There are many causes of local and global coral loss but human-induced climate change is one of the main and undeniable threats. Climate change is having negative effects on coral populations via at least three mechanisms.
First, ocean warming is directly reducing coral cover through coral bleaching. Reef-building corals contain plant-like organisms called zooxanthellae that live symbiotically within their tissue. Zooxanthellae provide their coral host with food and oxygen and in return, the zooxanthellae receive nutrients, carbon dioxide, and an enemy-free shelter. This symbiotic relationship evolved tens of millions of years ago and has been critical to the success and evolutionary radiation of corals and to the development of reef ecosystems. When summertime water temperatures are just a degree or two warmer than usual for a few weeks, this critical yet delicate symbiotic relationship breaks down and the zooxanthellae are expelled, often leading to the coral’s death. (The greater the magnitude or duration of the warming, the greater the mortality and effect on coral populations.) The phenomenon is called “coral bleaching” because the coral animal appears to turn white after the zooxanthellae loss. This is because without their zooxanthellae symbionts, which contain various photosynthetic pigments, corals are nearly transparent and the white, external calcium carbonate skeleton that the coral polyps live on becomes plainly visible.