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Trees in Australia's tropical rainforests have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, driven by rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which affects the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, as per recent research.

Forests typically absorb carbon during growth and release it upon decay and death. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they release – and this uptake is assumed to increase with rising atmospheric concentrations.

However, nearly 50 years of data gathered from tropical forests across northern Australia has revealed that this vital carbon sink may be at risk.

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and insufficient new growth, according to the research.

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

“We know that the humid tropical regions in Australia exist in 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 encounter 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 globally, and additional studies are required.

But if so, the findings could have significant implications for global climate models, CO2 accounting, and climate policies.

“This research is the initial instance that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for two decades,” stated an authority on climate science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under many climate models and policies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may underestimate global warming in the future. “This is concerning,” he added.

Continued Function

Even though the balance between gains and losses had shifted, 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 “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Data and Methodology

The analysis drew on a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but not the gains and losses in soil and roots.

Another researcher emphasized the value of gathering and preserving long term data.

“We thought the forest would be able to store more carbon because [CO2] is rising. But examining these decades of recorded information, we find that is not the case – it allows us to confront the theory with reality and better understand how these ecosystems work.”