From the surface, a coral reef can look almost impossibly beautiful. Beneath the water, however, those bright colors are part of a crowded living world where corals provide the structure, shelter and food that allow countless other species to survive.
Now scientists have delivered a warning that reaches far beyond the reefs themselves.
A global reassessment published in Nature found that 44% of assessed reef-building coral species are threatened with extinction. The finding comes as scientists confront a changing ocean, with marine heatwaves, repeated bleaching and other pressures leaving reefs with less time to recover.
The number sounds alarming, but it raises a more complicated question: what does it actually mean when scientists say a coral species is threatened with extinction, and what happens to the underwater communities that depend on it?
Nearly half of assessed corals are now at risk
The new assessment examined 892 reef-building coral species, making it the most comprehensive global reassessment of their extinction risk to date. Of the 774 species with enough information for an assessment, 340 were placed in one of the three IUCN threatened categories: Vulnerable, Endangered or Critically Endangered.
That represents 44% of the species for which sufficient data were available.
It is also a deterioration from the previous global assessment. In 2008, about one-third of assessed reef-building coral species were considered threatened, compared with 44% in the new assessment.
But there is an important detail hidden inside that number.
The assessment uses data through 2020, meaning it does not capture the full consequences of the extraordinary global coral bleaching event that unfolded from 2023 through 2025. The researchers therefore caution that the proportion of coral species currently facing elevated risk could be higher than the 44% recorded in the assessment.
Why does a coral reef matter so much?
A coral reef may occupy less than 1% of the ocean floor, but its influence is far larger.
IUCN says coral reefs support around a quarter of marine fish species and provide food, income and coastal protection for hundreds of millions of people. Reef structures also help break waves before they reach shore, reducing erosion and flooding in coastal areas.
For coastal communities, that makes a healthy reef more than a destination for divers and tourists. It can be part of the natural infrastructure protecting a village.
It can be a nursery for fish that later become food. It can support tourism jobs and local businesses. And for many coastal and island communities, reefs are also part of culture and identity. When the reef weakens, those connections can weaken with it.
What happens when the water gets too warm?
The biggest pressure identified in the new ecosystem assessment is marine heat stress.
Corals live in partnership with microscopic algae that provide much of the energy they need and contribute to their characteristic colors. When ocean temperatures become unusually high, corals can expel those algae, leaving the coral tissue pale or white.
This is known as coral bleaching.
Bleaching does not automatically mean the coral is dead. If temperatures return to safer levels quickly enough and other stresses are limited, some corals can recover.
The danger comes when heat stress is severe, prolonged or repeated before the reef has recovered.
NOAA says the fourth global coral bleaching event, which began in 2023, affected about 84% of the world's coral reef area through September 2025. The agency concluded in 2026 that the global event likely ended in mid-2025, but scientists continue to monitor reefs closely as ocean temperatures remain higher than they were decades ago.
That history is part of why the latest IUCN assessment is so significant. The concern is not simply that reefs can bleach.
It is that they may be asked to recover from one heat event only to encounter another.
Climate change is the biggest pressure, but it is not alone
Warming oceans are not the only problem facing coral reefs.
The IUCN assessment also identifies overfishing, pollution, coastal development, sediment, aquaculture, storms and disease among the pressures affecting reef ecosystems. In 63% of the assessed ecosystems, researchers found disruption in the balance among corals, algae and fish.
That local pressure matters because reefs need a chance to recover after bleaching.
When key herbivorous fish such as parrotfish disappear through overfishing, algae can spread across damaged reefs. Pollution and sediment can add further stress, making it harder for corals to regain the space they need.
This creates an important distinction in the climate story.
Reducing greenhouse-gas emissions is essential for limiting ocean warming, but protecting reefs from local pressures can also give surviving corals a better chance to recover.
The Western Indian Ocean is part of the story
The warning is not confined to Australia's Great Barrier Reef or the Caribbean.
The new IUCN assessment includes the Western Indian Ocean, stretching from Somalia and Kenya through Tanzania, Mozambique, Madagascar and the islands of the southwest Indian Ocean. Of the 13 ecosystems assessed in the region, 12 are classified as Critically Endangered overall, while one remains Data Deficient.
That gives the global story a particularly important African connection.
In 2024, when NOAA confirmed the fourth global bleaching event, widespread coral bleaching had already been documented in parts of the Western Indian Ocean, including Kenya and Tanzania.
For communities along the East African coast, the health of coral reefs is tied to fishing, tourism and coastal ecosystems.
What happens beneath the water does not necessarily stay beneath the water.
What happens when an underwater city loses its foundation?
A coral reef can look like a world of color from above and a maze of shapes from below.
But its beauty can hide how much life depends on its survival.
Fish need places to shelter. Young marine animals need nurseries. Coastal communities need healthy fisheries and natural protection from waves. Tourism economies need the reefs that attract visitors in the first place.
The new assessments do not say that all of those things are about to disappear.
They show something more uncomfortable: the conditions supporting them are becoming increasingly difficult for many reefs to withstand.
The ocean is warming, and repeated heatwaves are leaving less time for recovery. At the same time, local pressures can determine whether a damaged reef has the opportunity to rebuild.
That leaves humanity with a question that is both scientific and deeply human.
If coral reefs are the underwater cities supporting so much life, how much of those cities can we protect before the foundation becomes too difficult to rebuild?
The Bottom Line
Nearly half of assessed reef-building coral species are now classified as threatened with extinction, while a separate IUCN assessment puts 96% of assessed coral reef ecosystems in the Critically Endangered category when future heat stress is considered.
Those figures should not be read as a prediction that coral reefs will simply vanish.
They are a measure of risk.
And that distinction leaves room for something increasingly important in climate reporting: urgency without inevitability. The reefs are under extraordinary pressure, but scientists say many are still alive, still functioning and still capable of recovery if global warming is limited and local threats are reduced.
For now, beneath the waves, the cities are still there.
The question is what kind of cities we leave for the generations that come after us.
