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Trees in Australia's tropical rainforests have achieved a global first by transitioning from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and drier conditions.

The Tipping Point Discovered

This crucial shift, which affects the trunks and branches of the trees but does not include the root systems, started around a quarter-century back, as per recent research.

Trees naturally store carbon as they develop and emit it when they decompose. Generally, tropical forests are considered carbon sinks – taking in more carbon dioxide than they emit – and this absorption is assumed to increase with rising atmospheric concentrations.

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to show this symptom of change,” commented the lead author.

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will encounter in other parts of the world.”

Global Implications

One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are required.

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

“This paper is the first time 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 expert in climate change science.

On a global scale, the portion 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 many climate models and policies.

But should comparable changes – from sink to source – were detected in other rainforests, climate forecasts may underestimate global warming in the coming years. “Which is bad news,” it was noted.

Continued Function

Even though the equilibrium between gains and losses had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Data and Methodology

This study drew on a unique set of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It considered the carbon stored above ground, but excluded the gains and losses in soil and roots.

An additional expert highlighted the value of collecting and maintaining extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is rising. But examining these decades of recorded information, we find that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”