Estuaries at the mouths of rivers have in the past handled rising ocean levels. Sediment that accumulates along the edge of an estuary can raise the level of the land as the sea levels rise. And mangrove swamps, which buffer many a coastal zone around the world, flourish in brackish conditions. But because of our preference for living in coastal areas, and our habit of re-engineering our surroundings accordingly, humans make matters worse by preventing natural processes from managing the change. On the coast, we build roads and buildings, and replace natural buffers like mangrove swamps with dikes and bulkheads to control flooding, which make the problem worse by preventing beach sediment from collecting. And as we dam rivers and create reservoirs, we trap the sediment that would naturally flow down to the sea.