Coral Bleaching, Explained: Why Reefs Turn White and What Can Still Save Them
The world is currently living through the largest coral bleaching event ever recorded. Since early 2023, marine heatwaves have touched roughly five-sixths of the planet’s reef area — the fourth global bleaching event in history, and by far the most extensive. To understand what is at stake, you need to understand what bleaching actually is, because the name undersells it.
A healthy coral is a partnership. The animal — the coral polyp — hosts microscopic algae called zooxanthellae inside its tissue. The algae photosynthesize and hand over up to 90% of the coral’s food; in return they get shelter and the compounds they need. They are also what give coral its colour. When water gets too hot for too long, the partnership breaks: the stressed coral expels its algae, revealing the white limestone skeleton beneath. That is bleaching.
Bleached is not dead — yet
A bleached coral is starving, not dead. If the water cools within a few weeks, corals can re-acquire their algae and recover, though they emerge weakened and more vulnerable to disease. If the heat persists — or returns year after year, as it now does — the coral dies, and the reef begins eroding into rubble. Recovery of a dead reef is measured in decades; the heat events are now arriving years apart.
The stakes extend far beyond the reef itself. Reefs shelter about a quarter of all marine species, feed and protect roughly a billion people, buffer coastlines from storm waves, and support fishing and tourism economies worth hundreds of billions of dollars a year.
~84%
of global reef area hit by bleaching-level heat since 2023
4th
global bleaching event on record — and the largest
~25%
of all marine species depend on reefs
What can still save them
The IPCC’s numbers are unsparing: at 1.5°C of warming, 70–90% of tropical reefs are projected to be lost. At 2°C, more than 99%. That gap — between most reefs dying and essentially all of them — is decided by emissions, and it is the single most important lever.
Below the global lever sit real local ones. Reefs with clean water and intact fish populations bleach less and recover faster, so curbing overfishing, sewage and agricultural runoff buys time. Restoration science — coral nurseries, selective breeding of heat-tolerant strains, assisted gene flow — is advancing quickly, though it cannot yet operate at the scale of an ocean. The honest summary: local action keeps reefs alive long enough for decarbonization to matter.
