Half the world’s coral reefs were hit hard by extreme ocean heat, and an even more severe bleaching crisis is happening right now.

Coral reefs are far more than colorful underwater ecosystems. They support fisheries, drive tourism, shield coastlines from storms, and even contribute to the discovery of new medicines. Altogether, these benefits are valued at roughly $9.8 trillion each year.

Now, scientists report that a powerful global marine heatwave caused widespread coral bleaching, damaging about half of the world’s reefs. In the first global assessment of its kind, an international research team led by Smithsonian scientists measured the extent of bleaching worldwide. A new marine heatwave that began in 2023 is still underway. The findings were published in Nature Communications.

What Happens During Coral Bleaching

Corals survive through a close biological partnership. One partner is a small animal related to jellyfish that builds the reef’s hard skeleton. Living inside it are microscopic algae that use sunlight to produce energy, which feeds the coral.

When ocean temperatures climb too high, this partnership breaks down. The coral expels the algae that supply its energy and turns white. This process, known as bleaching, weakens corals, slowing growth and reproduction. If the heat stress is severe or prolonged, it can lead to widespread coral death.

Measuring the Third Global Coral Bleaching Event (2014–2017)

To understand the impact of the “Third Global Coral Bleaching Event” (2014–2017), researchers from dozens of countries collaborated on a massive data effort. The study was led by scientists at the Smithsonian Tropical Research Institute (STRI), James Cook University in Australia, and the former director of Coral Reef Watch at the U.S. National Oceanic and Atmospheric Administration (NOAA).

The team combined satellite measurements of ocean surface temperatures from the Coral Reef Watch system with underwater reef surveys and aerial observations collected around the world. This allowed them to link heat exposure detected from space with real damage observed on reefs.

“This is the most geographically extensive analysis of coral bleaching surveys ever done,” said Sean Connolly, a senior scientist at the Smithsonian. “Nearly 200 co-authors from 143 institutions in 41 countries and territories contributed data.”

Global Survey Reveals Severe Coral Damage

The researchers analyzed more than 15,000 reef surveys. Their results show that 80 percent of reefs experienced moderate or greater bleaching, while 35 percent suffered moderate or higher levels of coral mortality.

By establishing how measured heat stress translated into reef damage at surveyed sites, the team applied satellite heat data to estimate bleaching across reefs that were not directly observed. They concluded that more than 50% of coral reefs worldwide endured significant bleaching, and 15% experienced substantial coral death.

As coral reefs decline, so do the services they provide, including food supplies and tourism revenue that support communities around the globe.

“Levels of heat stress were so extreme during this event that Coral Reef Watch had to create new, higher bleaching alert levels that were not needed during prior events,” said first author C. Mark Eakin, former director of Coral Reef Watch and chief scientific advisor for the Netflix film Chasing Coral.

“Around half of reef locations affected by bleaching-level heat stress were exposed twice or more during the three-year event—often with devastating consequences,” said Scott Heron, professor of physics at James Cook University. “That included back-to-back events on Australia’s Great Barrier Reef. Three more bleaching events have happened there since. We are seeing that reefs don’t have time to recover properly before the next bleaching event occurs.”

Ocean Warming and the Fourth Global Bleaching Event

Over the past 30 years, the planet has lost about 50% of its corals. The oceans absorb most of the excess heat generated by burning fossil fuels. Without that heat absorption, global air temperatures would reach around 50 degrees Celsius (122 degrees Fahrenheit).

Data collected worldwide indicate that Earth is now experiencing a Fourth Global Coral Bleaching Event.

“Our results show that the Third Global Coral Bleaching Event was by far the most severe and widespread coral bleaching event on record,” Connolly said. “And yet, reefs are currently experiencing an even more severe Fourth Event, which started in early 2023.”

Why Global Coral Monitoring Matters

“Local, regional, and global economies rely heavily on the health of natural systems, such as coral reefs, but we often take them for granted,” said Joshua Tewksbury, the director of STRI. “It is vital that science communities come together, like this global team has done, to track how these critical systems are changing. Doing this well, and at scale, requires connecting geographies and combining technologies—from Earth observation satellites to in-the-water surveys that calibrate observations from space and show us the extent of the damage.”