What is bleaching?
Simply, it is the release of coral symbiotic zooxanthellae. A more in depth definition is the loss of all or some symbiotic algae and photosynthetic pigments by the coral animal resulting in their white calcium carbonate skeleton becoming visible through the now translucent tissue layer, making them appear white or ‘bleached’ of their original color  
Corals and symbiotic relationship
In many reefs, corals and a photosynthetic algae called zooxanthellae have a mutualistic relationship. A coral helps the zooxanthellae by providing the materials needed for photosynthesis as well as a safe environment to live. In return, the zooxanthellae help the coral remove waste by producing oxygen and supply the coral with the products of their photosynthesis. There are two classifications of coral species concerning zooxanthellae: zooxanthellate and azooxanthellate. Zooxanthellate corals are found covering much of the shallow water reefs in tropical regions. Corals that can survive under both classifications are extremely rare. Scleractinian corals provide an example of common zooxanthellae symbiosis. .
Expulsion of zooxanthellae
Normally, the zooxanthellae give the corals their color and provides the mutually beneficial relationship discussed above. Under stressors however, the symbiotic relationship between the coral animal and zooxanthellae can become harmful, and the presence of the symbiotic organism is actually toxic to the coral. When this relationship becomes toxic, the coral expels the zooxanthellae and the coral becomes transparent and appears white as a result of their calcium carbonate backbone structure showing through. While it is difficult for the coral to survive without the zooxanthellae, the release of them is not an instant death sentence like holding on to them would be. If normal or healthy conditions return in time, the corals are able to take in new zooxanthellae and start functioning normally again. 
There are several different causes of bleaching, and many of the causes of this phenomenon have yet to be studied or are not well understood. Most studies point to some form of stress that induces the coral to bleach. Stressors include:
Increasing sea surface temperatures (SST) have been seen throughout the Tropics for the majority of the recent past. Studies have shown that temperatures have increased by just under 1oC over the past century. Many people do not see the significance of a couple of degrees rise in SST. To put this rise in perspective, data suggests that tropical ocean temperatures have varied by less than 2oC over the past 18,000 years. Current variations are 1-2oC over only the past 100 years and SSTs are still rising! This temperature rise places corals in rare conditions they are not physically designed for; therefore corals are now living close to the upper limit of their ideal temperature range. Corals prefer temperatures between 18oC and 30oC, and current SSTs have risen and remained close to 28oC which causes stress to the corals.
Stress on corals
As mentioned above, corals are currently being forced to live at the upper end of their thermal (heat) tolerance. This is akin to intense heat waves experienced by humans that cause heat sickness, dehydration, and, in extreme cases, death. Corals are similarly stressed by consistently high temperatures which cause decreased productivity, bleaching, and possibly death. Ove Hoegh-Guldberg et al. expresses concern that if warming trends continue, corals will be pushed beyond their thermal tolerance.
Affects on symbiotic relationship
Studies show that increasing SSTs can have negative impacts on the symbiotic relationship between corals and zooxanthellae, mainly on the rates of photosynthesis. Corals receive a large amount of their energy from photosynthesis via zooxanthellae. As corals become stressed, rates of photosynthesis in the zooxanthellae decrease and reduce the amount of energy corals receive which causes further stress. Rates of photosynthesis decrease because the actual processes and chemical reactions that make up photosynthesis are damaged. There are two stages of photosynthetic reactions (light and dark reactions) and it was originally thought that the light reactions were being damaged. However, Jones et al. (1998) (from Ove Hoegh-Guldberg et al.) determined that damage occurred in the dark reactions because they are unable to process the influx of material from the light reactions and harmful products are produced. One main harmful product is oxygen free radicals, which can cause the cells to become toxic. Once the cells of zooxanthellae have become toxic, the symbiotic relationship between the zooxanthellae and the coral becomes detrimental to the corals’ health and the corals bleach.
Past, present, and future predictions
Past bleaching events have been correlated with El Niño Southern Oscillation (ENSO) warm cycles, commonly known as El Niño events. These events cause periods of high sea surface temperatures and movement of warm water over areas that coral reefs inhabit. A program that tracks the movement of warm water is called the “Hotspot” program, which is directed by the US National Oceanic and Atmospheric Administration (NOAA). This program tracks and predicts the movement of spots of unusually warm water (associated with El Niño events). This program has been used to accurately predict where mass bleaching events will occur days or even weeks before the bleaching event is observed. Evidence of the effectiveness of this bleaching ‘forecast’ can be seen with the discussion of the 1998 Great Barrier Reef bleaching event.
Future predictions of the effects of SSTs on corals are dire. Anthony et al. predict that intense bleaching events will increase in frequency to the point where they will become yearly events in 2050 worldwide. In 20-40 years, bleaching will no longer be triggered by high SSTs associated with El Niño events alone, but will be triggered by high SSTs due to seasonal temperature variations (essentially every summer). In the next 30-50 years, severe bleaching events will become commonplace and will occur annually. Overall, evidence shows that the frequency of bleaching events caused by SST will exceed the amount of time corals require to recover from bleaching events. This means a shift to algal dominated communities or the “complete loss of coral reefs on a global scale.”
While SST and bleaching events are highly correlated, there is still variability that is not entirely explained by increases in SST. One of which is the tendency, at a local scale, of coral colonies to show a gradation of bleaching, meaning the tops bleach first and more intensely than the sides of the coral. SST is uniform around the coral colony so is not a likely cause of this local bleaching trend. There must be other factors that affect coral bleaching and some are discussed below.
As a result of the 2015 El Niño, NOAA started monitoring the "third ever global coral bleaching event". Read More. Australia's Great Barrier Reef was one of the hardest hit areas. Read the Long-term Reef Monitoring Program - Annual Summary Report on coral reef condition for 2016/17
Impact of acidification on bleaching
As ocean acidity increases (lower pH), and calcium carbonate dissolves, it becomes harder and harder for corals to make their exoskeleton. This limits coral growth and causes increased stress and, as shown in experiments from Anthony et al., an increase in bleaching events. In their experiment, two primary reef builders (calcifying corraline algae, and Acropora or elkhorn coral) were dosed with low (control), intermediate, and high levels of CO2 for eight weeks. The CO2 pumped into the water lowered the pH and modeled the ocean acidification process. Anthony et al observed 40-50% bleaching in both species at high levels of CO2, meaning that high levels of CO2 (lower pH and increased acidity) caused 2 to 3 times more bleaching than the control group. An intermediate amount of CO2 led to 20-30% bleaching (1 to 2 times more bleaching than controls). Anthony et al. also observed greater bleaching when both temperature and CO2 concentrations were increased, suggesting that high SST amplifies the effects of acidification and increases bleaching even more. Anthony et al. warn that accurate future predictions must take into account this synergistic relationship between SST and acidification. They also express concern that possible adaptation of corals to higher SST may be offset (and therefore made negligible) by the effects of acidification. [[File:Pteropod.jpg|thumb|400x120px|Pteropod shells dissolving after 45 days in acidic water 
High solar irradiance
Light is required for corals to grow because their symbionts, zooxanthellae, require light to photosynthesize; however there is such a thing as too much light. Too much light can cause photoinhibition: the increased sensitivity of photosynthetic organisms to light which inhibits photosynthesis by damaging photosynthetic pathways. Studies have shown that high levels of ultraviolet radiation caused decreased growth rates, decreased photosynthesis rates, less cellular chlorophyll, and reduced carbon:nitrogen ratios which are essential for normal life. These effects on the zooxanthellae reduce their productivity to the point where it is better for the coral to expel them and bleaching occurs.
Corals thrive in a very specific environment that needs to be stable for maximum growth. Factors that can dramatically change coral environments include nutrient overload, sedimentation, chemical runoff, and reduced salinity. One of the major threats to coral ecosystems and environments are human activities. Ove Hoegh-Guldberg et al. reports that 50% to 70% of coral reefs worldwide are directly threatened by human activities. Some of the main activities include overfishing and exploitation of reef species, and destruction of the reef for economic gain through activities like cyanide and dynamite fishing.
Bleaching events can have detrimental effects on coral survivability. Rates of surviviability and rates of mortality are coupled; survivability is the number of organisms that live and mortality is the number of organisms that die. Scientists note that mass bleaching events can cause mortality of 0% (for mild bleaching events) to almost 100% mortality for severe bleaching events. The severity of bleaching events is directly correlated with mortality; therefore the more severe the bleaching is, the more corals will die. Bleaching severity is determined by how much temperatures increased and how long they remained high; it has been determined that the severity and intensity of bleaching events will increase, which will cause an increase in coral mortality and a decrease in survivability.
Coral reproduction is related to mortality. If bleaching events cause mortality of corals before they are able to reproduce, overall reproduction will decrease. Some corals take 4-5 years or more to reach reproductive age, but bleaching events are currently occurring every 4 years. If the frequency of bleaching events increases, which is expected, corals may be unable to reproduce at all and the species will become extinct. Even if corals survive to reproduce, high SSTs have been shown to reduce reproductive capacity (the number of offspring produced). An experiment studying colonies of reef flat corals observed the recovery of bleached colonies and their reproduction. This study found that bleached colonies did not contain eggs when they would normally have been preparing to spawn a few months later. Additionally, even after these corals recovered from the bleaching event, they were unable to spawn. If bleaching prevents coral spawning even when corals have recovered, how will coral colonies replenish the population lost from bleaching events alone?
Mass bleaching episodes can drastically change community structure. Death of corals due to bleaching could cause a shift to algal dominated reefs and change the habitat that so many species depend on. The majority of reefs primary productivity and primary shelter for marine organisms are provided by corals. Without corals, hundreds to thousands of species of crustaceans, mollusks, fish, marine mammals, and more will be impacted. The figure below shows pathways of disturbance caused by climate change:
Special Case Study
US Virgin Islands: Bleaching 2005
In the summer of 2005, unusually calm seas and record setting warm seawater temperatures induced the most devastating coral bleaching ever recorded in the U.S. Virgin Islands (USVI) with coral cover being more than 90% bleached and stretching from the surface to 20 meters deep.  This weather shock was followed by a breakout of disease on the reefs, which led to increase monitoring, but more partial or total coral death in the USVI.
Two corals that create much of the framework of reefs in USVI were heavily affected: Acropora palmata and Montastraea annularis.
- Montastraea annularis, while remaining very present on the reefs, dropped from 55.6 percent to 40.9 percent of coral cover over the 2 years following the bleaching event
- "Acropora palmate was less affected by disease, but 50% of the corals being watched in 2005 showed signs of bleaching, marking the first record of A. palmate bleaching in the USVI.
Researchers showed a correlation between temperature and disease, but only with the 2005 massive bleaching event. They found the size of white syndrome lesions, like white pox, increased as temperature increased for bleached colonies, but not other healthy colonies.  Even with cooler temperatures, the coral cover continued to decline, with the mean coral cover decreasing 61% through 2007. At peak disease after bleaching, “the mean number of lesions increased 51-fold and the mean area killed by disease increase 30 fold” in reference to levels prior to the bleaching event.  This is the only study to show such extreme declines in coral cover from disease covering such a large area. 
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What we are currently doing to sustain coral ecosystems
USGS “Science Based Strategies” example
Coral reefs are necessary in supporting many different ecosystems and populations, but these marine ecosystems and all the benefits that come with them are at risk. Scientific evidence continues to show that the threat to coral reefs is serious, and could cause severe large scale social and ecological consequences if not addressed. Although several threats and stressors are known, there are still strides that need to be made in how scientists respond to and explain both natural and human influenced changes in reef ecosystems. Science needs to improve how we respond to and/or prevent the current problems such as carbon sequestration, ocean temp, ocean acidification, sea level rise, atmospheric dust, land-derived impacts, pathogens and disease, overfishing and ecological integrity 
For more information see Protecting and Managing Coral Reefs
- “New Maps Depict Potential Worldwide Coral Bleaching by 2056”. The Daily Catch. February 27, 2013. http://theterramarproject.org/thedailycatch/new-maps-depict-potential-worldwide-coral-bleaching-by-2056/
- H. van Oppen, M. J., & Lough, J. M. (2009). Coral bleaching: Patterns, processes, causes and consequences. (Vol. 205). Springer Berlin Heidelberg. Retrieved from http://link.springer.com/book/10.1007/978-3-540-69775-6/page/1
- T. Tyrrell (2007). Calcium Carbonate Cycling in Future Oceans and its Influence on Future Climates. J. Plankton Res. (2008) 30 (2): 141-156. doi: 10.1093/plankt/fbm105
- Outlook for the Reef: Coral bleaching. Australian Government. Great Barrier Reef Marine Park Authority. http://www.gbrmpa.gov.au/outlook-for-the-reef/climate-change/what-does-this-mean-for-species/corals/what-is-coral-bleaching
- NOAA. (2008, March). National Ocean Service Education: Corals. Retrieved from http://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html
- Ove Hoegh-Guldberg et al. Climate Change Coral Bleaching and the Future of the World's Coral Reefs. University of Sydney. Amsterdam : Greenpeace International, ca. juni 1999. ISBN 90-73361-52-4.
- Alan Berman. “Coral Reefs in Crisis, a Review”. September 2012. http://ecologicalrenaissance.wordpress.com/2012/09/27/global-and-regional-threats-to-coral-reef-ecosystems/
- S. C. Doney. “The Dangers of Ocean Acidification.” Scientific American. March 2006. pg 62.
- K.R.N. Anthony, et al. (2008). Ocean acidification causes bleaching loss in coral reef builders. PNAS vol. 105 no. 45.
- O. Hoegh-Guldberg, et al. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidification. Science 318, 1737.
- USGS. (2008, June). Coral diseases following massive bleaching in 2005 cause 60 percent decline in coral cover and mortality of the threatened species, acropora palmata, on reefs in the u.s. virgin islands. Retrieved from http://pubs.usgs.gov/fs/2008/3058/pdf/fs2008-3058.pdf
- USGS. (2008, September). Science-based strategies for sustaining coral ecosystems. Retrieved from http://pubs.usgs.gov/fs/2009/3089/pdf/brewercoralfs3.pdf