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Coral bleaching occurs when coral polyps expel algae that lives inside their tissues. Algae living in the tissues of coral polyps ensure that the coral reef as a whole stays alive and healthy. However, the coral polyps will expel the algae if environmental factors prevent the coral from providing nutrients to the algae that are required for photosynthesis. Coral that has bleached will still be alive, but the coral is stressed and will die if conditions do not improve, as the algae provide nearly all of the coral's energy. Coral bleaching is an ongoing process, and bleaching events have been well documented. In the Great Barrier Reef alone, more than a third of the reef's coral has died out due to mass coral bleaching events in 1998, 2002, and 2016.

Causes

Coral polyps live in an endosymbiotic relationship with algae. The algae give chemical energy from photosynthesis to the coral, and in exchange, the coral shelters the algae and gives it nutrients. Without nutrients from the coral, the algae cannot perform photosynthesis, and in turn, cannot provide energy to the coral. Many triggers can prevent nutrients from getting to the algae, causing the coral to expel the algae from the tissues.

For mass bleaching events, an increase in water temperatures has been cited as the primary cause of concern. The water surrounding the coral gets warmer due to global warming and El Niño-related weather events. Corals living in warmer and shallower waters are the most at risk of severe bleaching, as temperatures in those areas rise to well above the threshold of survival.

Localized bleaching events may be triggered by other environmental factors, including:

• Ocean acidification
• El Niño weather events
• Climate change caused by global warming and human activity.
• Pollution
• Predators

Reefs who suffer from localized bleaching events are more likely to recover from bleaching. Mass-bleaching events, however, are more likely to result in the coral polyps dying.

Recovery

If coral polyps survive a bleaching event, then they need to recover to their pre-bleaching state in order to sustain the coral as a whole and the ecosystem that depends on it. In order to recover from bleaching, algae have to get back into the tissues of the coral polyps. Factors that contribute to coral reefs recovering have not been well understood until recently when mass coral bleaching events have become more obvious. Coral reefs can survive short-term disturbances, such as storms and predators if conditions are favorable, but in mass coral bleaching and die-off events, the reef's chances of survival slim, with some areas never recovering completely. Different species of coral polyps and algae are adapted to deal with bleaching in different ways. Coral algae that have evolved to gain resistance to warmer waters are more likely to recover from a mass-bleaching event, as it can carry out functions in temperatures that would normally lead to expulsion, even if human intervention is required.

Coral Death

If coral polyps die of starvation as a result of a bleaching event that it cannot recover from, then they will decay first, leaving behind the exoskeleton. The softer exoskeletons decay quickly. Macro algae (seaweed), will then take over the reef once the soft skeletons decay. The macro algae effectively blocks fish from accessing the reef, making then unable to use it as for food and shelter. Eventually, the harder skeletons erode away, causing the reef structure to collapse.

In the Great Barrier reef, more than 20 percent of the reef has been destroyed by coral death caused by bleaching. The reef is expected to keep bleaching in 2017, raising fears from scientists that more of the reef will die in the following years. In the Florida Keys, nearly all of area reefs in the region died out, with survivors recovering only because of genetic mutations and human intervention. Even with mutations and human intervention, however, the effects of bleaching in the Florida Keys and other regions remains to be seen. Currently, aerial inspections are being done over many coral reef areas around the world to understand the full extent of the death of coral reefs over large geographical areas.

Impact

Mass coral bleaching events have been well documented and have increased in frequency in recent years. In 2016, a mass-bleaching event hit Australia's Great Barrier Reef, killing more than 60 percent of the reef. This surpassed bleaching events that occurred in 1998 and 2002. In 2017, the coral in the reef bleached further into areas that previously was healthy and would not bleach. Other coral reefs have been affected by bleaching, including those off the coast of the Maldives, Sri Lanka, Seychelles, and along coastal areas in Kenya and Tanzania. In 2014, the first mass bleaching event in Hawaii was recorded, which biologists claimed was caused by warm waters associated with The Blob.

Economic impact

Most sea life requires the food and shelter coral reefs provide to survive. If coral reefs die due to bleaching, the entire ecosystem that supports fished marine life will collapse, causing marine life to disappear, devastating fisheries. It has been estimated that the costs of dying coral reefs to the fishing industry could be more than $49 billion. In addition, coral reefs are essential to the tourism industry, as people are attracted to coral reefs for recreation. If the coral reefs are lost due to mass coral bleaching, the estimated costs to the American tourism industry could end up being more than $2 billion.

In the Great Barrier Reef, it is estimated that damages to the reef associated with mass bleaching events would cost more than $3 billion a year, vastly affecting the Australian economy. Other areas are even more hard hit, as their entire economies depend on the reefs, and if they go, their economies, and possibly their societies, would collaspe.