Despite the importance of coral reefs, these wildlife habitats are imperiled throughout the world. A recent report estimated that 75 percent of remaining coral reefs are currently threatened, and many have already been lost. Even some of the most remote and pristine reefs are losing species.
Although there are many problems facing reefs today, rising seawater temperature as a result of climate change  is one of the most serious causes of stress to corals throughout the world. When temperatures are too high, the relationship between corals and their symbiotic microalgae breaks down. The algae are what give corals some of their bright colors, so when this happens, corals appear white or "bleached." Just one degree above the typical summer max is enough to bleach many corals. If the temperature is too high for too long, corals and their microalgae are unable to recover. Over the past 30 years, bleaching has become more frequent, more intense, and more widespread. This has led to massive die offs of corals throughout the world.
Warmer ocean temperatures cause even more problems when it comes to disease – high temperatures allow corals to become sick more easily, and allow disease-causing organisms to grow faster. There is a huge array of different diseases in corals. Most of them are named after how they change a sick coral’s appearance, like black band, white band, white spot and purple blotch diseases.
As more carbon dioxide (CO2) is released into the atmosphere, it also has adverse effects on the oceans. Recently, ocean acidification has emerged as another potentially serious threat to coral reefs. Seawater absorbs some of the excess CO2 from the atmosphere, causing the oceans to become more acidic. As a result, the oceans’ acidity has increased by 25 percent over the past 200 years. These acidic conditions dissolve coral skeletons, which make up the structure of the reef, and make it more difficult for corals to grow. If left unchecked, scientists estimate that the oceans could become 150 percent more acidic by the end of this century, making it very hard for corals to grow at all.
Coral reefs are also being degraded by many other factors. The list of problems can seem endless: overfishing, fishing using cyanide and dynamite, pollution from sewage and agriculture, massive outbreaks of predatory starfish, invasive species, and sedimentation from poor land use practices. Reefs and their wildlife across the world are also affected by destructive fishing and exploitation to supply the coral reef wildlife trade. Fish, corals, and various invertebrates are all taken from reef habitats to serve as aquarium pets or decorative items. Although this trade can be conducted sustainably, wildlife populations are often overexploited to feed the demand for these animals. Sometimes poisons like cyanide are dumped into the water to stun fish and make them easier to capture. Sadly, fishing with cyanide often kills fish, corals, and other forms of wildlife, while degrading the reef habitat itself.