By the end of this century, coral reefs in Australia and around the world could be slower to recover, structurally simpler, and increasingly dominated by fleshy algae as rising carbon dioxide reshapes ocean chemistry.
These are the predictions that new international research – published this week in Communications Biology – is warning against, as scientists present a volley of stark new findings about the current and long-term impact of a process known as ocean acidification.
As the oceans absorb more carbon dioxide from the atmosphere, they are becoming increasingly acidic – eroding the very calcium carbonate skeletons that build coral reefs. Yet despite decades of laboratory studies and ecosystem models, scientists have lacked real-world systems that reflect how entire reef communities respond to these long-term chemical shifts.
Researchers from the Australian Institute of Marine Science (AIMS) have now filled that gap by studying shallow-water reefs naturally bathed in volcanic CO₂. These reefs, located near remote submarine vents in Papua New Guinea’s Milne Bay Province, experience chronic exposure to elevated carbon dioxide, offering scientists a rare preview of the seascapes expected under future emissions scenarios.
“These unique natural laboratories are like a time machine,” said Dr. Katharina Fabricius, senior author of the study and a coral ecologist at the Australian Institute for Marine Science (AIMS). “The CO₂ seeps have allowed us to study the reefs’ tolerance limits and make predictions. How will coral reefs cope if emissions align with Paris Agreement targets? And what happens if they don’t?”
Fabricius first encountered the phenomenon in 2000 when she noticed streams of bubbles rising through coral gardens while surveying species diversity. Nearly a decade later – and spurred by growing concern over ocean acidification – she returned to analyse the gas and found it was almost pure CO₂.
It’s this discovery that launched a decade-long research initiative investigating how tropical marine ecosystems adjust, adapt, or falter after generations of exposure to acidified conditions.
The results have been sobering.
“These Papua New Guinea reefs are telling us that with every bit of increase in CO₂, we will see fewer corals and more fleshy algae,” said Dr. Sam Noonan, AIMS researcher and lead author of the study. “Critically, we also found far fewer baby corals. Without new recruits, reefs can’t grow or recover quickly – and that affects all the species that depend on them, including people.”
Today’s oceans remain slightly alkaline, with a pH of about 8.0, but acidity has already increased by roughly 30% since the industrial era. If emissions continue to rise, global ocean pH could fall to around 7.8 by 2100 – bringing it close to the values observed at the Milne Bay seep sites.
By surveying 37 reef sites along a 500-metre CO₂ gradient, the researchers documented a steady ecological transformation.
“There was no sudden tipping point,” Fabricius noted. “As CO₂ increased, fleshy algae gradually took over, smothering corals and other calcifying species.”
The implications of the study extend far beyond Papua New Guinea. Early signs of CO₂-driven change are already emerging on parts of the Great Barrier Reef. These seep sites – the latest to be analysed – show what may lie ahead.
“Ocean acidification is a massive global problem and one that has been understudied and underreported,” Fabricius continued. “This research offers the first field-based glimpse of how whole reef communities may transform under rising CO₂.
“The more carbon we emit, the greater the changes will be for reefs, for marine biodiversity, and for coastal communities that rely on them.”
Rob Hutchins, Oceanographic, 25 November 2025. Press release.
