Coral bleaching happens when corals turn white because they lose the algae that live with them and the pigments that help them make food. This loss can be caused by many factors, such as changes in water temperature, light, salt levels, or nutrients. When corals are bleached, they are stressed, more likely to starve or get sick, and may die. The main reason for coral bleaching is increasing ocean temperatures caused by climate change. Also, the ocean is becoming more acidic because it absorbs carbon dioxide from the air, which harms coral health. Large bleaching events can cause major damage to reef ecosystems, harming many sea animals and affecting people who rely on healthy reefs for food, jobs, and protection from strong ocean waves.

According to the United Nations Environment Programme, from 2014 to 2016, the largest recorded global bleaching events caused widespread coral death. In 2016, bleaching on the Great Barrier Reef killed 29 to 50 percent of the reef's coral. Bleaching happens in many places around the world, such as Australia, Hawaii, Japan, and others. Some corals naturally adapt to bleaching, and people can help by building reefs to reduce further harm.

Process

Corals that build large reef ecosystems in tropical seas rely on a special partnership with tiny, algae-like single-celled organisms called zooxanthellae. These organisms live inside the coral's tissues and give the coral its color. The zooxanthellae use sunlight to make food through a process called photosynthesis, which is essential because tropical waters are often clear and lack many nutrients. In return, the coral provides the zooxanthellae with carbon dioxide and ammonium, which are needed for photosynthesis.

Environmental problems, such as unusually warm or cool temperatures, too much sunlight, or some types of microbial diseases, can disrupt the partnership between corals and zooxanthellae. A study by D.J. Smith and others suggests that a process called photoinhibition may cause coral bleaching. The study also notes that hydrogen peroxide produced by zooxanthellae might signal them to leave the coral. To survive short-term stress, the coral may eat or push out the zooxanthellae. This causes the coral to lose its color and appear white, a process called "bleaching."

Under mild stress, some corals may turn bright blue, pink, purple, or yellow instead of white. This happens because the coral's own pigments become more visible when zooxanthellae are less present, a phenomenon called "colorful bleaching." Since zooxanthellae supply up to 90% of a coral's energy through photosynthesis, losing them can lead to starvation.

Corals can survive short-term stress, but if the conditions causing zooxanthellae loss continue, the coral's survival becomes unlikely. For recovery, zooxanthellae must re-enter the coral's tissues and restart photosynthesis to support the coral and the reef 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 reef structure may collapse as the skeletons erode.

Common triggers

Coral bleaching can happen because of many reasons. Some causes affect only small areas, but large-scale bleaching events in recent years have been mainly caused by global warming. As carbon dioxide levels rise in the 21st century, corals are likely to become less common in reef systems. Reefs in warm, shallow water with little water movement are more harmed than those in areas with stronger water flow. Marine heatwaves linked to the El Niño Southern Oscillation are a major cause of widespread coral bleaching and coral death.

Causes of coral bleaching include:
• higher or lower water temperatures (often from marine heatwaves, which are usually linked to global warming)
• increased sunlight (including visible light and ultraviolet light)
• more sediment from soil 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, which are ingredients in some sunscreens that do not break down easily
• ocean acidification from high CO2 levels caused by air pollution
• exposure to oil or chemical spills
• changes in water chemistry, especially when the balance of nitrate and phosphate levels is disrupted
• weather conditions.

Trends due to climate change

Extreme coral bleaching is connected to climate-related events that raise ocean temperatures, such as El Niño-Southern Oscillation (ENSO). Warmer ocean surface waters can cause corals to bleach, which may lead to serious harm or coral death. The IPCC Sixth Assessment Report from 2022 stated: "Since the early 1980s, the frequency and severity of large-scale coral bleaching events have increased sharply worldwide." Coral reefs and other coastal ecosystems, including rocky shores, kelp forests, seagrasses, and mangroves, have recently suffered widespread deaths due to marine heatwaves. Scientists predict that many coral reefs may experience "permanent changes" if global warming exceeds 1.5°C.

This issue was first highlighted in 2007 by the Intergovernmental Panel on Climate Change (IPCC) as the biggest threat to the world’s reef systems.

The Great Barrier Reef had its first major bleaching event in 1998. Since then, bleaching events have become more frequent, with three events occurring between 2016 and 2020. If global warming remains at 1.5°C, bleaching is expected to happen three times every 10 years on the Great Barrier Reef. If warming reaches 2°C, bleaching could occur every other year.

As coral bleaching increases globally, National Geographic reported in 2017: "In the past three years, 25 reefs—covering three-fourths of the world’s reef systems—experienced severe bleaching events, which scientists called the worst sequence of bleaching events ever recorded."

A study on the Hawaiian mushroom coral Lobactis scutaria found that higher temperatures and increased levels of photosynthetically active radiation (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 environments, as rising temperatures are making it harder for corals to reproduce.

Role of ocean acidification

The carbon cycle involves the movement of carbon between carbon sinks, such as forests and oceans, which store large amounts of carbon. When humans release carbon into the atmosphere by burning fossil fuels, much of this carbon dissolves into the oceans. This adds extra carbon dioxide to the water, disrupting the natural balance between carbon dioxide and carbonic acid, which helps keep the ocean's pH stable. This disruption causes the ocean's pH to decrease, a process called acidification. Ocean acidification combines with heat stress to harm coral reefs, leading to bleaching. For example, an 8-week study on Heron Island, Australia, found that 40-50% of certain corals turned white after being exposed to high levels of carbon dioxide. Acidification weakens corals' ability to form hard 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 absorb calcium carbonate. As a result, coral reefs become less strong and more likely to break down. Additionally, increased carbon dioxide levels can lead to overfishing of herbivores and nutrient pollution, which can change coral-dominated ecosystems into ones where algae grow more. A recent study from the Atkinson Center for a Sustainable Future found that if carbon dioxide levels continue to rise, coral reefs may not 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 decline in the number and types of herbivorous fish, which eat algae and help maintain reef health, has a big impact on coral reef ecosystems. As bleaching events become more common, fish populations tend to become more similar in type. Smaller, specialized fish that play important roles in the reef are often replaced by more generalist species. This loss of specialized fish may reduce the ability of coral reef ecosystems to recover after bleaching events.

According to Brian Skoloff of The Christian Science Monitor, "If reefs disappeared, experts say, hunger, poverty, and political instability could happen." Coral reefs are home to about a quarter of all ocean species. Many sea creatures rely on reefs for shelter and protection from predators. If reefs disappeared, this loss would create a chain reaction, affecting human communities that depend on reef fish for food and income. Experts estimate that coral reef services are worth up to $1.2 million per hectare, or about $172 billion per year globally. These benefits include protecting coastlines from waves, supporting life between ecosystems, helping maintain ocean chemistry, recording climate changes, and providing tourism and fishing opportunities. In the Florida Keys, coral reefs have declined by 44% over the past 20 years, and in the Caribbean, the decline has reached 80%.

Coral reefs serve as natural fish habitats, as many commercially important fish reproduce or spend their early lives in reefs. This makes reefs important for fishing, especially for small, local fisheries. When coral cover decreases due to bleaching, fish populations also decline, reducing fishing opportunities. A study by Speers et al. estimated that if greenhouse gas emissions remain high, direct losses to fisheries from reduced coral cover could reach $49–69 billion. However, if emissions are reduced, losses could be cut by about $14–20 billion.

These economic losses have political effects, as they disproportionately affect developing countries where reefs are located, such as in Southeast Asia and the Indian Ocean. These regions would 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 commercial value of reefs decreases by about 4% for every 1% loss in coral cover, due to reduced tourism and recreational activities. Restoring reefs can help these areas. A project in Maui increased annual visits by 47% and generated a $2.9 million welfare gain, averaging $26 per resident.

Coral reefs protect coastlines by reducing wave energy, which lowers damage from storms, erosion, and flooding. Reef crests absorb about 86% of wave energy, and with reef flats included, this can rise to 97%. Countries losing this natural protection face higher costs from increased storm damage. Combined with lost tourism revenue, these losses create major economic challenges.

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

Factors that influence bleaching outcomes include how well corals resist stress, their ability to survive without zooxanthellae (algae that live inside corals), and how quickly new coral grows. Local conditions, like shade or cooler water, can reduce bleaching. Larger coral colonies, such as Porites, are more resistant to temperature changes than fragile branching corals like Acropora. Corals regularly exposed to low stress may be more resilient to bleaching.

The first global coral bleaching event was in 1998, when 21% of reefs experienced heat stress. According to Clive Wilkinson of the Global Coral Reef Monitoring Network, the 1998 event in the Indian Ocean was caused by a 2°C rise in sea temperatures and a strong El Niño.

The 2023–2025 global bleaching event began in February 2023 and became the fourth major event. It 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 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, regions like Florida have already seen complete reef die-offs due to temperatures reaching 101°F (38.3°C). The Great Barrier Reef is now experiencing its fifth major bleaching event since 2016, showing the ongoing risks to these ecosystems.

The percentage of reefs affected by the four major bleaching events is estimated to be 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 at Carysfort Reef in the Florida Keys, affecting six reef-building species. A similar event occurred in 2014, with 61% of corals showing signs of white-plague disease at 14 sites off Florida’s coast. At least 13 species were affected.

Other bacteria, like Vibrio shiloi, can worsen bleaching by attacking zooxanthellae. This bacteria is active only during warm periods and uses a specific receptor in coral mucus to enter the coral. It then produces toxins that harm zooxanthellae by stopping photosynthesis and causing cell breakdown. In 2003, Mediterranean coral reefs showed increased resistance to V. shiloi, reducing further infections.

By region

The Great Barrier Reef along the coast of Australia had bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, 2006, 2016, 2017, 2022, 2024, and 2025. Some areas suffered serious damage, with up to 90% of the corals dying. The most widespread and severe events happened in the summers of 1998 and 2002. In 1998, 42% of reefs showed some level of bleaching, and 18% were strongly bleached. In 2002, 54% of reefs showed some level of bleaching, and 18% were strongly bleached. Between 1995 and 2009, new coral growth helped reduce some of the losses from earlier years. Overall, coral populations on the Great Barrier Reef decreased by 50.7% from 1985 to 2012. About 10% of this decline was due to bleaching, while the remaining 90% was caused roughly equally by tropical cyclones and predation by crown-of-thorns starfish. Since 2014, global coral bleaching has occurred because of the highest ocean temperatures ever recorded. These temperatures caused the most severe and widespread coral bleaching in the Great Barrier Reef. The worst bleaching in 2016 happened near Port Douglas, where more than half of the bleached corals died. In late November 2016, surveys of 62 reefs showed that long-term heat stress from climate change caused a 29% loss of shallow water coral. The most severe coral death and reef habitat loss occurred inshore and mid-shelf reefs around Cape Grenville and Princess Charlotte Bay. According to the IPCC, if summer temperatures rise by 2 °C by 2100, corals on the Great Barrier Reef will likely experience bleaching regularly.

A study in early 2024 tracked 462 coral colonies at One Tree Island after they were affected by heat stress. By July 2024, only 92 colonies were unaffected by bleaching, while 193 were dead and 113 showed signs of bleaching.

In 1996, Hawaii’s first major coral bleaching event occurred in Kaneohe Bay. Major bleaching events followed in the Northwest islands in 2002 and 2004. In 2014, scientists from the University of Queensland observed the first mass bleaching event and linked it to "The Blob," a warm ocean current. In 2014 and 2015, surveys in Hanauma Bay Nature Preserve on Oahu found that 47% of corals were bleached, and nearly 10% died. During the same time, 56% of coral reefs on the Big Island were affected by bleaching, and 44% of corals on west Maui were also bleached. In January 2019, scientists from The Nature Conservancy found that reefs had started to recover four years after the last major bleaching event. However, the Division of Aquatic Resources (DAR) reported that bleaching still occurred in 2019, with up to 50% of coral reefs on Oahu and Maui bleached and about 40% of corals on the Big Island’s Kona coast affected. The DAR noted that the recent bleaching events were not as severe as those in 2014–2015. In 2020, the National Oceanic and Atmospheric Administration (NOAA) released the first nationwide coral reef status report, stating that the northwestern and main Hawaiian islands were in "fair" condition, meaning the corals had been moderately affected.

In May 2018, Hawaii passed a law called "SB-2571," which banned the sale of sunscreen containing chemicals harmful to coral reefs. The law, signed by David Ige, a member of the Democratic Party, specifically targeted oxybenzone (and octinoxate), a chemical that becomes toxic to coral when exposed to sunlight. Up to one-tenth of the 14,000 tons of sunscreen polluting coral reef areas contains oxybenzone, putting nearly half of all coral reefs at risk. Corals exposed to high levels of oxybenzone show increased bleaching in both controlled and natural environments. Over time, oxybenzone in water weakens a reef’s ability to recover from other bleaching events, such as rising water temperatures. The law banned all non-prescription sunscreen products. Hawaii became the first U.S. state to implement this type of ban, which took effect in January 2021.

Jarvis Island, located in the Pacific Ocean, was originally surrounded by 100 meters of ancient coral reef. Between 1960 and 2016, eight severe and two moderate bleaching events occurred in the coral community on Jarvis Island. The 2015–2016 bleaching event was the most severe, reducing the level of hard coral from 18.7% in 2015 to 0.4% in 2016. This event caused a 98% decline in coral reef coverage.

In 2016, about 94% of corals on Japan’s Iriomote Island in the Ryukyu Islands were bleached during a major event. Before this, the region usually experienced typhoons in July and August. However, no typhoon was recorded until September, indicating a prolonged period of high seawater temperatures. A 2017 Japanese government report stated that nearly 75% of Japan’s largest coral reef in Okinawa had died from bleaching. The report also noted that 90% of the Sekisei reef, a popular scuba diving location, was bleached during this event. The same location experienced another bleaching outbreak in the summer of 2022, affecting 30 sites within the lagoon.

In the summer of 2024, rising sea temperatures caused another major bleaching event that killed 61.2% of corals off Amami-Oshima Island, Japan. The bleaching occurred because sea temperatures were 2° higher than in 2023.

Coral reef provinces have been permanently damaged by warm sea temperatures, especially in the Indian Ocean. During the 1997–1998 bleaching event, up to 90% of coral cover was lost in the Maldives, Sri Lanka, Kenya, Tanzania, and the Seychelles. In 1998, the Indian Ocean reported that 20% of its coral had died and 80% was bleached. Shallow tropical areas in the Indian Ocean are already experiencing ocean conditions predicted to occur worldwide in the future. Coral that survived in these areas may be suitable for restoration efforts in other parts of the world because they can withstand extreme conditions.

The Maldives has over 20,000 km of reefs, with more than 60%

Coral Adaptation

In recent years, climate change has been connected to a significant rise in coral deaths. Evidence also shows that bacteria living with corals may help them survive extreme heat. Scientists are working to improve coral strength against bleaching events. Since corals are essential to building coral reefs, their decline harms the stability and variety of reefs, which directly affects the animals that live there.

In 2010, researchers at Penn State found corals thriving in the warm waters of the Andaman Sea by using an unusual type of algae. Regular algae cannot survive such high temperatures, so this discovery was surprising. This suggests that as ocean temperatures rise due to global warming, corals might adapt by hosting algae that can survive heat. In 2010, scientists from Stanford University also observed corals near the Samoan Islands that endure sudden temperature spikes during low tide without bleaching or dying. Studies showed that corals near Ofu Island in American Samoa have developed the ability to tolerate heat. Researchers now wonder if corals from other areas can be trained to withstand heat by gradually exposing them to short, warm periods.

Some mild bleaching events cause corals to create pigments that act like sunscreen to protect themselves. These pigments can appear pink, blue, or purple, or glow brightly. Shallow-water corals produce these pigments when exposed to blue light. When corals bleach, blue light inside their tissues increases because the pigments from algae are no longer absorbing it. Instead, the light reflects off the white coral skeleton, triggering more pigment production. This makes bleached corals look colorful instead of white, a phenomenon sometimes called "colorful coral bleaching."

Rising sea temperatures cause the outer layer of corals to thin and the inner cells to die. This reduces the number of algae living inside the coral by up to 50% quickly. High temperatures or bright light cause corals to release harmful molecules called reactive oxygen species. If these molecules are not removed by the coral’s defenses, the coral may die. Studies show that corals exposed to heat stress become thinner than normal. When the algae inside the coral die due to heat, the coral must find new ways to get energy. Some corals that increase their ability to eat animals are more likely to recover from bleaching.

After the algae leave, algae often take over the coral structures because they need fewer resources to survive. There is little competition between the algae and the original algae, but without the original algae, the new algae thrive. Once the algae dominate and the coral cannot survive, the structures may break down because of ocean acidification. Ocean acidification happens when carbon dioxide dissolves in seawater, 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 predictions, corals are likely to break apart, and cooler winter months may not give them enough time to rebuild.

Helping Repair the Damage

The US National Oceanic and Atmospheric Administration (NOAA) tracks areas called "hot spots," where ocean surface temperatures rise 1°C or more above the usual monthly average. These hot spots show where heat stress is measured. Scientists use a method called Degree Heating Week (DHW) to monitor how much heat stress coral reefs experience. Satellite technology helps detect rising ocean temperatures earlier, which allows scientists to find coral bleaching events sooner. Monitoring high temperatures is important because coral bleaching harms coral reproduction, growth, and can lead to coral death. This system found the global coral bleaching event in 1998, which happened during the 1997–98 El Niño event. 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 rising ocean temperatures, early warnings help reef managers prepare for and raise awareness about future bleaching events. The first major global coral bleaching events were recorded in 1998 and 2010, both linked to El Niño, which caused ocean temperatures to rise and worsened coral conditions. The 2014–2017 El Niño was the longest and most harmful to coral, damaging over 70% of coral reefs. More than two-thirds of the Great Barrier Reef have been reported as bleached or dead.

To track how bleaching events spread and change, scientists use underwater photography to create detailed maps of coral reefs. They also use computer tools like TagLab to analyze images and assess coral health.

After a bleaching event, some reefs can recover by regaining their original state. Recovery happens when corals are recolonized by algae called zooxanthellae. However, other reefs may shift to a new state where thick layers of algae replace coral. This algae produces chemicals that prevent other organisms from settling and competes with corals for space and light. Algae communities become stable, making it hard for corals to grow again. Weakened reefs are more vulnerable to problems like poor water quality and loss of herbivore fish. Understanding why some reefs recover or remain resilient helps scientists protect coral more effectively.

A major area of research focuses on "super-corals," which live in naturally warm and acidic waters. These corals can help restore damaged reefs when transplanted. Emma Camp, a scientist, suggests super-corals might help repair reefs long-term, even as ocean temperatures and acidity rise. Research by Ruth Gates supports this idea, including studies on how corals survive in extreme environments.

Corals can recover from short-term problems like storms or invasions by a type of starfish. Fish populations often recover faster than corals after disturbances. However, bleaching events cause long-term changes in fish communities. Some studies show fish groups include more generalist species and fewer species that depend on coral. While rising temperatures and bleaching do not directly kill adult fish, they harm coral-associated fish due to habitat loss. Some herbivorous fish, which eat algae, increase in number because algae grow on dead coral. Better methods are needed to measure how disturbances affect coral resilience.

Until recently, scientists knew little about what helps coral reefs recover after bleaching. A study by Graham et al. (2015) examined 21 reefs in Seychelles and found that after losing over 90% of corals in 1998, about half recovered, and 40% shifted to algae-dominated states. Key factors for recovery included the number of young corals, reef structure, water depth, herbivorous fish populations, and nutrient levels. Reefs with complex structures and in deeper water were more likely to recover.

The roles of different fish species also influence reef recovery. Fish that erode dead coral, remove algae, or graze on algae help corals grow. Fewer grazing fish after bleaching in the Caribbean led to conditions similar to those dominated by sea urchins, which do not shift to algae-heavy states.

Changes in coral ecosystems may not always be obvious. Hidden losses or resilience can lead to unexpected shifts in reef communities. Better methods to assess reef health over time and improved conservation policies are needed to protect reefs.

Scientists are working to reduce coral deaths. Projects worldwide aim to restore reefs using methods like microfragmentation, coral farming, and relocation. Researchers grow corals in tanks that mimic natural ocean conditions to avoid harming wild reefs. They then transplant the healthy corals to damaged areas. Ruth Gates and Madelaine Van Oppen are experimenting with creating "super-corals" that can survive harsh conditions. Van Oppen is also developing a type of algae that may help corals thrive.