Coral Bleaching, Explained: Why Reefs Are Turning White

Coral reefs get their vivid color from a partnership with microscopic algae living inside their tissue. When ocean conditions turn hostile, corals break that partnership off, and the result is a stark, skeletal white. This is coral bleaching, and it’s now happening more often, and more severely, than at any point on record.

The Relationship Behind the Color

Coral polyps, the tiny animals that build reef structures, host microscopic algae called zooxanthellae inside their tissue. According to Coral Vita, these algae photosynthesize and share the resulting food with the coral, while the coral provides the algae a protected home in return. This partnership is also where most of a reef’s color comes from.

What Actually Happens During Bleaching

When ocean temperatures rise too high for too long, coral becomes stressed and expels the zooxanthellae living inside it. Without that algae, the coral’s tissue turns transparent, revealing the white calcium carbonate skeleton underneath, according to the Great Barrier Reef Foundation.

Bleached coral isn’t automatically dead coral. If temperatures drop back to normal quickly enough, corals can reabsorb algae and recover. But while bleached, most corals can’t feed themselves properly and become far more vulnerable to starvation and disease.

A Reef Under Repeated Stress

The Great Barrier Reef, the world’s largest coral reef system, has now experienced six mass bleaching events since 2016, according to the Australian Institute of Marine Science. Two of those events happened in consecutive years, 2016–2017 and again in 2024–2025, only the second time that’s ever occurred on record.

The 2024 event was the most severe ever recorded on the reef, with extreme bleaching across every region. Follow-up monitoring found regional coral losses between 14% and 30%, with some individual reefs losing more than 70% of their coral, according to Coral Vita.

This isn’t limited to Australia. A global bleaching event that began in February 2023 has affected reefs in at least 82 countries. By April 2025, the International Coral Reef Initiative reported that 84% of the world’s coral reefs had been exposed to bleaching-level heat stress, according to Wikipedia’s summary of the event, compared to about 67% during the previous major global bleaching event in 2014–2017.

Why Back-to-Back Bleaching Is So Dangerous

A single bleaching event is survivable for many reefs, given time to recover. The real danger is repetition. When bleaching strikes in consecutive years, reefs that were already weakened have no chance to rebuild their algae partnerships or regrow damaged tissue before being stressed again.

Some recovery is still possible even after severe events. Roughly 16% of tracked coral colonies on the Great Barrier Reef showed signs of recovery within months of the 2024 bleaching peak, according to Coral Vita, showing that reefs aren’t necessarily doomed even after major stress events.

What’s Being Done

Conservationists point to several strategies to help reefs survive the coming decades:

• Reducing global greenhouse gas emissions to slow ocean warming
• Expanding marine protected areas to reduce non-climate stressors
• Active coral restoration and reef replanting programs
• Selective breeding of more heat-tolerant coral strains

Frequently Asked Questions

Is bleached coral dead coral? Not necessarily. Bleached coral is alive but severely stressed, and can recover if favorable conditions return quickly enough.

What’s the main cause of coral bleaching? Elevated ocean temperatures are by far the leading cause, though pollution and increased UV radiation can worsen bleaching events.

As ocean temperatures continue climbing, scientists warn that the gap between bleaching events keeps shrinking, leaving less and less time for reefs to recover before the next one hits.