Three out of four reef-building coral species on the planet have now been individually assessed for extinction risk, and the result is worse than almost anyone expected. A major study published in *Nature* on October 15, 2026, found that 44% of assessed species face an elevated relative risk of extinction — up from roughly one-third when the first global assessment was conducted in 2008. What has changed in those 18 years is not the methodology. It is the ocean.

Lead author David Obura, a Kenyan reef scientist and chair of IPBES — the United Nations' expert panel on biodiversity — coordinated more than 60 coral specialists across institutions including the Australian Institute of Marine Science, the University of Queensland, the University of Technology Sydney, Curtin University, and the University of Portsmouth. The resulting paper in *Nature* is the most comprehensive update to the IUCN Red List status of reef-building corals since the baseline work was completed nearly two decades ago.

The Numbers Behind the Headline

Of the 774 reef-building coral species fully assessed in the study, 340 — precisely 44% — now sit at elevated risk. Within that group, the breakdown by severity is stark: 33 species are classed as Critically Endangered, 251 as Endangered, and 56 as Vulnerable. That is a meaningful internal gradient. Critically Endangered means the species is facing an extremely high probability of extinction in the near future under current conditions; Endangered means a very high probability. Most of the 340 at-risk species fall into the Endangered band.

The total species pool reassessed was 892, but the formal IUCN thresholds were applied to 774. The remaining species lacked sufficient data for a confident classification under IUCN criteria — itself a sign of how unevenly coral science is distributed across the global reef system.

The geographic split adds another layer. In the Atlantic, 50% of coral species are now classified as threatened. In the Indo-Pacific — which covers the vast arc of reefs from the Red Sea and Indian Ocean through Southeast Asia to the Great Barrier Reef and out into the Pacific — the figure is 43%. The Atlantic number is higher partly because Atlantic reefs have been hit harder, earlier, by disease and local degradation on top of warming. The Indo-Pacific number being nearly as bad reflects how far heat stress has spread across a region that was once considered more resilient.

Which Family Is in the Worst Shape

Not all coral groups are equally vulnerable. The study's most sobering finding for reef ecology concerns Acroporidae — the branching corals that create the three-dimensional architecture that most people picture when they think of a healthy reef. These species build the caves, the archways, the dense thickets that shelter juvenile fish and filter feeders. Acroporidae is also the most species-rich coral family. According to the study, 68% of Acroporidae species — 191 out of 279 — are now threatened with extinction and classified as Endangered.

Losing a dominant structural family is categorically different from losing scattered specialist species. When branching corals decline, the reef's physical complexity collapses. Fish communities that depend on that complexity follow. The ripple effects run through every trophic level on the reef and eventually reach the fishing communities and coastal populations whose food security the reef supports.

Heat Stress Drives Nine in Ten Cases

For 93% of the species placed at elevated risk, the most important driver was projected heat stress over the coming 30 years. Climate change ultimately determined the final classification for more than 90% of the highest-risk species. The study used a "middle-of-the-road" climate scenario — SSP2-4.5 — which assumes significant but not worst-case emissions through mid-century. Under that projection alone, heat stress alone is enough to place nearly half of all reef-building corals in the threatened categories.

The authors were careful about what they were measuring. Population trends are not tracked species-by-species for most corals, so the team used total coral cover data aggregated across species as a proxy for population change, combined with projections of annual severe bleaching under SSP2-4.5. This is a limitation they acknowledge openly — it means the 44% figure is an estimate with real uncertainty, not a precise species-by-species count. Importantly, though, the data runs only to 2020.

The 2020 Cutoff and What Has Happened Since

The study's authors are blunt about what the 2020 data cutoff means. Between 2023 and 2025, the planet experienced three consecutive record-breaking years of global temperature, including a global coral bleaching event that struck reefs across every major ocean basin. None of that is captured in the 44% figure. As Blogerroom's analysis of September 2026's record ocean temperatures showed, sea surface temperatures have continued breaking records since the study's data was collected.

Obura told AFP reporters on October 15 that the current situation is almost certainly worse than the published figure reflects, and that the 50% threshold — half of all reef-building species at elevated risk — is now within reach. "It is entirely possible that reassessing the status of corals just after this coming El Niño may show them to have crossed the 50% mark," he said.

The El Niño Factor

The 2026 super El Niño adds a specific near-term threat that the study did not model. Obura described the event as a "terrifying" layer of additional risk, citing the 1997–98 El Niño as a reference point. That event caused widespread bleaching across the Indo-Pacific and was responsible for significant coral mortality in locations that had previously been considered stable. The 2026 El Niño is forecast to be stronger.

NOAA now gives an 83% probability that this year's event will exceed every El Niño recorded since 1950. That warming pulse arrives on top of an ocean baseline that, as of September 2026, is already at all-time highs. The compounding effect — a background ocean that is already hot, hit by the strongest El Niño in decades — is exactly the kind of thermal stacking that the SSP2-4.5 projections don't fully capture, because those projections model gradual trends, not acute peak events layered onto warming baselines.

One observation worth making explicitly: the 2008-to-2026 jump from one-third to 44% threatened corresponds to roughly 18 years. If the trend continues at that rate, the 50% mark — not just possible after this El Niño but as a sustained classification — could arrive before 2030. The pace of deterioration is accelerating, not leveling off.

What the Study Does Not Say

The IUCN Red List measures relative extinction risk. It is not a prediction of imminent total collapse. Some corals listed as Endangered will survive if warming slows; some populations will recover partially between bleaching events. The Red Sea, particularly the northern Gulf of Aqaba, is already known to harbor unusually heat-tolerant coral populations that offer a limited refuge. The study also found species that improved in classification since 2008 — *Galaxea fascicularis*, for example, moved to Least Concern after updated analyses.

But the overall direction is not ambiguous. The study adds specific, peer-reviewed numbers to what field researchers have been observing for years: conditions for corals are getting worse faster than mitigation efforts are improving them, and the gap is widening. The 44% figure is not a prediction of what might happen under extreme warming. It is an assessment of where things already stood four years ago.