Coral bleaching is the process where corals become white due to loss of symbiotic algae and photosynthetic pigments. This loss of pigment can be caused by various stressors, such as changes in water temperature, light, salinity, or nutrients. A bleached coral is under stress, more vulnerable to starvation and disease, and at risk of death. The leading cause of coral bleaching is rising ocean temperatures due to climate change. Take up of carbon dioxide from the atmosphere also has been acidifying the oceans and affects coral health. Large-scale bleaching events can devastate entire reef ecosystems, affecting biodiversity and the livelihoods of coastal communities that depend on healthy reefs for food, tourism, and protection from storm surges.
According to the United Nations Environment Programme, between 2014 and 2016, the global bleaching events killed coral on an unprecedented scale for the time. In 2016, bleaching of coral on the Great Barrier Reef killed 29 to 50 percent of the reef's coral. Bleaching events impact many regions of the world, including Australia, Hawaii, Japan, and more. Natural corals adapted against bleaching and human assistance with reef building may help prevent further damage.
On April 15th, 2024, NOAA (National Oceanic and Atmospheric Administration) confirmed the world was in the midst of its fourth and largest global coral bleaching event. From 1 January 2023 to 30 September 2025, bleaching-level heat stress impacted ~84.4% of the worlds coral reef area and bleaching has been documented in at least 83 countries and territories.
Process The corals that form the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like single-celled flagellate protozoa called zooxanthellae that live within their tissues and give the coral its coloration. The zooxanthellae provide the coral with nutrients through photosynthesis, a crucial factor in the clear and nutrient-poor tropical waters. In exchange, the coral provides the zooxanthellae with the carbon dioxide and ammonium needed for photosynthesis. Negative environmental conditions, such as abnormally warm or cool temperatures, high light, and even some microbial diseases, can lead to the breakdown of the coral/zooxanthellae symbiosis. According to a study done by D.J. Smith et al., photoinhibition is a likely factor in coral bleaching. It also suggests that the hydrogen peroxide produced in zooxanthealle plays a role in signaling themselves to flee the corals. To ensure short-term survival, the coral-polyp then consumes or expels the zooxanthellae. This leads to a lighter or completely white appearance, hence the term "bleached". Under mild stress conditions, some corals may appear bright blue, pink, purple, or yellow instead of white, due to the continued or increased presence of the coral cells' intrinsic pigment molecules, a phenomenon known as "colourful bleaching". As the zooxanthellae provide up to 90 percent of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve.Coral can survive short-term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish. In order to recover from bleaching, the zooxanthellae have to re-enter the tissues of the coral polyps and restart photosynthesis to sustain the coral as a whole and the ecosystem that depends on it. If the coral polyps die of starvation after bleaching, they will decay. The hard coral species will then leave behind their calcium carbonate skeletons, which will be taken over by algae, effectively blocking coral regrowth. Eventually, the coral skeletons will erode, causing the reef structure to collapse.
Common triggers Coral bleaching may be caused by a number of factors. While localized triggers lead to localized bleaching, the large-scale coral bleaching events of recent years have been triggered by global warming. Under the increased carbon dioxide concentration expected in the 21st century, corals are expected to become increasingly rare on reef systems. Coral reefs located in warm, shallow water with low water flow have been more affected than reefs located in areas with higher water flow. Marine heatwaves caused by the El Nino Southern Oscillation have been found to be one of the main causes of widespread coral bleaching and consequent coral mortality.
List of triggers
increased water temperature (marine heatwaves, most commonly due to global warming), or reduced water temperaturesAnthony, K. 2007; Berkelmans increased solar irradiance (photosynthetically active radiation and ultraviolet light) increased sedimentation (due to silt runoff) bacterial infections changes in salinity herbicides extreme low tide and exposure cyanide fishing elevated sea levels due to global warming (Watson) mineral dust from African dust storms caused by drought pollutants such as oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene: four common sunscreen ingredients that are nonbiodegradable and can wash off of skin ocean acidification due to elevated levels of CO2 caused by air pollution being exposed to oil or other chemical spills changes in water chemistry, particularly an imbalance in the ratio of the macronutrients nitrate and phosphate meteorological conditions
Trends due to climate change
Extreme bleaching events are directly linked with climate-induced phenomena that increase ocean temperature, such as El Niño-Southern Oscillation (ENSO). The warming ocean surface waters can lead to bleaching of corals which can cause serious damage and coral death. The IPCC Sixth Assessment Report in 2022 found that: "Since the early 1980s, the frequency and severity of mass coral bleaching events have increased sharply worldwide".Cooley, S., D. Schoeman, L. Bopp, P. Boyd, S. Donner, D.Y. Ghebrehiwet, S.-I. Ito, W. Kiessling, P. Martinetto, E. Ojea, M.-F. Racault, B. Rost, and M. Skern-Mauritzen, 2022: Chapter 3: Oceans and Coastal Ecosystems and Their Services. In: Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 379–550, doi:10.1017/9781009325844.005. Coral reefs, as well as other shelf-sea ecosystems, such as rocky shores, kelp forests, seagrasses, and mangroves, have recently undergone mass mortalities from marine heatwaves. It is expected that many coral reefs will "undergo irreversible phase shifts due to marine heatwaves with global warming levels >1.5°C".This problem was already identified in 2007 by the Intergovernmental Panel on Climate Change (IPCC) as the greatest threat to the world's reef systems.
The Great Barrier Reef experienced its first major bleaching event in 1998. Since then, bleaching events have increased in frequency, with three events occurring in the years 2016–2020. Bleaching is predicted to occur three times a decade on the Great Barrier Reef if warming is kept to 1.5 °C, increasing to every other year at 2 °C.
With the increase of coral bleaching events worldwide, National Geographic noted in 2017, "In the past three years, 25 reefs—which comprise three-fourths of the world's reef systems—experienced severe bleaching events in what scientists concluded was the worst-ever sequence of bleachings to date."
In a study conducted on the Hawaiian mushroom coral Lobactis scutaria, researchers discovered that higher temperatures and elevated levels of photosynthetically active radiation (PAR) had a detrimental impact on its reproductive physiology. The purpose of this study was to investigate the survival of reef-building corals in their natural habitat, as coral reproduction is being hindered by the effects of climate change.
Role of ocean acidification The carbon cycle consists of fluxes between carbon sinks and the oceans of the Earth store large amounts of carbon. Because of this, when humans emit carbon into the atmosphere from burning fossil fuels a large sum of carbon will dissolve into the oceans. This addition disturbs the natural CO2-carbonic acid equilibrium that buffers the ocean and results in a decrease in pH or "acidification." Increasing ocean acidification also works synergistically with thermal stress to induce bleaching effects. For example, an 8-week experiment on Heron Island off the coast of Australia observed 40-50% of crustose coralline algae and Acropora to be bleached following high-CO2 dosing. Acidification affects the corals' ability to create calcareous skeletons, essential to their survival. This is because ocean acidification decreases the amount of carbonate in the water, making it more difficult for corals to absorb the calcium carbonate they need for the skeleton. As a result, the resilience of reefs goes down, while it becomes easier for them to erode and dissolve. In addition, the increase in CO2 allows herbivore overfishing and nutrification to change coral-dominated ecosystems to algal-dominated ecosystems. A recent study from the Atkinson Center for a Sustainable Future found that with the combination of acidification and temperature rises, the levels of CO2 could become too high for coral to survive in as little as 50 years.
Impacts
Coral bleaching events and the subsequent loss of coral coverage often result in the decline of fish diversity. The loss of diversity and abundance in herbivorous fish particularly affects coral reef ecosystems. As mass bleaching events occur more frequently, fish populations will continue to homogenize. Smaller and more specialized fish species that fill particular ecological niches that are crucial for coral health are replaced by more generalized species. The loss of specialization likely contributes to the loss of resilience in coral reef ecosystems after bleaching events.
Economic and political impact According to Brian Skoloff of The Christian Science Monitor, "If the reefs vanished, experts say, hunger, poverty and political instability could ensue."Skoloff, Brian (26 March 2010) Death of coral reefs could devastate nations , The Christian Science Monitor Coral reefs provide shelter to an estimated quarter of all ocean species. Since countless sea life depend on the reefs for shelter and protection from predators, the extinction of the reefs would ultimately create a domino effect that would trickle down to the many human societies that depend on those fish for food and livelihood. Experts estimate that coral reef services are worth up to $1.2 million per hectare which translates to an average of $172 billion per year. The benefits of coral reefs include providing physical structures such as coastal shoreline protection, biotic services within and between ecosystems, biogeochemical services such as maintaining nitrogen levels in the ocean, climate records, and recreational and commercial (tourism) services. There has been a 44% decline in coral reefs over the last 20 years in the Florida Keys and up to 80% in the Caribbean alone.