 Coral reefs provide food and habitat for a wide range of organisms and ecological goods and services. Warm water coral reefs are found in shallow, sunlit waters where they grow and calcify at high rates to build their calcium carbonate structures. Mesophotic coral reefs accumulate calcium carbonate at lower rates but remain important as habitat for a wide range of organisms, including those important for fisheries. Cold water coral reefs are found in the dark depths and are threatened by warming temperatures and ocean acidification. Evidence that coral reefs can adapt to rapid ocean warming and acidification is minimal, and corals are long lived with slow rates of evolution. Coral reefs are likely to degrade rapidly over the next 20 years, presenting fundamental challenges for the 500 million people who derive food, income, coastal protection and a range of other services from coral reefs. Unless rapid advances to the goals of the Paris Climate Change Agreement occur over the next decade, hundreds of millions of people are likely to face increasing amounts of poverty and social disruption, and, in some cases, regional insecurity.