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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

Critical Change Identified

This significant change, which impacts the stems and limbs of the trees but excludes the root systems, started around a quarter-century back, according to new studies.

Trees naturally store carbon during growth and emit it upon decay and death. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this absorption is expected to grow with rising atmospheric concentrations.

However, close to five decades 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 became a net emitter, with increased tree mortality and insufficient new growth, as the study indicates.

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

“It is understood that the moist tropics in Australia exist in 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 other parts of the world.”

Global Implications

A study contributor noted 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 significant implications for international climate projections, CO2 accounting, and environmental regulations.

“This research is the initial instance 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,” stated an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was expected to persist under many climate models and policies.

But if similar shifts – from sink to source – were detected in other rainforests, climate forecasts may underestimate global warming in the coming years. “This is concerning,” he added.

Continued Function

Even though the equilibrium between growth and decline had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid shift from carbon-based energy.

Data and Methodology

This study utilized a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but not the gains and losses below ground.

Another researcher highlighted the value of gathering and preserving extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these long term empirical datasets, we find that is incorrect – it allows us to confront the theory with reality and improve comprehension of how these systems work.”