The fourth global coral bleaching event on record began in early 2023 and is still, at the time of writing, running. It has affected reef systems across at least 84 countries and every ocean basin that contains them. It has now surpassed the previous longest event on record and shows no clear sign of ending.

What that means, in plain terms, is that most of the world’s coral reefs have been under continuous heat stress for more than two and a half years.

What bleaching actually is

A coral is an animal — a soft-bodied polyp — that lives in partnership with a photosynthetic alga inside its tissue. The alga provides most of the coral’s energy through photosynthesis; the coral provides shelter and nutrients. The colours of a healthy reef come from those algae.

When water temperatures rise even a small amount above the local summer maximum for long enough, the relationship breaks down. The coral expels the alga. Without it, the polyp becomes translucent and the white calcium carbonate skeleton beneath shows through — the bleaching.

Bleached coral is not dead. If temperatures return to normal within a few weeks, the alga can recolonise and the coral survives, sometimes weakened. If the heat stress continues, or if bleaching events come too close together for recovery, the coral dies. Repeat bleaching is what turns a reef into a ruin.

The scale of this event

The National Oceanic and Atmospheric Administration’s Coral Reef Watch program declared the fourth global bleaching event in April 2024, retroactive to January 2023. By late 2025 it had affected more than 80 per cent of reef area worldwide — a number that has continued to climb.

The Great Barrier Reef in Australia has recorded consecutive mass bleachings — 2016, 2017, 2020, 2022, 2024 and 2025 — a pattern that until this decade had not been seen. The reefs of the Caribbean, the western Indian Ocean, the Red Sea and the Persian Gulf have all been badly affected. Some of these are systems for which no significant historical bleaching record exists at all.

Why it matters beyond reefs

Reefs occupy less than one per cent of the ocean floor. They support around a quarter of marine species. Roughly a billion people benefit from them directly — through fisheries that depend on reef nurseries, through the tourism economies of small island states, and through the coastal protection reefs provide against storm surges. When a reef degrades, that protection weakens, and every hurricane afterwards does more damage than it otherwise would.

The economics are not marginal. Reef degradation is a slow, distributed cost that shows up as smaller fish catches, more expensive coastal defence, and lost income in places with few alternatives. It is a decline nobody has to declare; it just happens.

What could stop it

The honest answer is only two things, and one of them is very slow.

The proximate cause of every mass bleaching is ocean heat, and the ultimate cause of ocean heat rising faster than at any point in the observational record is atmospheric greenhouse gases. Reducing emissions eventually stabilises ocean temperatures. Eventually.

The other lever is local: reducing pollution, controlling overfishing, protecting the fish species that graze algae, restricting anchoring on reefs. These do not stop bleaching, but they help reefs recover between events. Recovery becomes harder each time bleaching returns before the reef has caught its breath, which is the pattern that has now become normal.

A great deal of research money and hope is being spent on more speculative interventions — assisted evolution, larval seeding, cloud brightening, artificial reef structures. Some of it is promising at small scale. None of it is anywhere near the scale that would matter.

Sources

NOAA Coral Reef Watch program declarations and updates; International Coral Reef Initiative reporting; published research on mass bleaching frequency and reef recovery; Great Barrier Reef Marine Park Authority.