On a tiny island off Panama’s Caribbean coast, about 300 families are packing their belongings in preparation for a dramatic change. Generations of Gunas who have grown up on Gardi Sugdub in a life dedicated to the sea and tourism will trade that next week for the mainland’s solid ground.

They go voluntarily — sort of.

The Gunas of Gardi Sugdub are the first of 63 communities along Panama’s Caribbean and Pacific coasts that government officials and scientists expect to be forced to…