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John Bruno kicks off another part of his Reef Science coral series, excerpting a modified version of an article he published last year on the Earth Portal about Coral Reefs and Climate Change. He covers global patterns of coral loss and several of the mechanisms through which anthropogenic climate change is contributing to this trend including coral bleaching, disease and ocean acidification. Let him know know if there are particular topics you want to hear about. You can reach him at [email protected]

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.

Coral Bleaching

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 that 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.

Carbon dioxide, methane, and other greenhouse gases trap heat, leading to global warming. The increase in ocean temperature is variable and quite subtle: on the order of 1° C over the last several decades. But even such modest changes have caused mass-coral mortality events around the world during some of the especially warm summers we have all experienced over the last ten years. In 1998 when an intense El Niño greatly warmed much of the western Pacific and Indian Oceans, coral bleaching was widespread, causing mass coral mortality in many countries. For example, in Palau, more than 90% of the corals on some reefs bleached and at least 50% perished. Even some isolated reefs were impacted. In the Maldives, in the east Indian Ocean, bleaching caused coral cover to plummet to only about 5%.

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