The risk of severe coral bleaching—a condition in which corals lose their symbiotic algae, called zooxanthellae—is five times more frequent today than it was forty years ago. Coral bleaching is a direct result of global warming, where rising temperatures cause marine heat waves, which place stress on the living coral animals, as well as the photosynthetic algae on which they depend for energy. This heat stress causes the algae to malfunction, at which point they are expelled by the corals, causing the organisms to lose their color and appear white, thus the term coral ‘bleaching.’
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Source: Phys.org