Australian tropical rainforest trees have become the first worldwide by shifting from serving as a CO2 absorber to turning into a carbon emitter, driven by increasingly extreme temperatures and drier conditions.
Critical Change Discovered
This significant change, which impacts the stems and limbs of the trees but does not include the underground roots, began approximately 25 years ago, according to new studies.
Trees naturally store carbon as they develop and release 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 increase with higher CO2 levels.
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
Roughly 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and insufficient new growth, as the study indicates.
“It’s the first tropical forest of its kind to display this sign of transformation,” commented the principal researcher.
“It is understood that the humid tropical regions in Australia occupy a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will experience in other parts of the world.”
Worldwide Consequences
One co-author noted that it remains to be seen whether Australia’s tropical forests are a harbinger 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, CO2 accounting, and climate policies.
“This paper is the initial instance that this critical threshold 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 two decades,” stated an authority on climate science.
Worldwide, 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 many climate models and policies.
But should comparable changes – from absorber to emitter – were observed in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” he added.
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 reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, 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 excluded the changes in soil and roots.
An additional expert emphasized the importance of gathering and preserving extended datasets.
“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these decades of recorded information, we discover that is not the case – it allows us to confront the theory with reality and improve comprehension of how these systems work.”
