Coral Reefs Facts: Bleaching, Climate Change, and Why They Matter

Coral reefs facts begin with a striking contrast: these ecosystems cover a tiny share of the ocean, yet they support a huge share of marine life. Built by colonies of coral polyps held together by calcium carbonate, reefs have been part of Earth’s oceans since 485 million years ago, at the dawn of the Early Ordovician.

What are coral reefs?

Coral reefs are underwater ecosystems formed by colonies of coral polyps held together by calcium carbonate. Shallow coral reefs occupy less than 0.1% of the world’s ocean area, but they provide a home for at least 25% of all marine species. They also shelter an estimated quarter of all ocean species, making reefs one of the most concentrated habitats in the sea.

These reefs are most commonly found at shallow depths in tropical waters. Deep-water and cold-water coral reefs also exist, though on smaller scales in other areas. Coral reefs first appeared 485 million years ago, which places their origin at the dawn of the Early Ordovician.

Why do coral reefs matter to people?

Coral reefs deliver ecosystem services for tourism, fisheries, and shoreline protection. They act as a protective barrier for coastlines by reducing wave impact, and they help coastal communities by limiting the force of storm surges.

Healthy reefs also matter for food and local livelihoods. Large-scale reef damage can affect communities that depend on reefs for fisheries, tourism, and protection. That human connection is part of why reef decline is more than an environmental story; it is also a story about coastlines, work, and food.

What causes coral bleaching?

The leading cause of coral bleaching is rising ocean temperatures due to climate change. When coral bleaches, it is under stress. A bleached coral becomes more vulnerable to starvation and disease, and it is at risk of death.

Bleaching can spread beyond individual corals. Large-scale bleaching events can devastate entire reef ecosystems and affect the livelihoods of coastal communities that depend on healthy reefs for food, tourism, and protection from storm surges. In other words, the loss is ecological and practical at the same time.

How serious has bleaching been?

According to the United Nations Environment Programme, between 2014 and 2016, global bleaching events killed coral on an unprecedented scale for the time. In 2016, bleaching on the Great Barrier Reef killed 29 to 50 percent of the reef’s coral.

The scale of bleaching has continued to stand out. On April 15, 2024, NOAA confirmed the world was in the midst of its fourth and largest global coral bleaching event. From 1 January 2023 to 30 September 2025, bleaching-level heat stress impacted about 84.4% of the world’s coral reef area.

How much reef has been lost?

Shallow tropical coral reefs have declined by 50% since 1950. That decline matters because reefs already occupy less than 0.1% of the world’s ocean area while supporting at least 25% of all marine species. When a habitat so small carries so much life, even partial losses can reshape marine ecosystems.

Coral reefs connect climate, oceans, and people in one system. They began hundreds of millions of years ago, support vast marine diversity, and protect shorelines today.

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