— Protecting the beaches is about to get tougher and costlier, and the consequences worse.

The coastal mainland

Today’s coast is about where it was 120,000 years ago:

20 million years ago, the coast was at the fall zone — the point in the state where river rapids fall to the coastal plain, as far inland as Columbia.

18,000 years ago, the coast was the Continental Shelf more than 50 miles offshore.

Today a 10-foot scarp, or inland rim that slopes steeply to the marsh, marks the upland boundary inland along Mount Pleasant and James Island. If sea rise and storms overwhelm the barrier islands, that could become the next coast.

The recent battle between environmentalists and officials trying to build a groin at the county park here is only the tip of a melting iceberg.

Compromise on both sides ended the stalemate, maybe in time to save the severely eroding Folly Beach County Park. That sort of give and take will decide how much of the rest of the coast gets saved.

The seas are rising in front of your eyes, more than a foot from 1926 to 2006, according to the National Oceanic and Atmospheric Administration. Experts expect it to rise at least a foot in the next 50. Measures now indicate that that rate is accelerating, and researchers expect to have data in five to 10 years to say how fast.

Over the next 20 years, more sea walls will be needed, more beach renourishment demanded because of storms, flood tides and erosion. Every spot protected might well mean the one next door erodes quicker.

The list just keeps growing, and millions of dollars in costs piling up — the Folly park groin re-cutting Capt. Sam’s Inlet and developing the erosive spit on Kiawah Island, and renourishing the stripped-away Wild Dunes beach on Isle of Palms, to name a few.

The Charleston peninsula is staving off flooding, but homes along the estuaries are gradually being underwashed by storm tide waves.

The question is: How long can the Lowcountry keep putting fingers in the dike?