Queensland Rainforest Forests Switch from Carbon Sink to Emissions Source in Global Milestone

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 drier conditions.

The Tipping Point Identified

This significant change, which affects the trunks and branches of the trees but does not include the underground roots, started around a quarter-century back, according to recent research.

Trees naturally store carbon during growth and release it when they decompose. Overall, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to increase with higher CO2 levels.

However, close to five decades of data gathered from tropical forests across northern Australia has shown 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, as the study indicates.

“It’s the first tropical forest of its kind to show this symptom of transformation,” commented the principal researcher.

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a coming example for what tropical forests will experience 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 worldwide, and further research are needed.

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

“This paper is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for two decades,” remarked an authority on climate science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was assumed to continue under numerous projections and policies.

But should comparable changes – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Continued Function

Even though the balance between growth and decline had changed, these forests were still playing an important role in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

The analysis utilized a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but excluded the changes in soil and roots.

Another researcher highlighted the value of collecting and maintaining extended datasets.

“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 incorrect – it allows us to confront the theory with reality and improve comprehension of how these systems work.”