What causes coral bleaching? Here's how it threatens ocean and human life

National Geographic

08-07-2025

Sea water divides the sunset and a ghost land of corals in New Ireland, Papua New Guinea. Ocean acidification has increased coral bleaching around the world. Photograph by Juergen Freund, Nature Picture Library
Coral reefs aren't just marine biodiversity hotspots, they're critical to human life. When reefs die, the impact ripples across food systems, local economies, and climate resilience, especially in coastal communities.
Here's everything you need to know about bleaching, its impacts on marine life and human communities, and how we can save our reefs. What is coral bleaching?
Tropical coral reefs are known for their rainbow of reds, oranges, pinks, and purples, which are produced by a microscopic algae that lives inside the coral tissue.
'Corals have this partnership with a tiny little algae called zooxanthellae,' says Molly Timmers, a marine ecologist for National Geographic Society's Pristine Seas project. In this symbiotic relationship, the algae inside the coral converts sunlight into food through photosynthesis, and shares this energy with its host. Up to 90 percent of a coral's energy comes from the zooxanthellae, also known as algal symbionts.
Certain changes, especially increased ocean temperature, can upset this delicate balance. Prolonged heat stress causes corals to expel the algae living in their tissues and turn white, becoming highly vulnerable.
'When coral gets stressed, it's like you and I getting sick,' says Timmers. 'We sweat when we're recovering from something.' The coral expels the algae as a stress response. Without it, the coral loses its color and main source of food.
(These photos show what happens to coral reefs in a warming world.)
When coral bleaches, it isn't dead—yet. 'They're on life support,' says Michael Sweet, professor of aquatic biology at University of Derby in the United Kingdom.
Bleaching impacts a coral's ability to reproduce and to create mucus, making it more susceptible to disease. In the ocean, 'corals are bathed in this microbial soup,' he says. Like snot in a human's nose, mucus helps them capture and get rid of harmful bacteria. 'The mucus is the first line of defense.'
If normal environment conditions return quickly, the algae do too. If not, the coral can quickly starve to death. 'It can shut down and just give up, and then it dies quite instantly,' says Sweet. Bleached corals line the reef in Koh Mak, Thailand. Global warming is driving higher temperatures, warming coral so much that they expel the zooxanthellae living in their tissues and turn white. Photograph by Napat Wesshasartar, Reuters/Redux Bailey Thomasson, a restoration coordinator with the Coral Restoration Foundation, scuba dives above bleached corals at Looe Key Reef. Located off Florida's Big Pine Key, Looe Key was the epicenter of massive coral reef bleaching event in summer of 2023 with nearly 100 percent of corals dying. Photograph by Jason Gulley, The New York Times/Redux A school of Cavalla swim above bleached Stony Coral near Heron Island, Great Barrier Reef, Australia. Two-thirds of the Great Barrier Reef have been impacted by coral bleaching as it disrupts the mutually beneficial relationship between corals and the algae that live inside them. Photograph by Gary Bell, Oceanwide/Minden Pictures
Coral reefs support 25 percent of the world's marine life. Their structures provide a home, feeding, breeding, and nursery grounds for many fish—housing that's a good deal more efficient compared to the flat seabed. 'You can house more people in a 20-story apartment building than a one-story building,' says Timmers.
When corals die and animals lose their home, mobile species migrate and those who can't move might die out—disrupting the food web. 'Things get out of whack,' she says.
Coastal communities lose their main food source as well as livelihoods dependent on tourism and hospitality.
Their loss can have a cultural impact for Indigenous communities who value natural ecosystems. The Hawai'ian story of creation tells that polyps—the individual organisms that make up the coral colony—were the first animals created. Corals being the very first thing to appear from the darkness demonstrates their importance to the community.
(How trash from ancient humans is protecting these coastal islands today.)
The disappearance of coral also puts coastal infrastructure at risk. Reefs act as natural breakwaters that can reduce wave energy by 97 percent. Without reefs buffering the shoreline, waves hitting land are more powerful. 'Seafaring people know that when you have fringing reefs, the wave energy is stopped before your community,' says Timmers.
Stronger waves pummeling the coastline also increases the risk of erosion and flood damage. Sediment covers part of this Brain coral as it experiences coral bleaching. Healthy coral colonies are a green-brown color and clearly show the coral's grooves. Photograph by Norbert Wu, Minden Pictures
Scientists are developing various ways of protecting corals from bleaching. One method involves shading corals from the hot sun using underwater parasols made from cloth. Some experts are preserving species in controlled 'biobanks' to keep them safe from extreme conditions in the wild. Others are supporting restoration efforts by breeding or moving heat-tolerant corals to new areas. Marine protected areas, fisheries management, and pollution measures are also important.
Some researchers are even experimenting with a method known as known as cloud brightening, or manipulating the clouds above reefs to make them more reflective and therefore keeping the waters cooler.— However, critics worry about potential unintended consequences, such as changes to weather patterns.
'Prevention is better than cure,' says Sweet. 'We need to tackle climate change. That should always be front and center.'
Experts say if we take urgent action now, reefs around the world can recover and thrive. 'It is devastating, what is happening,' says Timmers, 'but there's still hope.'