By Will McCarry
September 23, 2025
What on Earth is ‘coral bleaching’?
7 min
Editor’s note: From “blue carbon” to “ecosystem services,” environmental jargon is everywhere. Conservation International looks to make sense of it in an explainer series we’re calling “What on Earth?"
In this installment, we explore coral bleaching, a crisis impacting the world’s reefs.
I keep seeing headlines about coral reefs dying. What’s happening?
Climate change is happening. And it’s placing the world’s reefs in peril.
Why does heat cause corals to bleach?
Well, remember, corals aren’t just colorful rocks — they’re living things, made of hundreds or thousands of individual animals called polyps. Each polyp is only a few millimeters across and is shaped a bit like a miniature sea anemone.
Like most animals, they’re sensitive to big temperature swings. But there’s a strange twist behind the bleaching: Polyps host even tinier organisms inside them — microscopic algae that use sunlight to make food. The algae feed the corals, while the corals give the algae nutrients and shelter.
Dead coral in the Pacific Ocean.
What happens to the reef after corals bleach?
When a coral bleaches, the polyps inside are either starving or already dead. What’s left behind is a kind of skeleton — the stony structures it built while alive. At first glance, a tourist visiting a famous reef may not even notice the difference, with all the branching structures still in place. But without the living coral, it’s a graveyard.
But wait. Coral reefs are home to lots of fish. What happens to all of them?
When reefs crumble, vibrant ecosystems vanish with them.
Take the reef away, and you lose that abundance. That’s bad for nature, but it’s also bad for people.
Coral reefs support a diversity of marine life.
What do coral reefs have to do with people?
Reefs also act like natural seawalls. Their limestone structures break up waves before they hit the shore, helping to protect coastlines from erosion and storm surges. Without them, coastal flooding would be far worse.
So, what in the world do we do about it?
Look, it’s simple: we have to stop heating the planet.
Even a small increase in temperature can be catastrophic for coral reefs.
Excuse me? If it gets hot enough pretty much all reefs could vanish?
It’s heartbreaking, but yes.
While remote reefs look very different from their colorful shallow-water counterparts, they are still critical refuges for marine life.
A garden of coral in the Pacific Ocean 2,465 meters (8,090 feet) below the surface.
How do we hold the line for coral reefs while the world sorts out climate change?
Protecting existing reefs from overfishing and pollution keeps them healthy, while restoration projects can nurse damaged corals back to life.
It’s a powerful reminder that while the threats are grave, healthy reefs — and the vibrant life they support — are still within our reach if we act now.
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