🔗 Share this article

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

Critical Change Identified

This crucial shift, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately 25 years ago, according to recent research.

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

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

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

“This marks the initial rainforest of its kind to display this sign of transformation,” stated the lead author.

“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 encounter in global regions.”

Global Implications

A study contributor noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

But if so, the findings could have significant implications for international climate projections, carbon budgets, and climate policies.

“This research is the first time that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for two decades,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide absorbed by forests, trees, and plants has been quite stable 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 observed in other rainforests, climate forecasts may underestimate global warming in the future. “This is concerning,” it was noted.

Ongoing Role

Although the balance between growth and decline had changed, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate 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 monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the gains and losses in soil and roots.

Another researcher highlighted the importance of collecting and maintaining long term data.

“It was believed the forest would be able to store more carbon because [CO2] is increasing. But examining these long term empirical datasets, we discover that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”