Coral bleaching happens when corals turn white. This is because they lose algae and colors that help them make food. These changes can be caused by stress, such as water that is too hot or cold, too bright or dark, too salty or not salty enough, or too many or too few nutrients. When corals bleach, they are stressed and more likely to starve, get sick, or die. The main reason corals bleach is because ocean temperatures are rising because of climate change. When humans add carbon dioxide to the air, it makes ocean water more acidic, which harms corals. Large bleaching events can hurt whole reef ecosystems. This harms many types of sea life and affects people who live near reefs and depend on them for food, jobs, and protection from strong waves.

According to the United Nations Environment Programme, from 2014 to 2016, the largest global bleaching events in history caused widespread coral death. In 2016, bleaching on the Great Barrier Reef killed between 29% and 50% of the reef’s corals. Bleaching happens in many places around the world, such as Australia, Hawaii, Japan, and others. Some corals have adapted naturally to survive bleaching, and people can help by working to rebuild reefs.

Process

Corals that build the large reef systems in warm ocean waters have a special partnership with tiny algae-like organisms called zooxanthellae. These organisms live inside the coral's tissues and give the coral its color. The zooxanthellae use photosynthesis to provide the coral with nutrients, which is especially important in the clear, nutrient-poor waters of tropical seas. In return, the coral gives the zooxanthellae carbon dioxide and ammonium, which they need for photosynthesis.

If environmental conditions become harmful, such as when water temperatures are too high or too low, or when there is too much sunlight or certain diseases, the partnership between the coral and zooxanthellae can break down. A study by D.J. Smith et al. suggests that a process called photoinhibition may cause coral bleaching. It also states that hydrogen peroxide produced by the zooxanthellae may signal them to leave the coral. To survive short-term, the coral may eat or push out the zooxanthellae. This causes the coral to lose its color, turning white, which is why this process is called "bleaching."

Under mild stress, some corals may appear bright blue, pink, purple, or yellow instead of white. This happens because the coral's own pigment molecules remain visible. This is called "colorful bleaching." Since the zooxanthellae provide up to 90% of the coral's energy through photosynthesis, the coral may begin to starve after losing them.

Corals can survive short-term stress, but if the harmful conditions continue, their chances of survival decrease. To recover from bleaching, the zooxanthellae must return to the coral's tissues and restart photosynthesis to support the coral and the ecosystem. If the coral dies from starvation, it will decay. Hard coral species will leave behind their calcium carbonate skeletons, which algae may take over, blocking new coral growth. Over time, the skeletons will erode, causing the reef structure to collapse.

Common triggers

Coral bleaching can happen because of many different reasons. Local causes may lead to bleaching in 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 expected to become less common in reef systems. Reefs in warm, shallow waters with little water movement have been more affected than those in areas with stronger water flow. Marine heatwaves, which are 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 (especially from marine heatwaves, often caused by global warming)
– increased sunlight (including light that helps plants grow and ultraviolet light)
– more dirt and sand in water (from soil runoff)
– bacterial infections
– changes in salt levels in the water
– chemicals used in farming, such as herbicides
– being exposed to air during very low tides
– fishing methods that use cyanide
– rising sea levels caused by global warming (Watson)
– dust from African dust storms during droughts
– harmful chemicals like oxybenzone, butylparaben, octyl methoxycinnamate, and enzacamene, which are found in sunscreens and do not break down in the environment
– ocean acidification from high carbon dioxide levels caused by air pollution
– contact with oil or other chemical spills
– changes in water chemistry, especially when the balance of nitrate and phosphate levels is disrupted
– weather patterns and other atmospheric conditions.

Trends due to climate change

Severe coral bleaching events are closely connected to climate-related events that raise ocean temperatures, such as El Niño-Southern Oscillation (ENSO). When ocean surface temperatures rise, it can cause corals to lose their color and health, which may lead to serious harm or death. The IPCC Sixth Assessment Report from 2022 states that since the early 1980s, the number and intensity of large-scale coral bleaching events have increased significantly worldwide. Coral reefs, along with other coastal ecosystems like rocky shores, kelp forests, seagrass beds, and mangroves, have suffered major losses due to marine heatwaves. Scientists expect that many coral reefs may experience long-term changes if global warming exceeds 1.5°C.

This issue was first recognized in 2007 by the Intergovernmental Panel on Climate Change (IPCC) as the greatest risk 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 major events occurring between 2016 and 2020. If global warming is limited to 1.5°C, scientists predict that the Great Barrier Reef may experience three bleaching events every decade. If warming reaches 2°C, bleaching could occur every other year.

In 2017, National Geographic reported that over the past three years, 25 reefs—covering three-fourths of the world’s reef systems—underwent severe bleaching events, which scientists called the worst series of bleaching events recorded.

A study on the Hawaiian mushroom coral Lobactis scutaria found that higher ocean temperatures and increased levels of light that help plants grow (photosynthetically active radiation, or PAR) negatively affected the coral’s ability to reproduce. The study aimed to understand how climate change impacts 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 and the Earth's oceans, which store large amounts of carbon. When humans add carbon to the air by burning fossil fuels, much of this carbon dissolves into the oceans. This change disrupts the natural balance between carbon dioxide and carbonic acid in the ocean, causing the water's pH to decrease, a process called acidification. Ocean 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 carbon dioxide. Acidification weakens corals' ability to build calcareous skeletons, which are vital 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 needed for their skeletons. 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 excess nutrients, which can change coral-dominated ecosystems into ones dominated by algae. A recent study from the Atkinson Center for a Sustainable Future found that if acidification and rising temperatures continue, CO2 levels might become too high for coral to survive within 50 years.

Impacts

Coral bleaching events and the loss of coral coverage often lead to fewer types of fish living in the area. The decrease in the number and variety of fish that eat plants, especially, harms coral reef ecosystems. As bleaching events happen more often, fish populations become more similar. Smaller and more specialized fish that play specific roles in the ecosystem are replaced by fish that can live in a variety of environments. Losing these specialized fish may make coral reef ecosystems less able to recover after bleaching events.

According to Brian Skoloff of The Christian Science Monitor, "If reefs disappeared, experts say, hunger, poverty, and political problems could happen." Coral reefs provide homes to about a quarter of all ocean species. Many sea animals depend on reefs for shelter and protection from predators. If reefs disappeared, it could cause a chain reaction that affects human societies that 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 benefits include protecting coastlines from waves, helping ecosystems work together, keeping ocean nitrogen levels balanced, storing climate information, and supporting tourism and fishing. In the Florida Keys, coral reefs have decreased by 44% in the past 20 years, and in the Caribbean, the decline has reached up to 80%.

Coral reefs also act as natural fish habitats, as many fish that are commonly caught live or reproduce in coral reefs near the tropics. This makes reefs important fishing areas and a source of income for local fishers. When coral reefs are damaged by bleaching, fish populations decline, which reduces fishing opportunities. A study by Speers et al. estimated that if greenhouse gas emissions remain high, losses to fisheries could reach $49–69 billion. However, if emissions are reduced, these losses might decrease by about $14–20 billion.

These economic losses affect developing countries where reefs are located, such as in Southeast Asia and the Indian Ocean. These areas would face higher costs to replace lost income and food sources, as well as lose benefits like ecotourism. A study by Chen et al. found that each 1% decrease in coral cover reduces the commercial value of reefs by almost 4% due to losses in tourism and outdoor activities. Restoring reefs can help these regions. A project in Maui showed a 47% increase in annual visits and a $2.9 million welfare gain for the island, averaging $26 per resident.

Coral reefs also protect coastlines by reducing wave energy, which lessens damage from storms, erosion, and flooding. Reef crests are estimated to reduce wave energy by 86%, and reef flats can reduce it by up to 97%. Countries losing this natural protection may face higher costs from increased storm damage. Combined with lost tourism revenue, these effects could have major economic impacts.

High sea temperatures are the main cause of mass coral bleaching. Between 1979 and 1990, 60 major bleaching events occurred, affecting reefs worldwide. In 2016, the longest recorded bleaching event happened. The most damaging event was linked to the El Niño from 2014 to 2017, during which over 70% of global coral reefs were damaged.

Factors that influence how bleaching events affect reefs include how well corals resist stress, their ability to survive without their algae (zooxanthellae), and how quickly new coral grows to replace dead coral. Local conditions, like shade or cooler water, can reduce bleaching. Coral and algae health, as well as genetics, also play a role. Large coral colonies, like Porites, are more likely to survive extreme heat, while fragile corals, like Acropora, are more vulnerable. Corals regularly exposed to low stress may be more resistant to bleaching.

The first globally recognized bleaching event occurred in 1998, with 21% of reefs experiencing 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, becoming the fourth such event. It has affected reefs in at least 82 countries and all major ocean basins. By April 2024, NOAA confirmed the event, and 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. Although a shift to La Niña may help, regions like Florida have already seen complete reef die-offs, with temperatures reaching 101°F (38.3°C). The Great Barrier Reef is experiencing its fifth major bleaching event since 2016, showing the ongoing risks to these ecosystems.

The share of affected coral reefs by each of the four major bleaching events has been estimated as 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 a 61% prevalence 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. This bacterium attacks the algae in Mediterranean corals, causing bleaching. Vibrio shiloi is active only during warm periods, and higher temperatures increase its ability to infect corals. It attaches to receptors in coral m

By region

The Great Barrier Reef near the coast of Australia has had bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, 2006, 2016, 2017, 2022, 2024, and 2025. Some areas had severe damage, with up to 90% of corals dying. The most widespread and intense events happened in the summers of 1998 and 2002. In 1998, 42% of reefs were bleached to some degree, and 18% were strongly bleached. In 2002, 54% of reefs were bleached to some degree, and 18% were strongly bleached. Between 1995 and 2009, new coral growth helped reduce some coral losses. Overall, coral populations on the Great Barrier Reef dropped by 50.7% from 1985 to 2012. About 10% of this decline was due to bleaching, while the rest was caused 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 worst and most widespread coral bleaching ever recorded in the Great Barrier Reef. The most severe 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 highest coral death and reef habitat loss was inshore and mid-shelf reefs around Cape Grenville and Princess Charlotte Bay. A report from the IPCC predicted that corals on the Great Barrier Reef will likely experience summer temperatures high enough to cause bleaching regularly by 2100.

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 coral colonies were not affected by bleaching, while 193 were dead and 113 showed signs of bleaching.

In 1996, Hawaii’s first major coral bleaching happened 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 a warm ocean current called "The Blob." In 2014 and 2015, surveys in Hanauma Bay Nature Preserve on Oahu found 47% of corals suffering from bleaching, with nearly 10% dying. During the same period, 56% of the coral reefs on the Big Island were affected, and 44% of corals on west Maui were bleached. In January 2019, scientists found that reefs had begun to stabilize nearly four years after the last bleaching event. However, the Division of Aquatic Resources reported that up to 50% of coral reefs on Oahu and Maui were still bleached in 2019, and about 40% of corals on the Big Island’s Kona coast were bleached. 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 corals were 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 Democratic official, banned oxybenzone and octinoxate, chemicals that become toxic to coral when exposed to sunlight. Up to 10% of the 14,000 tons of sunscreen polluting coral reef areas contains oxybenzone, which puts 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, like rising water temperatures. The law banned all sunscreen products except prescription ones. Hawaii became the first U.S. state to implement this type of ban, which took effect in January 2021.

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

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

In summer 2024, rising sea temperatures caused a major bleaching event that killed 61.2% of corals off Amami-Oshima Island, Japan. The event was caused by sea temperatures 2°C higher than in 2023.

Coral reef provinces have been permanently damaged by warm sea temperatures, especially in the Indian Ocean. Up to 90% of coral cover was lost in the Maldives, Sri Lanka, Kenya, Tanzania, and the Seychelles during the 1997–1998 bleaching event. In 1998, 20% of coral in the Indian Ocean died, and 80% was bleached. The shallow tropical areas of the Indian Ocean are already experiencing ocean conditions predicted for the future. Coral that survived in these areas may be useful for restoring reefs elsewhere because they can withstand extreme conditions.

The Maldives has over 20,000 km of reefs, with more than 60% of coral bleached in 2016. The Maldivian reefs also face risks from tourism, coastal construction, land reclamation, and diseases.

Coral reef ecosystems are important along the western shoreline of the Gulf of Thailand. Bleaching events occurred in 1998 and 2010, with varying effects on coral

Coral Adaptation

In recent years, climate change has been connected to a significant rise in coral deaths. Studies show that bacteria living with corals help them survive extreme heat. Scientists have worked to make corals stronger against events called bleaching, which harm reefs. Since corals are the main part of coral reefs, their decline greatly affects the health and makeup of reefs, which in turn harms the animals that live there.

In 2010, scientists at Penn State found corals surviving in the warm waters of the Andaman Sea by using an unusual type of algae. Normal algae cannot live in such high temperatures, so this discovery was surprising. This suggests that as ocean temperatures rise due to global warming, corals might adapt by using algae that can survive heat. In 2010, researchers from Stanford University also found corals near the Samoan Islands that survive daily temperature spikes during low tide. These corals do not bleach or die despite the heat. Studies showed that corals near Ofu Island have learned to handle high temperatures. Scientists now wonder if corals from other areas can be trained to survive heat by slowly exposing them to short periods of warmth.

During mild bleaching events, corals produce pigments that act like sunscreen to protect themselves. Some pigments have pink, blue, or purple colors, while others glow brightly. These pigments are made more when corals are exposed to blue light. When corals bleach, blue light inside their tissues increases because the algae that normally absorb the light are gone, and the light reflects off the white coral skeleton. This causes more pigments to form, making bleached corals look colorful instead of white—a phenomenon called "colorful coral bleaching."

Higher ocean temperatures cause the outer layer of corals to thin and the inner cells to die. This loss of cells leads to a sharp drop in the number of algae living inside the coral. Under high heat or bright light, corals produce harmful chemicals called reactive oxygen species. If these chemicals are not removed by the coral’s defenses, the coral dies. Studies show that corals exposed to heat become thinner than those not exposed. When algae leave corals during heat stress, the corals must find new ways to get energy. Some corals that eat more animals are better at recovering from bleaching.

After algae leave, other types of algae often take over coral structures because they need fewer resources to survive. These algae outcompete the original algae. Once algae dominate, corals struggle to survive. Ocean acidification, which happens when carbon dioxide dissolves in seawater, reduces the amount of carbonate ions needed for corals to build their skeletons. Corals naturally build and break down their skeletons daily and yearly due to temperature changes. Under current climate predictions, corals may not have enough time to rebuild their structures during cooler winter months, leading to their breakdown.

Helping Repair the Damage

The US National Oceanic and Atmospheric Administration (NOAA) tracks areas called "bleaching hot spots," which are places where ocean temperatures rise 1°C or more above the usual average for that time of year. These hot spots show where heat stress affects coral reefs. Scientists use a system called Degree Heating Week (DHW) to measure how much heat stress corals experience. Satellites help detect rising ocean temperatures earlier, which allows scientists to notice coral bleaching events sooner. Monitoring high temperatures is important because coral bleaching harms coral reproduction, growth, and can lead to coral death. NOAA detected the global coral bleaching event in 1998, which happened during the 1997–98 El Niño. Today, NOAA monitors 190 reef sites worldwide and sends alerts to scientists and reef managers through the NOAA Coral Reef Watch (CRW) website. By tracking ocean warming, 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 the El Niño weather pattern, which raised ocean temperatures and worsened coral health. The 2014–2017 El Niño was the longest and most harmful to corals, damaging over 70% of coral reefs globally. More than two-thirds of the Great Barrier Reef have been reported as bleached or dead.

To better understand how bleaching events spread, scientists use underwater photography and AI tools to create detailed maps of coral reefs. These maps help identify the health of coral reefs.

After a bleaching event, some reefs can recover by regaining their algae, called zooxanthellae, which provide corals with energy. Other reefs may shift to being dominated by algae, which makes it harder for corals to grow. Algae can block sunlight and release chemicals that stop corals from settling. This weakens coral reefs and makes them more vulnerable to other problems, like poor water quality and loss of fish that eat algae. Scientists are studying why some reefs recover while others do not, as this knowledge helps protect corals.

A key area of research is "super-corals," which are corals that live in naturally warmer and more acidic waters. These corals are more resilient and may help restore damaged reefs. Scientists like Emma Camp are studying how these corals might help rebuild reefs in the future. While it can take 10–15 years to restore reefs, super-corals may help reefs survive as oceans get warmer and more acidic. Research by Ruth Gates has also explored how corals survive in extreme environments with low oxygen.

Corals can recover from short-term problems, such as storms or invasions by sea stars. However, coral bleaching causes more lasting damage. Fish populations often recover better than corals after disturbances, but bleaching can change the types of fish that live on reefs. Studies show that after bleaching, reefs have fewer fish that depend on corals and more generalist species. While rising ocean temperatures do not directly kill adult fish, they harm coral-dependent fish by reducing their habitat. Some herbivorous fish, which eat algae, increase in number because algae grow on dead coral. Scientists need better ways to measure how disturbances affect coral recovery.

Until recently, little was known about what helps coral reefs recover from bleaching. A study by Graham et al. (2015) looked at 21 reefs in Seychelles and found that after losing over 90% of their corals in 1998, about half of the reefs recovered, while 40% shifted to being dominated by algae. Factors that helped recovery included the number of young corals, the complexity of reef structures, water depth, the amount of herbivorous fish, and nutrient levels on the reef. Reefs with complex structures and deeper water were more likely to recover.

The types of fish living on reefs also affect recovery. Fish that remove dead coral, algae, or sediment help corals grow. Fewer grazing fish after bleaching can lead to more algae, which harms coral. In the Caribbean, fewer grazing fish have been linked to sea-urchin-dominated systems, which do not shift to algae-dominated conditions.

Scientists are still learning about hidden changes in coral communities that may affect recovery. These changes can lead to unexpected shifts in reef ecosystems. Better methods to assess reef health and long-term changes are needed to protect reefs in the future.

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 creating "super-corals" that can survive harsher conditions. Van Oppen is also developing algae that can help corals survive in changing environments.