LONDON, Oct. 30 (Xinhua) -- Warming oceans pose mounting risks to coral species and reef ecosystems, recent global assessments have warned, raising concerns for marine biodiversity and coastal livelihoods.
An assessment of 892 reef-building coral species found that 340 were threatened with extinction, according to a study published in Nature in October.
Marine ecologist and co-chair of the International Union for Conservation of Nature (IUCN) SSC Coral Specialist Group Francoise Cabada-Blanco is one of the lead authors of the study. She told Xinhua that species extinction and ecosystem collapse are distinct but interconnected risks.
The former concerns the risk of losing individual coral species, while the latter concerns whole reef systems collapsing, with living coral largely replaced by algae, rubble and other organisms, she said.
A separate IUCN assessment released on Oct. 22 classified 122 of 127 shallow-water coral reef ecosystems, or 96 percent, as Critically Endangered when projected climate change was included.
Under a scenario projected by the Intergovernmental Panel on Climate Change, marine heatwaves will become so frequent over the next 50 years that reefs may lack sufficient recovery time, IUCN explained.
However, the classification does not mean these ecosystems have already collapsed. Most still retain living coral and continue to function as reef ecosystems. "There is still time to act, but the window is closing," IUCN said.
For Cabada-Blanco, the growing frequency of bleaching is particularly worrying. The phenomenon, whereby corals eject algae and can eventually die, is caused by warming waters. "We are now moving towards near-annual bleaching, which leaves no recovery window at all."
The study published in Nature combined coral-cover trends, projections of severe bleaching, and biological traits affecting vulnerability. Nevertheless, its underlying data ended in 2020, before the global bleaching event of 2023-2025.
Cabada-Blanco said conditions had most likely worsened since 2020, as bleaching events have become more widespread and longer-lasting.
LOSSES BEYOND THE REEF
Coral reefs occupy less than 1 percent of the ocean floor but support around a quarter of all marine fish species, according to IUCN. Their contribution to food supplies, tourism and coastal protection makes reef degradation a concern whose implications reach far beyond conservation.
Coral reef decline is depriving countless species of the food, shelter and breeding grounds they depend on, said Grethel Aguilar, IUCN Director General, noting that it is also threatening the safety and livelihoods of coastal communities, and undermining key economic sectors such as fisheries, aquaculture and coastal tourism.
"Reef-building corals are ecosystem engineers," Cabada-Blanco said. "When they die, reefs flatten, and the fish and invertebrates that depend on them lose their homes." Small-scale reef fisheries often feed local families, she said.
Regional findings show that even reefs that appear healthier reefs face substantial risks. In East Asia, 22 out of 25 ecoregions are under threat, despite comparatively high coral cover. All 25 are Critically Endangered when future climate projections are included, IUCN said.
Cabada-Blanco said overall coral cover could remain stable while sensitive species were replaced by tougher ones. "Coral cover alone can therefore mask a real loss of diversity, and it may hide how close a reef is to a sudden decline."
The Nature study provided a warning over that vulnerability: 191 of 279 Indo-Pacific species in the Acroporidae family, which includes branching and plating corals that shape reef structure, are classified as Endangered.
For communities relying on reefs, recovery could also come too late. Cabada-Blanco said reef structures could rebuild over centuries, but not on timescales relevant to people depending on them today. "Some losses would be irreversible. Extinction is permanent."
ACTION STILL MATTERS
IUCN said that, based only on observed changes over the past 50 years, 24 percent of the assessed reef ecosystems would be Critically Endangered, compared with 96 percent when future climate projections were included. The contrast highlights how strongly future warming influences the outlook.
"Emissions reductions come first. No local action can protect reefs from unlimited warming," Cabada-Blanco said. Every tenth of a degree of warming avoided would help retain more coral species and reef function, she underlined.
Local pressures also affect reefs' ability to withstand heat. IUCN found disrupted ecological relationships among corals, algae and fish in 63 percent of assessed ecosystems. Overfishing, pollution and sediment can weaken the balance needed for recovery.
Mishal Gudka, scientific lead of the global IUCN Red List of Coral Reef Ecosystems assessment, urged governments to cut emissions while keeping coastal waters clean and making fishing sustainable.
Scaling up restoration and improving coral heat tolerance are important areas of research, Cabado-Blanco said, but warned that a single heatwave could erase years of restoration work if warming continues. "Restoration is a complement, not a substitute," she stressed.
According to IUCN, reefs in areas such as the Northern Red Sea, Papua New Guinea, and Bermuda have maintained living coral and a healthy ecological balance despite repeated heatwaves, making them priorities for protection.
Scientific cooperation is another essential part of the response. Cabada-Blanco said that although her team's assessment involved more than 60 experts, only 15 of the 892 coral species had population data across their ranges. Open data and taxonomic research would improve future assessments, she emphasized.
David Obura, Cabada-Blanco's fellow co-chair of the IUCN SSC Coral Specialist Group, said that major knowledge gaps must be addressed alongside urgent efforts to reduce the pressures that are driving the decline of species and ecosystems.
"Reefs have repeatedly shown they can recover when given the chance," Cabada-Blanco said. "Our task is to give them that chance." ■
