Half the World’s Coral Reefs Bleached in a Single Three-Year Event, Landmark Study Confirms

The numbers are in, and they are worse than previous estimates suggested. A massive international study published February 10, 2026, in Nature Communications has produced the first comprehensive global estimate of coral reef damage during the Third Global Coral Bleaching Event (2014–2017), and the findings confirm what reef scientists had feared: roughly half the world’s coral reefs suffered significant bleaching during a single three-year marine heat wave.

The research team, led by scientists at the Smithsonian Tropical Research Institute, James Cook University in Australia, and NOAA’s Coral Reef Watch, analyzed data from 15,066 reef surveys conducted across dozens of countries during the event. Across all surveyed reefs, 80% experienced moderate or greater bleaching (affecting more than 10% of corals), and 35% suffered moderate or greater mortality.

To estimate global impact beyond just the surveyed sites, the team calibrated bleaching response curves using the survey data and applied them to comprehensive satellite measurements of ocean heat stress. Those models predicted that 51% of the world’s coral reefs endured moderate or worse bleaching, and approximately 15% experienced significant mortality, during one or multiple years of the event. Both figures surpass damage from any previously recorded global bleaching event.

The 2014–2017 event was the first global bleaching episode to last substantially longer than a single year. The two prior global events, in 1998 and 2010, had each lasted roughly one year. The extended duration meant that some reef locations were hit by repeated heat stress across the three-year period, leaving little time for recovery between episodes.

“This is the most geographically extensive analysis of coral bleaching surveys ever done,” said Sean Connolly, a senior scientist at the Smithsonian and one of the study’s lead authors. He added a pointed warning: “And yet, reefs are currently experiencing an even more severe Fourth Event, which started in early 2023.”

Coral bleaching occurs when heat stress causes corals to expel the symbiotic algae (zooxanthellae) that provide them with energy and color. Without these partners, the coral turns white and becomes nutritionally compromised. If temperatures do not return to tolerable levels, the coral starves and dies. Recovery of reef structure, when it happens at all, can take decades.

The economic context intensifies the urgency. Benefits to society from coral reefs, including fisheries, tourism, coastal protection, and pharmaceutical discovery, are estimated at roughly $9.8 trillion per year, according to the Smithsonian’s press release accompanying the study.

Scott Heron, professor of physics at James Cook University and a co-author, underscored the accelerating pace: “We are seeing that reefs don’t have time to recover properly before the next bleaching event occurs.”

With heat stress levels during the ongoing Fourth Global Coral Bleaching Event already comparable to or exceeding those from 2014–2017 in some regions, NOAA has expanded its Bleaching Alert system to capture more extreme thresholds. The study’s authors are clear about the trajectory: without dramatic reductions in greenhouse gas emissions, large-scale and possibly irreversible degradation of coral reef ecosystems is not a question of if, but when.

Written by: Junior Thanong Aiamkhophueng

Attribution: This article draws on the peer-reviewed study “Severe and widespread coral reef damage during the 2014–2017 Global Coral Bleaching Event” published in Nature Communications (February 10, 2026), led by the Smithsonian Tropical Research Institute, James Cook University, and NOAA Coral Reef Watch; reporting from Phys.org and the Smithsonian’s media release on the study findings; reef valuation data from the Smithsonian; and supplementary scientific context on the Fourth Global Coral Bleaching Event from NOAA Coral Reef Watch. Bleaching comparison imagery from Kiritimati (Christmas Island) by Kieran Cox and Kristina Tietjen. For further reading, visit Nature Communications (DOI: 10.1038/s41467-025-67506-w) and the Smithsonian Tropical Research Institute.