Coral bleaching happens when corals lose their color and turn white because they no longer have algae that live with them or the pigments that help them make food. This loss of color can be caused by stress from changes in water temperature, light, salt levels, or nutrients. When corals bleach, they become weak, more likely to starve or get sick, and may die. The main reason for coral bleaching is increasing ocean temperatures caused by climate change. The ocean also becomes more acidic when it absorbs carbon dioxide from the air, which harms coral health. Large bleaching events can harm entire reef ecosystems, reducing the variety of life in the ocean and affecting people who rely on reefs for food, tourism, and protection from strong waves.

According to the United Nations Environment Programme, between 2014 and 2016, the longest global bleaching events in history caused widespread coral death. In 2016, bleaching on the Great Barrier Reef killed 29 to 50 percent of its coral. Bleaching events happen in many places around the world, such as Australia, Hawaii, Japan, and others. Some corals naturally adapt to bleaching, and human efforts to help build reefs may reduce future damage.

Process

Corals form large reef ecosystems in tropical seas. These corals have a special partnership with tiny, algae-like organisms called zooxanthellae that live inside their tissues and give the coral its color. The zooxanthellae use photosynthesis to create nutrients, which are essential for the coral to survive in the clear, nutrient-poor waters of the tropics. In return, the coral provides the zooxanthellae with carbon dioxide and ammonium needed for photosynthesis.

Environmental conditions such as unusually warm or cool temperatures, high light levels, or some microbial diseases can break the partnership between corals and zooxanthellae. A study by D.J. Smith et al. suggests that photoinhibition may be a reason for coral bleaching. It also states that hydrogen peroxide produced by zooxanthellae may signal them to leave the coral. To survive short-term, the coral-polyp may consume or expel the zooxanthellae. This causes the coral to lose its color, appearing lighter or completely white, a process called "bleaching."

Under mild stress, some corals may show bright colors like blue, pink, purple, or yellow instead of white. This happens because the coral’s own pigment molecules remain visible, a phenomenon called "colorful bleaching." Since zooxanthellae provide up to 90% of the coral’s energy through photosynthesis, losing them can lead to starvation.

Corals can survive short-term disturbances, but if the stressful conditions continue, their survival becomes unlikely. To recover from bleaching, zooxanthellae must re-enter the coral polyps and restart photosynthesis to support the coral and the ecosystem. If the coral dies from starvation, it will decay, leaving behind calcium carbonate skeletons. These skeletons may be taken over by algae, which can block new coral growth. Over time, the coral skeletons will erode, causing the reef structure to collapse.

Common triggers

Coral bleaching can be caused by many different factors. Local events may lead to small areas of bleaching, but the large coral bleaching events that have happened recently are mainly caused by global warming. Scientists predict that as carbon dioxide levels rise in the 21st century, corals may become less common in reef systems. Reefs in warm, shallow waters with little water movement have been more affected than reefs in areas with more water flow. Marine heatwaves, which are often linked to El Niño weather patterns, have been found to be a major cause of widespread coral bleaching and coral death.

Common causes of coral bleaching include:
• higher or lower water temperatures (especially marine heatwaves, often caused by global warming)
• more sunlight (especially ultraviolet light)
• more sediment (from silt runoff)
• bacterial infections
• changes in salt levels in the water
• herbicides
• extreme low tides that expose corals to air
• fishing that uses cyanide
• higher sea levels caused by global warming (Watson)
• dust from African dust storms during droughts
• pollutants like oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene (four common ingredients in sunscreens that do not break down easily)
• ocean acidification from high carbon dioxide levels caused by air pollution
• exposure to oil or other chemical spills
• changes in water chemistry, especially an imbalance in the amounts of nitrate and phosphate in the water
• weather conditions.

Trends due to climate change

Severe coral bleaching is connected to climate-related events that raise ocean temperatures, such as El Niño-Southern Oscillation (ENSO). Warmer surface ocean waters can cause corals to lose their color and health, leading to serious harm or death. The IPCC Sixth Assessment Report from 2022 stated: "Since the early 1980s, the number and strength of mass coral bleaching events have increased sharply worldwide." Coral reefs and other coastal ecosystems, including rocky shores, kelp forests, seagrass beds, and mangroves, have suffered large-scale deaths due to marine heatwaves. Scientists predict that many coral reefs may experience long-term changes that are hard to reverse if global warming exceeds 1.5°C.

This issue was recognized as the greatest threat to the world’s reef systems by the Intergovernmental Panel on Climate Change (IPCC) in 2007. The Great Barrier Reef had its first major bleaching event in 1998. Since then, bleaching events have become more frequent, with three major events occurring between 2016 and 2020. If global warming remains at 1.5°C, bleaching is expected to happen three times every decade on the Great Barrier Reef. If warming reaches 2°C, bleaching could occur every other year.

In 2017, National Geographic reported that in the past three years, 25 reefs—three-fourths of the world’s reef systems—experienced severe bleaching events. Scientists called this the worst series of bleaching events recorded.

A study on the Hawaiian mushroom coral Lobactis scutaria found that higher temperatures and increased levels of sunlight (photosynthetically active radiation, or PAR) harmed the coral’s ability to reproduce. The study aimed to understand how climate change affects the survival of reef-building corals in their natural environment.

Role of ocean acidification

The carbon cycle involves the movement of carbon between different parts of the Earth. The oceans and other carbon sinks store large amounts of carbon. When humans release carbon into the atmosphere by burning fossil fuels, a large amount of this carbon dissolves into the oceans. This adds carbon to the ocean and disrupts the natural balance between CO2 and carbonic acid, which helps keep the ocean stable. This disruption causes the ocean's pH to decrease, a process called acidification. Acidification works with heat stress to cause coral bleaching. For example, an 8-week study on Heron Island, Australia, found that 40-50% of crustose coralline algae and Acropora coral became bleached after being exposed to high levels of CO2. Ocean acidification weakens corals' ability to build skeletons made of calcium carbonate, which are essential for their survival. This happens because acidification reduces the amount of carbonate in the water, making it harder for corals to take in calcium carbonate. As a result, coral reefs become less able to recover from damage and more likely to erode. Also, increased CO2 levels can lead to overfishing of herbivores and too many nutrients, which can change coral-dominated ecosystems into ecosystems where algae grow more than coral. A recent study from the Atkinson Center for a Sustainable Future found that if acidification and rising temperatures continue, CO2 levels could become too high for coral to survive in as little as 50 years.

Impacts

Coral bleaching events and the loss of coral coverage often lead to a decrease in the variety of fish species. The loss of herbivorous fish, which help keep coral reefs healthy, can harm these ecosystems. As bleaching events happen more often, fish populations become more similar. Smaller and more specialized fish that play important roles in coral health are replaced by fish that can live in a wider range of environments. This loss of specialized species may reduce the ability of coral reefs to recover after bleaching events.

According to Brian Skoloff of The Christian Science Monitor, "If the reefs vanished, experts say, hunger, poverty and political instability could ensue." Coral reefs provide shelter to about one-quarter of all ocean species. Many sea creatures depend on reefs for safety from predators. If reefs disappear, it could cause a chain reaction that affects people who rely on fish for food and income. Experts estimate that coral reef services are worth up to $1.2 million per hectare, which is about $172 billion each year. These services include protecting coastlines from waves, supporting life in and between ecosystems, maintaining ocean chemistry, recording climate changes, and offering tourism and recreation opportunities. In the past 20 years, coral reefs in the Florida Keys have declined by 44%, and in the Caribbean, the decline has reached 80%.

Coral reefs also act as natural fish habitats, as many commercially important fish spend part of their lives in coral reefs. This makes reefs important for fishing, especially for small, local fisheries. When coral reefs are damaged by bleaching, fish populations decline, which affects fishing opportunities. A study by Speers et al. estimated that if greenhouse gas emissions remain high, losses to fisheries could be about $49–69 billion. However, if emissions are reduced, these losses could be reduced by about $14–20 billion.

Economic losses from coral reef decline have political effects, as they affect developing countries where reefs are located, such as in Southeast Asia and the Indian Ocean. These countries may face higher costs to replace lost income and food sources, as well as other services like ecotourism. A study by Chen et al. found that the value of reefs decreases by about 4% for every 1% loss of coral cover due to reduced tourism and recreation. Restoring reefs can help these areas. A study in Maui showed that a reef restoration project increased annual visits by 47% and provided a welfare gain of $2.9 million, or $26 per resident.

Coral reefs protect coastlines by reducing wave energy, which lowers damage from storms, erosion, and flooding. Reef crests can reduce wave energy by about 86%, and with reef flats, this can reach up to 97%. Countries that lose this natural protection may face higher costs from storm damage. Combined with lost tourism revenue, these losses can have major economic effects.

High sea temperatures are the main cause of coral bleaching. Between 1979 and 1990, 60 major bleaching events occurred, affecting reefs worldwide. The longest and most damaging bleaching event happened in 2016, linked to a strong El Niño from 2014 to 2017. During this time, over 70% of coral reefs were damaged.

Factors that influence bleaching outcomes include how well corals resist stress, how quickly they recover after losing their symbiotic algae (zooxanthellae), and how fast new coral grows. Local conditions like shade or cooler water can reduce bleaching. Coral and algae health, as well as genetics, also affect bleaching. Large coral colonies, like Porites, are more resistant to temperature changes, while fragile corals like Acropora are more vulnerable. Corals that experience low stress regularly may be more resistant to bleaching.

The first global bleaching event was in 1998, with 21% of reefs experiencing heat stress. According to Clive Wilkinson of the Global Coral Reef Monitoring Network, this event was caused by a 2°C rise in sea temperatures and a strong El Niño in 1997–1998.

The 2023–2025 global bleaching event began in February 2023, becoming the fourth such event. It has affected reefs in at least 82 countries and all major ocean basins. By April 2025, 84% of reefs had been exposed to bleaching-level heat. This event caused severe damage, with coral mortality reaching up to 93% in areas like the Pacific coast near Mexico. Coral reefs contribute about $2.7 trillion annually to the global economy, including $36 billion from tourism. While a future La Niña phase may help, some areas, like Florida, have already seen complete coral die-offs due to high temperatures. The Great Barrier Reef is now experiencing its fifth major bleaching event since 2016, showing the ongoing risks to these ecosystems.

The estimated share of affected coral reefs by each of the four major bleaching events is 20%, 35%, 56%, and 54%.

After bleaching events, coral disease outbreaks have increased. Weakened corals are more likely to be infected by disease-causing pathogens. The first large coral disease outbreak was in 1975 in the Florida Keys, affecting six reef-building species. A similar event occurred in 2014, with a white-plague disease affecting 61% of corals at 14 sites off Florida’s coast. At least 13 species were affected.

Other bacteria, like Vibrio shiloi, can worsen bleaching by attacking coral algae. This bacteria is active only during warm periods. High temperatures increase its ability to infect corals by attaching to specific receptors on their surface. Vibrio shiloi then enters the coral, multiplies, and produces toxins that harm the algae. In 2003, some Mediterranean coral reefs showed resistance to this pathogen, reducing further infection.

By region

The Great Barrier Reef near Australia had coral bleaching in 1980, 1982, 1992, 1994, 1998, 2002, 2006, 2016, 2017, 2022, 2024, and 2025. Some areas had very serious damage, with up to 90% of corals dying. The largest and most severe bleaching happened in the summers of 1998 and 2002. In those years, 42% and 54% of reefs were bleached to some degree, and 18% were strongly bleached. Between 1995 and 2009, new corals grew to replace some of the lost ones. Overall, coral populations on the Great Barrier Reef dropped by 50.7% from 1985 to 2012. About 10% of this loss was due to bleaching, while the rest was caused equally by tropical storms and attacks by crown-of-thorns starfish. Since 2014, global coral bleaching has increased because of the highest ocean temperatures ever recorded. These temperatures caused the worst coral bleaching in the Great Barrier Reef. In 2016, near Port Douglas, more than half of the bleached corals died. In late November 2016, surveys of 62 reefs showed that long-term heat from climate change caused a 29% loss of shallow water coral. The most damage happened in inshore and mid-shelf reefs near Cape Grenville and Princess Charlotte Bay. Scientists predict that corals on the Great Barrier Reef will often face summer temperatures high enough to cause bleaching by 2100.

A study in early 2024 tracked 462 coral colonies at One Tree Island after they were stressed by heat. By July 2024, only 92 colonies were not bleached, while 193 were dead and 113 showed signs of bleaching.

In 1996, Hawaii’s first major coral bleaching happened in Kaneohe Bay. Major bleaching also occurred in the Northwest islands in 2002 and 2004. In 2014, scientists from the University of Queensland found the first large-scale bleaching event and linked it to "The Blob," a warm ocean area. In 2014 and 2015, surveys in Hanauma Bay on Oahu found 47% of corals bleached and nearly 10% dead. On the Big Island, 56% of coral reefs were affected, and 44% of corals on west Maui were bleached. In 2019, scientists found that reefs had started to recover four years after the 2014–2015 bleaching. However, in 2019, up to 50% of reefs on Oahu and Maui were still bleached, and about 40% of corals on the Big Island’s Kona coast were affected. The 2014–2015 events were more severe than recent ones. In 2020, NOAA released the first nationwide coral reef report, stating that the northwestern and main Hawaiian islands were in "fair" condition, meaning corals were moderately impacted.

Hawaiian Sunscreen Policy
In May 2018, Hawaii passed a law (SB-2571) banning the sale of sunscreen containing chemicals harmful to corals. The law, signed by Governor David Ige, banned oxybenzone and octinoxate, which become toxic to corals when exposed to sunlight. Up to 10% of the 14,000 tons of sunscreen polluting reefs contains oxybenzone, threatening nearly half of all coral reefs. Corals exposed to oxybenzone bleach more easily in both lab and natural settings. Over time, oxybenzone in water weakens reefs’ ability to survive other stressors like warm temperatures. SB-2571 only allowed prescription sunscreens. Hawaii was the first U.S. state to ban these chemicals, which took effect in January 2021.

Jarvis Island, in the Pacific Ocean, once had a coral reef 100 meters deep. Between 1960 and 2016, eight severe and two moderate bleaching events occurred. The 2015–2016 event was the worst, with hard coral cover dropping from 18.7% to 0.4%, a 98% decline.

In 2016, about 94% of corals on Japan’s Iriomote Island bleached. Before this, typhoons often hit the area in July and August, but in 2016, no typhoon occurred until September, leading to prolonged high temperatures. A 2017 Japanese report said 75% of Okinawa’s largest reef died from bleaching, with 90% of the Sekisei reef bleached. The same area had another bleaching in 2022, affecting 30 sites.

In summer 2024, rising sea temperatures caused a major bleaching event that killed 61.2% of corals near Amami-Oshima, Japan. Temperatures were 2°C higher than in 2023.

Coral reefs in the Indian Ocean have been permanently damaged by warm waters. Up to 90% of coral cover was lost in the Maldives, Sri Lanka, Kenya, Tanzania, and the Seychelles during the 1997–1998 bleaching. In 1998, 20% of Indian Ocean coral died, and 80% was bleached. These shallow tropical areas now face ocean conditions predicted for the future. Surviving corals here may help restore reefs elsewhere.

The Maldives has over 20,000 km of reefs, with more than 60% bleached in 2016. Risks include tourism, coastal construction, land reclamation, and diseases.

Coral reefs in Thailand’s Gulf of Thailand are important. Bleaching occurred in 1998 and 2010, with 2010 being more severe. In 2010, 70% of coral in the Andaman Sea was affected, with 30% to 95% of bleached corals dying.

Acropora corals, common in Indonesian reefs, are very vulnerable to stress. A 2010 study found that recovery after bleaching depends on event severity and frequency. Frequent moderate events harm Porites corals, while rare but strong events mainly affect Acropora. Acropora corals regrow

Coral Adaptation

In recent years, climate change has been connected to a rise in coral deaths. Research shows that bacteria living with corals may help them survive high temperatures. Scientists have tried to make corals more resistant to bleaching, which occurs when corals lose their color and health. Since corals are the main part of coral reefs, their decline harms the reefs and the animals that live there.

In 2010, researchers at Penn State found corals in the warm waters of the Andaman Sea that were healthy. These corals used a type of algae that is unusual and can survive high temperatures. This discovery surprised scientists because most algae cannot live in such heat. It suggests that corals might adapt by using other algae types that can survive rising ocean temperatures. Around the same time, Stanford University researchers found corals near the Samoan Islands that survive daily temperature spikes during low tide. These corals do not bleach or die, even when exposed to high heat. Studies show these corals have become used to the heat. Scientists now wonder if corals from other areas can be trained to survive higher temperatures by gradually exposing them to heat.

During mild bleaching, corals produce pigments that act like sunscreen to protect themselves. These pigments can appear pink, blue, or purple and glow brightly. Shallow-water corals make more of these pigments when exposed to blue light. When corals bleach, blue light inside their tissue increases because the algae that normally absorb it are gone, and the white coral skeleton reflects the light. This causes more pigments to form, making bleached corals look colorful instead of white.

Higher ocean temperatures cause the outer layer of coral to thin and some cells inside the coral to die. This leads to a loss of algae that live within the coral, reducing their numbers by up to half quickly. When corals face high heat or bright light, they create harmful chemicals called reactive oxygen species. If these chemicals are not removed, they can kill the coral. Studies show that corals exposed to heat stress become thinner than healthy corals. When the algae that provide energy to corals leave during heat events, corals must find new ways to get energy. Some corals that eat more animals are more likely to recover from bleaching.

After the algae leave, algae often take over coral structures because they need fewer resources to survive. There is little competition between algae and the original algae, but without the original algae, the new algae thrive. When algae take over, corals can no longer support themselves, and their structures begin to break down due to ocean acidification. Ocean acidification happens when carbon dioxide is absorbed into the ocean, reducing carbonate ions, which corals need to build their skeletons. Corals build and break down their skeletons daily and seasonally based on temperature changes. Under current climate change predictions, corals may not have enough time to rebuild during cooler winter months, leading to their collapse.

Helping Repair the Damage

The US National Oceanic and Atmospheric Administration (NOAA) watches for coral bleaching "hot spots," which are areas where ocean temperatures rise 1°C or more above the usual monthly average. These hot spots show where corals experience heat stress. Scientists use a system called Degree Heating Week (DHW) to track how long corals are exposed to high temperatures. Satellites help detect rising ocean temperatures earlier, allowing scientists to warn about coral bleaching before it happens. High temperatures harm coral reefs by stopping their normal growth and reproduction, and can eventually kill them. NOAA discovered the global coral bleaching event in 1998, which happened during the strong El Niño event of 1997–98. Today, NOAA monitors 190 reef locations worldwide and sends alerts to scientists and reef managers through the NOAA Coral Reef Watch (CRW) website. By tracking ocean warming, scientists can warn reef managers about upcoming bleaching events so they can prepare. The first major global coral bleaching events were recorded in 1998 and 2010, both linked to El Niño events that raised ocean temperatures and hurt coral health. The 2014–2017 El Niño was the longest and most harmful to corals, damaging over 70% of coral reefs. More than two-thirds of the Great Barrier Reef have been reported as bleached or dead.

To study coral bleaching, scientists use underwater photography and artificial intelligence tools like TagLab to create detailed maps of coral reefs and assess their health. After a bleaching event, some reefs can recover by regaining their symbiotic algae, called zooxanthellae. However, other reefs may shift to being dominated by algae, which makes it harder for corals to grow. Algae produce chemicals that stop corals from settling and compete with them for space and light. This weakens corals and makes reefs more vulnerable to other problems, like poor water quality and fewer herbivorous fish. Understanding why some reefs recover while others do not is important for protecting coral ecosystems.

Scientists are studying "super-corals," which are corals that live in naturally warm and acidic waters. These corals may help restore damaged reefs when transplanted. Research by scientists like Emma Camp and Ruth Gates shows that super-corals could help reefs survive longer even as ocean temperatures and acidity increase. Corals can recover from short-term problems like storms or invasions by a type of starfish, but they struggle more with bleaching. Fish populations often recover faster than corals after disturbances, but bleaching can change the types of fish that live on reefs. Some studies show that after bleaching, reefs have more generalist fish and fewer fish that depend on corals.

Until recently, scientists knew little about what helps reefs recover after bleaching. A study of 21 reefs in the Seychelles found that about half recovered after losing most of their corals in 1998, while the other half shifted to algae-dominated reefs. Factors that helped recovery included the number of young corals, the complexity of the reef structure, water depth, the amount of herbivorous fish, and nutrient levels. Reefs that were deeper and had complex structures were more likely to recover.

Different types of fish also affect reef recovery. Bioeroding fish break down dead coral, scraping fish remove algae and sediment, and grazing fish eat algae. The presence of these fish helps corals grow by making space for new corals to settle. In some areas, fewer grazing fish after bleaching have led to conditions similar to those dominated by sea urchins, which do not shift to algae-covered reefs.

Scientists are still learning about hidden changes in coral communities that may affect their ability to recover. These changes can lead to unexpected shifts in reef ecosystems. Better ways to measure reef health and long-term changes are needed to protect coral reefs.

Researchers are working to slow coral loss by restoring reefs. Methods include growing coral fragments in tanks, farming corals, and transplanting them to damaged areas. Scientists like Ruth Gates and Madelaine Van Oppen are experimenting with "super-corals" that can survive harsh conditions and developing algae that may help corals thrive in changing oceans.