The term "coral reef" generally refers to a marineecosystemin which the main organisms are corals that house algal symbionts within their tissues. These ecosystems require: 1) fully marine waters; 2) warmtemperatures; and 3) amplesunlight. They are therefore restricted to shallow waters of tropical and subtropical regions. The more technical definition of "coral reef" includes an additional geological requirement that the reef organisms produce enough calcium carbonate to build the physical reef structure.
Coral reefs are often considered the marine counterpart to rainforests because of their highbiodiversity(driven in large part because the reef itself provides a tremendous amount of structural complexity), the elaborate specializations of the resident species, and the many co-evolved relationships between species.
Climate change threatens coral reefs and their incredible biodiversity in two ways: higher ocean temperatures and ocean acidification. Coral reefs are vulnerable to elevatedtemperature, which causes corals to expel their symbiotic algae in a process called coral bleaching. Mass coral bleaching episodes have increased dramatically over the last 2-3 decades. Corals can recover from bleaching, but high mortalities are often reported over huge reef areas; some 16% of the world’s coral reefs experienced bleaching in 1997-1998, and about half of those have not recovered. A second global threat to corals and coral reefs isocean acidification, or a lowering of oceanpHdue to its uptake of atmospheric carbon dioxide and the subsequent formation of carbonic acid. Ocean acidification has been shown to decrease the rate at which corals and coralline algae secrete their calcium carbonate skeletons.