Australian Rainforest Trees Switch from Carbon Sink to Carbon Emitter in World First

Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to increasingly extreme temperatures and drier conditions.

The Tipping Point Identified

This crucial shift, which affects the stems and limbs of the trees but excludes the root systems, started around 25 years ago, as per new studies.

Forests typically absorb carbon during growth and release 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 rising atmospheric concentrations.

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 trunks and branches 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,” stated the lead author.

“It is understood that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a coming example for what tropical forests will encounter in other parts of the world.”

Global Implications

One co-author mentioned that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are required.

But should that be the case, the results could have major consequences for international climate projections, carbon budgets, and environmental regulations.

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

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and policies.

But if similar shifts – from sink to source – were detected in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” he added.

Ongoing Role

Even though the equilibrium between growth and decline had shifted, 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.

Research Approach

The analysis drew on a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the changes below ground.

An additional expert emphasized the value of gathering and preserving extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is rising. But examining these long term empirical datasets, we find that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”