected to persist under many climate models and strategies.

But if similar shifts – 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 balance between gains and losses had changed, 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 shift from carbon-based energy.

Research Approach

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

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

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these decades of recorded information, we find that is incorrect – it enables researchers to confront the theory with reality and better understand how these ecosystems work.”