Louisiana’s barrier islands don’t look like much. Dozens of the long, narrow masses form a loose chain around the state’s southeastern coastline, and many rise only a couple of feet above the surface of the ocean. Erosion is an inevitable foe for any sandy expanse, but in recent years these islands have begun to contract at an alarming rate—so quickly that the thousands-year-old features may disappear entirely by the end of the century. Losing these wisps of land would be disastrous. Without them, powerful storms would slam coastal towns, seaports, and wetlands. The calm waters behind their protective front would vanish, and with them the nurseries where fish, shrimp, crab, and oysters reproduce and raise their young, and where 100 million birds live, nest, or stop to rest and refuel…