An estimated half of the world’s coral reefs experienced significant bleaching during the 2014–2017 global marine heatwave, according to a major international study led by researchers at the Smithsonian Tropical Research Institute (STRI). The findings, published in Nature Communications, represent the most geographically extensive analysis of coral bleaching ever conducted.
The study concludes that more than 50% of coral reefs worldwide suffered substantial bleaching during what scientists call the “Third Global Coral Bleaching Event,” with approximately 15% experiencing significant coral mortality.
Researchers warn that a fourth global bleaching event, which began in 2023, is already underway.
Coral reefs provide enormous benefits to society, supporting fisheries, tourism, coastal protection and pharmaceutical research. These services are estimated to be worth nearly $9.8 trillion annually.
However, coral bleaching threatens these ecosystems at their foundation.
“It takes two partners to make a coral,” scientists explain. A coral is composed of a tiny animal related to jellyfish and microscopic algae that live inside its tissues. The algae convert sunlight into energy that sustains the coral.
When ocean temperatures rise beyond normal levels, this delicate partnership breaks down. The coral expels its algal symbionts, losing its primary energy source and turning white a process known as bleaching. If heat stress persists, the coral may die.
Bleaching reduces coral growth and reproduction and can trigger widespread reef decline when repeated or prolonged.
To measure the extent of damage during the 2014–2017 event, scientists combined satellite-derived ocean temperature data from NOAA’s Coral Reef Watch system with more than 15,000 reef surveys conducted in water and via aerial assessments around the world.
The effort involved nearly 200 co-authors from 143 institutions across 41 countries and territories.
“This is the most geographically extensive analysis of coral bleaching surveys ever done,” said Sean Connolly, senior scientist at the Smithsonian.
The results were stark:
• 80% of surveyed reefs experienced moderate or greater bleaching.
• 35% experienced moderate or greater coral mortality.
• Globally, more than half of all coral reefs suffered significant bleaching.
After calibrating field survey data with satellite heat measurements, researchers were able to estimate bleaching severity even in areas without direct observations.
Extensive mortality was documented in locations such as Kiritimati (Christmas Island), Kiribati, where reefs showed dramatic decline following record heat stress.
The 2014–2017 event was so intense that NOAA’s Coral Reef Watch system had to introduce new, higher alert levels to account for unprecedented heat stress.
“Levels of heat stress were so extreme during this event that new bleaching alert categories were required,” said C. Mark Eakin, former director of Coral Reef Watch and the study’s lead author.
In many regions, reefs were exposed to bleaching-level heat stress more than once during the three-year event, compounding damage.
“Around half of reef locations affected by bleaching-level heat stress were exposed twice or more,” said Scott Heron, professor at James Cook University in Australia. “Reefs don’t have time to recover properly before the next bleaching event occurs.”
Australia’s Great Barrier Reef, for example, experienced back-to-back bleaching during the study period and has endured three additional bleaching events since.
Entering a fourth global bleaching event
Over the past three decades, Earth has lost roughly 50% of its corals, largely because the oceans absorb most of the excess heat generated by greenhouse gas emissions from fossil fuel use.
Scientists note that without the ocean absorbing this heat, average air temperatures would be dramatically higher potentially reaching levels incompatible with human life.
“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.”
Marine heatwaves are also affecting reef fish populations and broader marine ecosystems, adding to the cascading impacts.
The importance of global collaboration
Researchers emphasize that understanding reef decline at a planetary scale requires coordinated international effort and advanced technology.
“It is vital that science communities come together to track how these critical systems are changing,” said Joshua Tewksbury, director of STRI. “Doing this well, and at scale, requires connecting geographies and combining technologies from Earth observation satellites to in-the-water surveys.”
The findings underscore growing concerns that repeated heat stress events are reducing the resilience of coral reefs worldwide, threatening biodiversity, coastal economies and food security.
As a fourth global bleaching event continues, scientists warn that without rapid reductions in greenhouse gas emissions, coral reef ecosystems may face increasingly severe and frequent heat-driven crises.