Coral bleaching

• 1. Introduction

  ›

  Coral bleaching is the process by which a coral turns ghostly white, stripped of the colour that once made it one of the most visually striking ecosystems on Earth. The cause is not a disease or a predator, at least not in the familiar sense. It is a collapse of a partnership that has sustained tropical reefs for millions of years.

  Between 2014 and 2016, the United Nations Environment Programme documented the longest recorded global bleaching event in history. In 2016 alone, bleaching killed between 29 and 50 percent of the coral on the Great Barrier Reef. Then came a second record: the 2023-2025 bleaching event, confirmed as the fourth global event, touched reefs in at least 82 countries and territories. By April 2025-84 percent of the world's reefs had been exposed to bleach-level heat.

  What exactly happens inside a coral when temperatures rise? Why does a reef that survived ice ages buckle under a single unusually warm summer? And what happens to the quarter of all ocean species that shelter within those reefs when the structure they depend on begins to crumble? Those are the questions this documentary sets out to answer.

• 2. The Partnership That Powers the Reef

  ›

  Inside every reef-building coral lives a single-celled organism called a zooxanthella. These microscopic algae-like flagellate protozoa are not passengers. They provide up to 90 percent of a coral's energy needs through photosynthesis. In exchange, the coral supplies carbon dioxide and ammonium, the raw materials the zooxanthellae need to photosynthesize in the nutrient-poor, sunlit waters of the tropics.

  This arrangement works beautifully under stable conditions. The zooxanthellae give the coral its colour. They fuel its growth. They are, in almost every meaningful sense, what makes a coral reef possible.

  When the partnership breaks down, a bleached coral is not simply pale. According to research by D.J. Smith and colleagues, photoinhibition plays a likely role in the rupture: the zooxanthellae themselves may signal their departure through the hydrogen peroxide they produce. To survive in the short term, the coral consumes or expels the algae. Without them, the animal begins to starve.

  Mild stress produces a stranger result. Some corals, rather than turning white, fluoresce in blue, pink, purple, or yellow. These colours come from the coral's own intrinsic pigment molecules, a phenomenon researchers call "colourful bleaching." Blue light penetrates the now-algae-free tissue and stimulates the production of sun-screening pigments. The result is a reef that can look, briefly, more vivid than it ever did in health, even as the coral beneath is fighting to survive.

• 3. What Triggers a Bleach

  ›

  Elevated sea surface temperature is the leading cause of mass bleaching events. Between 1979 and 1990, sixty major episodes of coral bleaching were recorded, affecting reefs in every part of the world. Marine heatwaves driven by the El Nino Southern Oscillation have been identified as one of the main engines of widespread bleaching and consequent coral mortality.

  The list of other triggers is long and partly surprising. It includes increased solar radiation, changes in salinity, bacterial infections, sedimentation from silt runoff, cyanide fishing, herbicides, and extreme low tide. Several common sunscreen ingredients, among them oxybenzone, butylparaben, octyl methoxycinnamate, and enzacamene, are nonbiodegradable and wash off swimmers' skin directly onto reefs. Up to one-tenth of the estimated 14,000 tons of sunscreen entering coral reef areas contains oxybenzone, putting close to half of all coral reefs at risk of exposure. In May 2018, Hawaii responded by passing bill SB-2571, banning the sale of sunscreens containing oxybenzone and octinoxate on the islands. The bill was signed by Governor David Ige and took effect in January 2021, making Hawaii the first U.S. state to introduce such a ban.

  Ocean acidification operates through a separate but reinforcing mechanism. When humans burn fossil fuels, carbon dioxide dissolves into the sea and disturbs the natural equilibrium that buffers the ocean's pH. A more acidic ocean strips carbonate ions from the water, making it harder for corals to absorb the calcium carbonate they need to build and maintain their skeletons. An eight-week experiment at Heron Island off the coast of Australia found that 40-50 percent of crustose coralline algae and Acropora bleached following high-CO2 dosing. Acidification and thermal stress do not merely coexist. They work together, each amplifying the damage the other causes.

• 4. A Planet-Scale Pattern

  ›

  The IPCC Sixth Assessment Report of 2022 stated plainly that since the early 1980s, the frequency and severity of mass coral bleaching events have increased sharply worldwide. The report projected that many reefs will undergo irreversible changes under global warming levels above 1.5 degrees Celsius. The IPCC had already identified bleaching as the greatest threat to the world's reef systems in 2007.

  The Great Barrier Reef experienced bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, 2006, 2016, 2017, 2022, 2024, and 2025. Some locations suffered mortality up to 90 percent. Near Port Douglas in 2016, more than half of bleached corals died on average. A survey of 462 coral colonies at One Tree Island, tracked from early 2024 through July 2024, found that 193 had died and 113 were showing active signs of bleaching, while only 92 remained unaffected.

  In Japan, about 94 percent of corals on Iriomote Island bleached in 2016. Almost 75 percent of Japan's largest coral reef in Okinawa subsequently died. In the summer of 2024, sea temperatures 2 degrees higher than those recorded in 2023 killed 61.2 percent of corals off Amami-Oshima island.

  In the Caribbean, hard coral cover fell from an average of 50 percent in the 1970s to roughly 10 percent in the early 2000s, a loss of about 80 percent. A 2013 study following a 2010 bleaching event in Tobago found that within a single year, the dominant coral species declined by about 62 percent and coral abundance fell by about 50 percent.

  The 2023-2025 event reached temperatures as high as 101 degrees Fahrenheit (38.3 degrees Celsius) in Florida, where some reefs experienced complete die-offs. The economic implications are substantial: coral reefs contribute approximately $2.7 trillion annually to the global economy, including $36 billion from tourism alone.

• 5. Infectious Aftermath

  ›

  A bleached coral is not simply weakened. It becomes a target. Bleaching events regularly trigger surges in infectious disease among coral populations, because the stress that drives out the zooxanthellae also lowers the coral's defences against pathogens.

  The first large disease outbreak in corals was observed in 1975 at Carysfort Reef in the Florida Keys, affecting six reef-building species. After the 2014 bleaching event off the shores of Florida, white-plague disease reached a prevalence of 61 percent across 14 survey sites, and at least 13 coral species were affected.

  Some bacteria go further than exploiting weakened corals. The species Vibrio shiloi is the bleaching agent of Oculina patagonica in the Mediterranean Sea. It attacks the zooxanthellae directly. The bacterium is only infectious during warm periods. Rising temperature increases its virulence, allowing it to adhere to a receptor in the coral's surface mucus, penetrate the epidermis, multiply inside the tissue, and produce toxins that inhibit photosynthesis and cause the zooxanthellae to lyse. During the summer of 2003, corals in the Mediterranean appeared to develop resistance to V. shiloi, and infection was not observed again. Researchers believe the most likely explanation is the presence of protective symbiotic bacteria living within the coral tissues.

• 6. After the Bleach

  ›

  Recovery from bleaching is not simply a matter of temperatures cooling. The zooxanthellae must re-enter the coral's tissues and restart photosynthesis. If conditions persist too long, the coral polyps die of starvation. Hard coral species then leave behind calcium carbonate skeletons, which algae colonise, actively blocking coral regrowth. Eventually, even the skeletons erode, and the physical structure of the reef collapses.

  Research by Graham and colleagues in 2015 tracked 21 reefs around the Seychelles in the Indo-Pacific following the loss of more than 90 percent of corals in 1998. About 50 percent of the reefs recovered; roughly 40 percent experienced regime shifts to macroalgae. The study identified five factors that distinguished recovering reefs from those that shifted: the density of juvenile corals, initial structural complexity, water depth, the biomass of herbivorous fish, and nutrient conditions. Structural complexity and greater depth were the strongest predictors of resilience.

  Fish populations respond differently from the coral itself. Research by Bellwood and colleagues found that while species richness and abundance in reef fish assemblages changed little after bleaching, the communities shifted toward generalist species and away from coral-dependent specialists. Smaller, specialised fish that fill narrow ecological niches crucial to reef health are replaced by more generalist species, reducing the reef's capacity to recover from the next disturbance.

  A restoration project in Maui illustrated what recovery can mean in practice: the project led to a 47 percent increase in annual visits to the area and an island-wide welfare gain of $2.9 million, averaging to $26 per resident. The commercial value of reefs, according to a study by Chen and colleagues, falls by almost 4 percent for every 1 percent decline in coral cover, driven primarily by losses in ecotourism and other outdoor recreational activities.

• 7. Building Resilience

  ›

  Ruth Gates spent years asking whether corals could be coached into surviving the conditions that now kill them. Working at the Gates Coral Lab at the Hawaii Institute of Marine Biology, Gates and collaborator Madelaine Van Oppen ran experiments attempting to produce what they called "super corals" capable of withstanding the stressors currently driving coral to extinction. Van Oppen also worked on developing a strain of algae that could tolerate sustained water temperature fluctuations and maintain a symbiotic relationship with coral. Growing corals in research tanks is slow. It takes at least 10 years for corals to fully mature and begin breeding. Following Gates' death in October 2018, her team continued the work. By 2019, the lab had concluded that large-scale restoration techniques were not yet practical, and that localised, individual-reef efforts were more realistic while broader solutions were still being developed.

  In 2010, researchers at Penn State discovered corals thriving in the warm waters of the Andaman Sea using an unusual species of symbiotic algae that normal zooxanthellae cannot tolerate. Researchers from Stanford University found corals near Ofu Island in American Samoa that survived dramatic daily temperature spikes during low tide. Those corals had, in effect, been conditioned by repeated exposure to high heat.

  In 2020, scientists reported evolving 10 clonal strains of a common coral microalgal endosymbiont at elevated temperatures over four years, increasing their thermal tolerance. Three of those strains raised the bleaching tolerance of coral host larvae after reintroduction. In 2021, researchers demonstrated that probiotics can help corals mitigate heat stress, suggesting another potential tool for resilience.

  At the policy level, Marine Protected Areas managed by government have produced a 62 percent increase in coral populations in some Caribbean sites, according to research published in 2018. MPAs defend reefs from overfishing, which allows herbivorous fish populations to control seaweed density and give young corals room to grow. NOAA currently monitors 190 reef sites around the globe through its Coral Reef Watch programme, sending early-warning alerts to researchers and reef managers when sea surface temperatures rise 1 degree Celsius or more above long-term monthly averages, giving those on the ground time to prepare before the worst damage sets in.