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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, driven by rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which impacts the trunks and branches of the trees but excludes the underground roots, started around a quarter-century back, as per new studies.

Trees naturally store carbon as they develop and emit it upon decay and death. Overall, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is assumed to grow with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across northern Australia has revealed that this vital carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, according to the research.

“It’s the first tropical forest of its kind to display this sign of change,” commented the lead author.

“We know that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Global Implications

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

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

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

Worldwide, 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 expected to persist under many climate models and strategies.

But if similar shifts – from sink to source – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “Which is bad news,” he added.

Continued Function

Although the balance between growth and decline had shifted, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an even more rapid transition away from fossil fuels.

Data and Methodology

This study utilized a distinct collection 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 changes in soil and roots.

An additional expert highlighted the importance of gathering and preserving long term data.

“It was believed the forest would be able to store more carbon because [CO2] is rising. But looking at these long term empirical datasets, we discover that is not the case – it enables researchers to confront the theory with reality and improve comprehension of how these ecosystems work.”