Coastal regions around the world have become increasingly crowded, intensively developed, and severely exploited. Hundreds of millions of people living in these low-lying areas are subject to short-term coastal hazards such as cyclones, hurricanes, and destruction due to El Niño, and are also exposed to the long-term threat of global sea-level rise. These massive concentrations of people expose often-fragile coastal environments to the runoff and pollution from municipal, industrial,...