Coral reefs are beautiful, vibrant ecosystems and a cornerstone of a healthy ocean. Often called the “rainforests of the sea,” they support an extraordinary diversity of marine life from fish and crustaceans to mollusks, sea turtles and more. Although reefs cover less than 1% of the ocean floor, they provide critical habitat for roughly 25% of all ocean species.
Coral reefs are also essential to human wellbeing. These structures reduce the force of waves before they reach shore, providing communities with vital protection from extreme weather such as hurricanes and cyclones. It is estimated that reefs safeguard hundreds of millions of people in more than 100 countries.
What is coral bleaching?
A key component of coral reefs are coral polyps—tiny soft bodied animals related to jellyfish and anemones. What we think of as coral reefs are actually colonies of hundreds to thousands of individual polyps. In hard corals, these tiny animals produce a rigid skeleton made of calcium carbonate (CaCO3). The calcium carbonate provides a hard outer structure that protects the soft parts of the coral. These hard corals are the primary building blocks of coral reefs, unlike their soft coral relatives that don’t secrete any calcium carbonate.
Coral reefs get their bright colors from tiny algae called zooxanthellae. The coral polyps themselves are transparent, and they depend on zooxanthellae for food. In return, the coral polyp provides the zooxanethellae with shelter and protection, a symbiotic relationship that keeps the greater reefs healthy and thriving.
When corals experience stress, like pollution and ocean warming, they can expel their zooxanthellae. Without the zooxanthellae, corals lose their color and turn white, a process known as coral bleaching. If bleaching continues for too long, the coral reef can starve and die.
Ocean warming and coral bleaching
Human-driven stressors, especially ocean warming, threaten the long-term survival of coral reefs. An alarming 77% of the world’s reef areas are already affected by bleaching-level heat stress.
The Great Barrier Reef is a stark example of the catastrophic impacts of coral bleaching. The Great Barrier Reef is made up of 3,000 reefs and is home to thousands of species of marine life. In 2025, the Great Barrier Reef experienced its sixth mass bleaching since 2016. It should also be noted that coral bleaching events are a new thing because of ocean warming, with the first documented in 1998.
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How you can help
The planet is changing rapidly, and the stakes have never been higher. The ocean has absorbed roughly 90% of the excess heat caused by anthropogenic greenhouse gas emissions, and the consequences, including coral die-offs, are already visible. With just 2℃ of planetary warming, global coral reef losses are estimated to be up to 99% — and without significant change, the world is on track for 2.8°C of warming by century’s end.
To stop coral bleaching, we need to address the climate crisis head on. A recent study from Scripps Institution of Oceanography was the first of its kind to include damage to ocean ecosystems into the economic cost of climate change – resulting in nearly a doubling in the social cost of carbon. This is the first time the ocean was considered in terms of economic harm caused by greenhouse gas emissions, despite the widespread degradation to ocean ecosystems like coral reefs and the millions of people impacted globally.
This is why Ocean Conservancy advocates for phasing out harmful offshore oil and gas and transitioning to clean ocean energy. In this endeavor, Ocean Conservancy also leads international efforts to eliminate emissions from the global shipping industry—responsible for roughly 1 billion tons of carbon dioxide every year.
But we cannot do this work without your help. We need leaders at every level to recognize that the ocean must be part of the solution to the climate crisis. Reach out to your elected officials and demand ocean-climate action now.
The post What is Coral Bleaching and Why is it Bad News for Coral Reefs? appeared first on Ocean Conservancy.
What is Coral Bleaching and Why is it Bad News for Coral Reefs?
This fall, Ocean Conservancy is celebrating its 41st annual International Coastal Cleanup® (ICC), once again bringing together volunteers to protect the beaches and waterways we all love. Ocean Conservancy recently hosted flagship events in Long Beach and Washington, D.C. that celebrated this global movement close to home.
Over the past four decades, nearly 20 million volunteers have rolled up their sleeves to remove more than 405 million pounds of trash from beaches and waterways around the world. And their impact extends far beyond what they collect. ICC volunteers have documented nearly 440 million individual items, creating the world’s largest marine debris database and providing invaluable information that helps Ocean Conservancy and others better understand—and tackle—ocean plastic pollution.
This year’s celebration continued that incredible legacy. At Alamitos Beach in Southern California, more than 150 volunteers gathered on September 19 and removed 1,109 pounds of trash, alongside members of Team USA Artistic Swimming and Olympic and Paralympic athletes. In Washington, D.C., 439 volunteers gathered on September 26 to remove 2,182 pounds of trash from Anacostia Park along the Anacostia River.
“I am so grateful to all of the volunteers who came out today for our ocean—and every volunteer who has made the last 41 years of impact possible,” said Ocean Conservancy’s Senior Director of Conservation Cleanups, Allison Schutes. “Every piece of plastic collected helps protect marine animals and habitats and contributes to research and advocacy.”
And the impact doesn’t stop when the cleanup ends. During International Coastal Cleanup events, volunteers do more than remove trash: They document what they find, helping build a clearer picture of the plastic pollution threatening our waterways and wildlife. Through the Clean Swell® app or paper data sheets, ICC volunteers contribute to Ocean Conservancy’s online, open-access database, the world’s largest repository of marine debris data. These records help scientists, conservation groups and governments understand pollution trends and identify solutions.
That data can also drive meaningful policy change. ICC data were recently used in a peer-reviewed study showing that plastic bag bans lead to reduced plastic bag pollution. The data have also helped advance policies ranging from California’s SB 54 to Florida’s balloon release ban and the federal Farewell to Foam Act. From a neighborhood cleanup to statewide action, every piece of documented trash helps build the evidence needed to tackle plastic pollution at its source.