Understanding Coral Bleaching: What Divers Need to Know

Understanding Coral Bleaching: What Divers Need to Know

If you've dived tropical reefs in the last decade, you've likely seen bleached coral — white, skeletal, or a pale ghostly grey where there should be colour. Coral bleaching is one of the most visible consequences of ocean warming, and understanding what's happening, and what the prognosis is, has become part of every informed diver's baseline knowledge.

What Bleaching Is

Corals are animals — tiny polyps that build calcium carbonate skeletons — but they derive most of their energy from symbiotic algae called zooxanthellae that live inside their tissue. The algae photosynthesize sunlight and share the resulting sugars with the coral. In return, the coral provides shelter and the CO₂ the algae need.

When water temperature rises above the coral's tolerance threshold — typically 1–2°C above the seasonal maximum for a sustained period — the coral expels its zooxanthellae. Without the algae, the coral's tissue becomes transparent, revealing the white skeleton beneath. This is bleaching.

A bleached coral is not dead. It is stressed and starving. If temperatures return to normal within a few weeks, the algae can recolonise and the coral recovers. If elevated temperatures persist, the coral starves and dies — leaving a white skeleton that is eventually colonized by algae, turning it grey and then green.

Why It's Getting Worse

Coral bleaching has always occurred. The problem is frequency. The Great Barrier Reef experienced its first documented mass bleaching event in 1998. Between 1998 and 2010, major bleaching events occurred approximately every 5–7 years — long enough for corals to partially recover between events. Since 2016, bleaching events on the GBR have occurred in 2016, 2017, 2020, 2022, and 2024 — with insufficient recovery time between them.

The 2016 event killed approximately 50% of the shallow-water corals on the northern Great Barrier Reef. Many of those corals have not recovered.

The Reefs Diving on Bleached Coral

Bleached reef looks different from healthy reef, and what 'different' means varies:

• Freshly bleached: Vivid white, sometimes luminescent — temporarily striking, immediately alarming
• Recently dead: White turning grey, beginning to be colonised by filamentous algae
• Long-dead: Grey-green carpet of algae, occasional small corals attempting to recolonise; fish diversity significantly reduced

Divers in Okinawa (Japan) in recent years have encountered full sections of reef that have shifted from category one to category three between consecutive annual visits. The trajectory at poorly protected sites is clear.

What Divers Can Do

The honest answer is: the actions of individual divers have limited direct impact on ocean temperature. The structural response is political and economic. That said:

• Use reef-safe sunscreen (or none) — oxybenzone and octinoxate harm coral even at low concentrations
• Never touch coral — the oil from hands damages the mucous layer; bleached coral is especially fragile
• Support marine protected areas — the evidence that MPAs increase reef resilience to bleaching events is strong
• Contribute to monitoring programs — citizen science programs like CoralWatch train divers to report bleaching observations systematically
• Choose operators with conservation commitments — operators who train crew in coral monitoring and eliminate anchor damage make a measurable difference

The Prognosis

At 1.5°C of global warming (the Paris Agreement threshold), approximately 70–90% of coral reefs are projected to decline significantly. At 2°C, over 99% of tropical coral reefs are projected to experience annual bleaching — too frequently to recover. The scientific consensus is that avoiding the worst outcomes requires immediate and substantial emissions reductions. The reefs divers are visiting now are not the reefs their children will visit at current trajectories.