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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, due to rising heat extremes and drier conditions.

The Tipping Point Identified

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

Trees naturally store carbon during growth 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 expected to increase with rising atmospheric concentrations.

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

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, 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 humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on other continents, and therefore it might serve as a future analog for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

A study contributor noted that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and additional studies are needed.

But if so, the findings could have significant implications for international climate projections, carbon budgets, and environmental regulations.

“This research is the initial instance that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for 20 years,” stated an expert in climate change science.

Worldwide, the share 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 should comparable changes – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” he added.

Continued Function

Even though the balance between growth and decline had changed, these forests were still serving a vital function in soaking up CO2. But their diminished ability to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated shift from carbon-based energy.

Research Approach

The analysis drew on a unique set of forest data starting from 1971, including records tracking approximately 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but not the gains and losses in soil and roots.

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

“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 enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”