Australian Tropical Forests Switch from Carbon Sink to Carbon Emitter in Global Milestone

Trees in Australia's tropical rainforests have become the first worldwide by transitioning from serving as a CO2 absorber to turning into a carbon emitter, driven by rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which affects the stems and limbs of the trees but does not include the underground roots, began approximately a quarter-century back, according to new studies.

Trees naturally store carbon as they develop and emit it when they decompose. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is assumed to increase with higher CO2 levels.

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

Study Insights

Roughly 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and insufficient new growth, as the study indicates.

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

“It is understood that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will encounter in global regions.”

Worldwide Consequences

A study contributor noted 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 findings could have major consequences for global climate models, CO2 accounting, and climate policies.

“This research 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 merely temporarily, but for 20 years,” remarked an authority on climate science.

On a global scale, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” it was noted.

Ongoing Role

Although the balance between growth and decline had shifted, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and require an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a unique set of forest data starting from 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It considered the carbon stored in trunks and branches, but not the gains and losses in soil and roots.

Another researcher highlighted the value of collecting and maintaining long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. 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 systems work.”