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Trees in Australia's tropical rainforests have become the first worldwide by transitioning from acting as a carbon sink to becoming a source of emissions, due to increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This significant change, which affects the stems and limbs of the trees but does not include the root systems, began approximately a quarter-century back, as per 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 release – and this absorption is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across Queensland has shown that this vital carbon sink could be under threat.

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with increased tree mortality and inadequate regeneration, as the study indicates.

“It’s the first tropical forest of its kind to display this sign of change,” stated the principal researcher.

“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 could act 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 worldwide, and additional studies are required.

But should that be the case, the results could have significant implications for international climate projections, CO2 accounting, and climate policies.

“This research is the initial instance that this critical threshold 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 20 years,” stated an expert in climate change science.

On a global scale, the share 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 numerous projections and policies.

But if similar shifts – from sink to source – were observed in other rainforests, climate projections may underestimate global warming in the future. “Which is bad news,” he added.

Continued Function

Although the balance between gains and losses had shifted, these forests were still playing an important role in soaking up CO2. But their diminished ability 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 tracking roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the gains and losses in soil and roots.

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

“It was believed the forest would be able to absorb additional CO2 because [CO2] is increasing. But examining these decades of recorded information, we find that is incorrect – it allows us to confront the theory with reality and better understand how these systems work.”