When corals lose their vivid colours and turn white, this is known as coral bleaching. There's a lot more to it than that, though. Coral is brilliant and vivid thanks to zooxanthellae, a type of microscopic algae. The zooxanthellae live in a mutually beneficial relationship within the coral, one assisting the other to survive. However, when the ocean environment changes—for example, if it becomes too hot—the coral becomes stressed and expels the algae. The coral fades when the algae dies, giving it the appearance of being bleached. If the temperature remains high, the coral will not allow the algae to return, and it will die.

The greatest global threat to coral reef ecosystems is climate change. Scientific evidence now clearly shows that the Earth's atmosphere and oceans are warming, and that this warming is mostly caused by greenhouse gases emitted by human activities.

Mass coral bleaching episodes and infectious disease outbreaks are becoming more common as global temperatures rise. Furthermore, carbon dioxide absorbed from the atmosphere has already begun to impair calcification rates in reef-building and reef-associated animals through modifying seawater chemistry through pH changes. Ocean acidification is the term for this phenomenon.

Climate change will affect coral reef ecosystems, through sea level rise, changes to the frequency and intensity of tropical storms, and altered ocean circulation patterns. When combined, all of these impacts dramatically alter ecosystem function, as well as the goods and services coral reef ecosystems provide to people around the globe.