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Australian tropical rainforest trees have become the first worldwide by transitioning from acting as a carbon sink to turning into a carbon emitter, driven by increasingly extreme temperatures and drier conditions.

The Tipping Point Discovered

This significant change, which affects the stems and limbs of the trees but does not include the root systems, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon during growth and emit it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed 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 may be at risk.

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and inadequate regeneration, according to the research.

“It’s the first tropical forest of its kind to show this symptom of change,” 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 experience in global regions.”

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 additional studies are needed.

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

“This paper is the first time 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 20 years,” stated an authority on climate science.

Worldwide, the portion of carbon dioxide taken in by forests, trees, and plants has been relatively constant 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 detected in other rainforests, climate forecasts may underestimate global warming in the future. “Which is bad news,” it was noted.

Continued Function

Although the balance between gains and losses had shifted, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

This study utilized a unique set of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It considered the carbon stored in trunks and branches, but not the changes 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 rising. But examining these long term empirical datasets, we discover that is incorrect – it allows us to confront the theory with reality and better understand how these systems work.”