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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, due to rising heat extremes and arid environments.

Critical Change Discovered

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

Trees naturally store carbon as they develop and emit it when they decompose. Generally, tropical forests are considered 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 Queensland has revealed that this vital carbon sink could be under threat.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and insufficient new growth, as the study indicates.

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

“It is understood that the moist tropics in Australia exist in a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Global Implications

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 required.

But should that be the case, the findings could have major consequences for global climate models, CO2 accounting, and climate policies.

“This research is the first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for 20 years,” stated an authority on climate science.

On a global scale, 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 should comparable changes – from absorber to emitter – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “This is concerning,” it was noted.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and require an even more rapid shift from carbon-based energy.

Data and Methodology

The analysis drew on a distinct collection of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the gains and losses below ground.

An additional expert emphasized the value of gathering and preserving extended datasets.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we discover that is incorrect – it enables researchers to compare models with actual data and improve comprehension of how these systems work.”