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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, driven by rising heat extremes and arid environments.

Critical Change Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the root systems, started around a quarter-century back, according to new studies.

Trees naturally store 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 uptake is assumed to grow with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland 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 inadequate regeneration, as the study indicates.

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

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a coming example for what tropical forests will experience in global regions.”

Global Implications

One co-author mentioned that it remains to be seen whether Australia’s tropical forests are a precursor 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, carbon budgets, and environmental regulations.

“This paper is the first time 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,” remarked an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist 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,” it was noted.

Continued Function

Even though the balance between growth and decline had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Research Approach

This study drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the gains and losses in soil and roots.

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

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these decades of recorded information, we discover that is incorrect – it enables researchers to compare models with actual data and better understand how these systems work.”