‘This was a terrible idea’: the incident that broke Republicans’ DeSantis fever | Ironically named Great Outdoors Initiative that would pave over acres of state parks sent steadfast allies over edge

I like your comment because it could be interpreted in either way. Of course less than 30 years for some communities.

Rising sea level is one thing, and that goes together with damaging tropical storms. All it takes is one or two storms to devastate a coastal city. Then either the federal government says there will be no disaster relief funds here in the future, or the insurance companies will refuse to cover the area. Either of those would effectively end serious rebuilding efforts.

That's not to say everyone will immediately disappear. If you build a concrete bunker in a relatively high place, you could stay there for decades to come. But the bulk of the community would leave and never return. If you're looking for a related example, check out the small communities built near Kilauea in Hawaii.