Australian Rainforest Forests Shift from CO2 Absorber to Emissions Source in World First

Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and drier conditions.

Critical Change Identified

This crucial shift, which impacts the stems and limbs of the trees but excludes the underground roots, started around 25 years ago, as per new studies.

Forests typically absorb carbon as they develop and release it when they decompose. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is expected to grow with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across Queensland has revealed that this essential carbon sink may be at risk.

Research Findings

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.

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

“It is understood that the humid tropical regions 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.”

Worldwide Consequences

One co-author noted that it is yet unclear 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 international climate projections, CO2 accounting, and climate policies.

“This research is the initial instance that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” stated an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under numerous projections and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” he added.

Ongoing Role

Although the equilibrium between growth and decline had shifted, these forests were still playing an important role in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and require an even more rapid transition away from fossil fuels.

Data and Methodology

This study utilized a distinct collection of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes below ground.

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

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But looking at these decades of recorded information, we discover that is incorrect – it allows us to compare models with actual data and improve comprehension of how these systems work.”