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Australian tropical rainforest trees have become the first worldwide by transitioning from acting as a carbon sink to becoming a source of emissions, driven by rising heat extremes and arid environments.

The Tipping Point Discovered

This significant change, which impacts the stems and limbs of the trees but does not include the root systems, started around 25 years ago, as per new studies.

Trees naturally store carbon during growth and release it upon decay and death. Generally, 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 shown that this essential carbon sink could be under threat.

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, as the study indicates.

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

“We know that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Global Implications

A study contributor mentioned 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 findings could have major consequences for international climate projections, carbon budgets, 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 identified clearly – not just for one year, but for two decades,” remarked an expert in climate change 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 many climate models and strategies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate projections may underestimate global warming in the coming years. “Which is bad news,” he added.

Continued Function

Even though the balance between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

This study utilized a unique set of forest data dating back to 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the gains and losses in soil and roots.

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

“We thought the forest would be able to store more carbon because [CO2] is increasing. But looking at these decades of recorded information, we discover that is not the case – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”