Home values could fall significantly. Banks could stop lending to flood-prone communities. Towns could lose the tax money they need to build sea walls and other protections.

ON THE MEKONG RIVER — When the Chinese came to the village of Lat Thahae, perched on a muddy bend of a Mekong River tributary, they scrawled a Chinese character on the walls of homes, schools and Buddhist temples. No one in this isolated hamlet in northern Laos could read what it said. But the...