84% of coral reefs are bleaching as oceans warm

What you probably already know: Earth’s colorful reefs are transforming into bone-white expanses at an alarming pace as oceans warm. In April, the U.S. National Oceanic and Atmospheric Administration (NOAA) and the International Coral Reef Initiative (ICRI) announced that nearly 84% of the world’s coral reef area across at least 83 countries and territories has been impacted by bleaching-level heat stress since January 2023. It marks the largest coral bleaching event ever recorded and scientists aren’t sure when, or if, it will end.

Why? Bleaching occurs when corals are stressed, causing them to expel the microscopic algae living within their tissues. The algae, zooxanthellae, provide coral with food through photosynthesis. Without them, corals become transparent and are vulnerable to disease and starvation, slowing their recovery and impeding their ability to reproduce. Bleaching doesn’t outright kill corals: they can recover if conditions improve and their symbiotic algae return. While the phenomenon can be triggered by changes in light or nutrients, warming ocean temperatures are the leading cause of mass bleaching events. An increase of one degree Celsius for just four weeks can trigger the response. Scientists have recorded four global bleaching events caused by heat stress to date, including the one that began in 2023. The first and second mass bleachings occurred in 1998 and 2010, respectively. The third took place from 2014 to 2017 and set the former record, with approximately two thirds of the world’s reef area experiencing bleaching-level heat stress.

What it means: NOAA said the latest bleaching event impacted Florida, the Caribbean, Brazil, the eastern Tropical Pacific, large swaths of the South Pacific, the Red Sea, the Persian Gulf, the Gulf of Aden, and parts of the Indian Ocean. It also affected Australia’s Great Barrier Reef, the world’s largest coral reef system and one of the richest, most complex ecosystems on Earth. Though coral reefs cover just 1% of the planet, about a quarter of all marine species depend on them for food, shelter, and reproduction, earning reefs the nickname “rainforests of the sea.” They also protect coastlines from erosion and storm damage. Reefs support millions of people whose livelihoods revolve around fisheries, tourism, and other marine industries. The Great Barrier Reef alone supports over 64,000 jobs, contributing $6.4 billion to Australia’s economy, according to Deloitte Access Economics. Corals are a keystone species; their collapse would be felt by marine and terrestrial life around the world.

What happens next: There is no end in sight for the current global bleaching event. 2024 was Earth’s hottest year on record, with surface temperatures temporarily reaching 1.5 degrees Celsius warmer than the preindustrial era average, thus hitting the cautionary limit set forth by the 2015 Paris Agreement as a global standard for curbing climate change. Ocean heat content reached record highs during each of the last eight years, with the rate of ocean warming more than doubling from 2005 to 2024 compared to the period from 1960 to 2005, according to the World Meteorological Organization. Since 2009, the Global Coral Reef Monitoring Network estimates that Earth has lost 14% of its corals. “We may never see the heat stress that causes bleaching dropping below the threshold that triggers a global event,” Mark Eakin, corresponding secretary for the International Coral Reef Society and retired chief of NOAA’s Coral Reef Watch program, told the Associated Press. “We’re looking at something that’s completely changing the face of our planet and the ability of our oceans to sustain lives and livelihoods.”