Bleaching is frequently described as coral dying, which is not quite accurate and obscures what is actually happening.
The partnership
Reef-building corals are animals hosting single-celled algae in their tissues.
The algae photosynthesise and provide the coral with a large proportion of its energy.
The coral provides shelter and nutrients.
Which is why reef corals grow in shallow clear water — the algae need light.
The algae also provide most of the colour, since coral tissue itself is largely transparent over a white skeleton.
What bleaching is
Under stress, the coral expels the algae or the algae lose their pigments.
The white skeleton becomes visible through the transparent tissue, which is the bleached appearance.
The coral is alive at this point but has lost its main energy source.
Which means it can survive for a period on stored reserves and by feeding, and it will die if conditions do not improve.
The trigger
Elevated water temperature is the principal cause, and the threshold is surprisingly narrow.
Sustained temperatures a small amount above the normal summer maximum are sufficient.
Which is why monitoring uses accumulated heat stress over time rather than peak temperature, and forecasting systems publish these.
Other stressors — pollution, sedimentation, extreme low tides, disease — also cause bleaching, generally at more local scales.
Recovery
If temperatures return to normal within weeks, corals can regain algae and recover.
Recovery takes years for the reef structure, since growth is slow and mortality removes established colonies.
Which means the interval between bleaching events determines whether recovery is possible.
Events that once occurred decades apart now occur within years in several regions, which is the more consequential change than any individual event.
Variation in tolerance
Coral species differ substantially in heat tolerance.
Fast-growing branching corals are generally more susceptible. Slower massive corals are more resistant.
Which means bleaching changes reef composition rather than simply reducing it, and reefs that recover frequently do so with a different community.
Some algal strains confer greater heat tolerance, and corals hosting them bleach less, which is an active research area.
Intervention research
Several approaches are being investigated.
Selective breeding for heat tolerance, and assisted gene flow moving tolerant individuals to other reefs.
Manipulating the algal community toward more tolerant strains.
Larval reseeding of damaged areas.
And local shading or cooling, which is only feasible at very small scales.
All are at research or trial stage, and none addresses the cause.
Local pressures
Reefs facing less local stress recover better from thermal events.
Which is why water quality management, fishing regulation and reducing physical damage remain worthwhile even though they do not address temperature.
Herbivorous fish in particular prevent algae overgrowing damaged reef, which is required for coral to recolonise.
What the evidence indicates
Local action improves resilience and recovery.
The frequency and severity of thermal stress is determined by global temperature.
Which means both matter and they operate on different timescales, and neither substitutes for the other.
Measuring the stress
Satellite sea surface temperature data allows accumulated heat stress to be calculated and forecast.
Which produces alerts giving weeks of warning, used for monitoring deployment and for management responses.
The metric accumulates the amount by which temperature exceeds a threshold over time, which captures duration as well as magnitude.
Other pressures on reefs
Ocean acidification reduces the availability of the carbonate corals use to build skeletons, which is a separate consequence of the same underlying cause.
Nutrient runoff promotes algal growth that competes with coral.
Sedimentation smothers and blocks light.
Destructive fishing damages structure directly.
Crown-of-thorns starfish outbreaks consume coral, and outbreak frequency has been linked to nutrient enrichment.
What reefs provide
Coastal protection, with wave energy reduction that has been quantified and valued.
Fisheries supporting substantial numbers of people.
Tourism revenue.
Which means reef loss has direct human consequences, and the economic valuations have been used in policy arguments.
Deep and mesophotic reefs
Coral communities at greater depths experience less thermal stress.
Which has generated interest in whether they could act as refuges and reseed shallow reefs.
Evidence is mixed — the species composition differs substantially, so deeper reefs may not contain the shallow species that need reseeding.
Cold water corals
Reef-forming corals also exist in deep cold water without symbiotic algae.
Which means they do not bleach and are threatened by bottom trawling and by acidification.
They are far less studied than tropical reefs and support substantial biodiversity.
Restoration efforts
Coral gardening — growing fragments in nurseries and transplanting them — has been widely deployed.
Which produces measurable local results at small scale and cannot operate at the scale of reef loss.
Its value is contested between those who see it as buying time and those who see it as a distraction from the cause.
Which is a reasonable disagreement, and both positions accept that emissions determine the long-term outcome.