A reef can go from a riot of colour to a ghostly white in a matter of weeks. Divers come back from those sites describing the quiet as much as the colour: the constant clicking and chattering of a healthy reef falls away, and what is left looks like a landscape under frost. Bleaching is one of the clearest signals that the sea is under stress, and it is happening more often than reefs can comfortably absorb. It is also a process we understand well — with warning signs you can learn to read, and real work that UK supporters can do about it.

What bleaching actually is

Corals are animals. A reef-building coral is a colony of tiny polyps that build limestone cups around themselves, and most of them live in partnership with microscopic algae called zooxanthellae. Those algae live inside the coral's tissue, photosynthesise, and pass most of the energy they make to their host. In return they get shelter and nutrients. They also supply the coral's colour.

When a coral is stressed — most often by water that stays too warm for too long — that partnership breaks down and the coral expels its algae. Its tissue becomes transparent, and what you are looking at is the white limestone skeleton beneath. That is bleaching. The coral is not necessarily dead, but it is running on empty: without its algae it loses its main food supply and must survive on what it can catch from the water.

Bleaching is not the same as death

If conditions settle within a few weeks, many corals take in new algae from the surrounding water and regain their colour. Some swap in types of algae that tolerate heat better, which is one reason a recovered reef can look different from before. But the repair work costs energy. Growth slows, reproduction pauses, and corals become more vulnerable to disease. A reef that bleaches once and recovers is bruised; a reef that bleaches every other year is being worn down.

Bleaching is not a disease. It is a coral's attempt to survive a bad hand — a gamble on finding tougher algae before its reserves run out.

Why it happens: heat first, other pressures close behind

Marine heatwaves are the main trigger. When sea temperatures sit a degree or two above the usual summer maximum for weeks at a time, the algae inside coral tissue start producing damaging compounds and the coral evicts them. That is why bleaching events often follow strong El Niño years — and why they now turn up in ordinary years too, as background ocean temperatures rise.

Heat is rarely the only problem. Sediment and nutrient runoff from poor land management cloud the water and encourage algae that smother young corals. Overfishing removes the grazing fish that keep algae in check. Coastal construction, dredging, anchor damage and disease all chip away at a reef's resilience.

That combination matters, because the same heatwave does not hit every reef equally. A reef with clean water, intact fish populations and complex structure often bleaches less and recovers faster than one already under pressure. Tackling local threats will not cool the ocean, but it buys a reef time.

Signs to look for on a reef

Bleaching announces itself in stages, and knowing the sequence helps you judge what you are seeing.

• Fading colour. Corals lose depth of colour and turn pastel — a yellow brain coral looks washed out, brown corals go beige.
• Fluorescent patches. Some corals produce bright pink, blue or purple pigments as a stress response, so a struggling colony can look vivid rather than pale. Beautiful, but a warning.
• Stark white colonies. Fully bleached coral is bone white and you can see the skeleton's structure through the tissue. It looks painted rather than alive.
• Partial bleaching. Often the sunlit top of a colony goes white while the shaded underside keeps its colour. Branching corals such as staghorn types are usually first to go.
• Algal fuzz. If the heat continues, dead skeletons are quickly colonised by fine turf algae. Bare white turning fuzzy brown or green is a bad sign, not a recovery.

Not every pale coral is bleached. Sediment, disease, freshwater runoff and naturally light-coloured species can all look similar. Photograph what you find, note the depth and the date, and report it — those records are genuinely useful.

How this connects to the UK

Britain has no tropical reefs of its own, but it has reefs. Cold-water coral reefs built by species such as Lophelia pertusa grow in deep, dark water off Scotland and across the wider north-east Atlantic, and they are vulnerable to trawling damage and changing ocean chemistry in much the same way.

UK waters are warming too. Recent summers have brought marine heatwaves around the British Isles, with unusually high sea surface temperatures recorded in the north Atlantic. And through the British Overseas Territories — the Cayman Islands, British Virgin Islands, Turks and Caicos, Anguilla, Montserrat, Bermuda and the Chagos Archipelago — the UK carries responsibility for a significant share of the world's coral reefs. Decisions made here, from climate policy to marine protection funding, reach those reefs.

What UK reef supporters can do

1. Cut the heat where you can. Home energy, travel and diet are the biggest levers for most households. Insulation and a genuinely renewable electricity tariff, fewer short-haul flights, and more plant-based meals all reduce the emissions driving marine heatwaves.
2. Back reef organisations. Join or donate to a UK-based marine conservation charity, and support protection work in the overseas territories, including the UK's Blue Belt programme. Steady small donations help more than one-off bursts.
3. Volunteer your eyes. Divers and snorkellers can log records with Seasearch; shore volunteers can join beach cleans and intertidal surveys run by local wildlife trusts. Knowing what lives where helps the people arguing for protection.
4. Travel carefully. Pick operators who brief divers on buoyancy and no-touch rules, who moor instead of anchoring, and who never feed fish. Skip coral and shell souvenirs, and steer clear of reef fish you cannot identify as sustainably caught.
5. Think about sunscreen and clothing. Cover up with long-sleeved tops and leggings, and where you need sunscreen, choose a mineral-based product. Several dive destinations have banned ingredients such as oxybenzone for good reason.
6. Use your voice

   Photo: PublicDomainPictures / Pixabay