Coral reefs are very complex communities of many living creatures: corals and fish, invertebrates like crabs and snails, as well as plants. Though they may cover less than 1% of the world’s oceans, they hold over 25% of marine species. Pollution can hit coral reefs hard, but global warming appears to have caused 10% of the world’s reefs to die with a brief rise in ocean temperature of 1 degree C.