Hurricane Irma (copy) (copy) (copy) (copy)

The High Battery was hit by waves as Tropical Storm Irma hit Charleston's peninsula in 2017. File/Brad Nettles/Staff

Building walls to hold off sea rise — for just the next 20 years — would cost South Carolina's coastal and estuary communities nearly $20 billion.

Charleston alone would face more than $1 billion in costs.

Those are the jolting conclusions from a Center for Climate Integrity study released Thursday.

Coastal communities around the country would face more than $400 billion to build the walls — a figure that suggests the cost in smaller communities could be as much as $1 million per person, according to the "High Tide Tax" study.

Charleston County as a whole would face $6.3 billion in costs, the report said. Beaufort County, home to Hilton Head Island, would face $6.1 billion.

The center is a Washington, D.C.-based advocacy group that is calling for fossil fuel industries to pay for the environmental damages caused by burning the fuels.

The study was paid for by donors to Institute for Governance & Sustainable Development, a nonprofit that bills itself as promoting sustainable societies and protecting the environment. The center is run by the institute.

The study used building sea walls as a standard to convey the magnitude of the cost of sea rise, and the authors said other defenses or retreat could be far more expensive.

The figures are large but not shocking.

In 2017, the city of Charleston estimated it would take $2 billion and a generation to address its myriad of flooding and drainage challenges.

That estimate included what would be envisioned as completed various drainage improvements: raising the Low Battery by 2½ feet and extending the seawall along Lockwood Boulevard to Brittlebank Park.

The city figured it would have to raise about half that sum, or $1 billion, in addition to getting state and federal help.

The study's numbers "are probably consistent with the figures we looked at," said Mark Wilbert, the city of Charleston's chief resilience officer. "Our strategy consists of a lot more than just building a sea wall, but perimeter protection is certainly a part of it."

To determine how high and extensively walls would have to be built, the study used local tidal gauge readings and sea level histories to produce a range of sea rise and storm surge that varied location to location.

For Charleston, they came up with 6 inches of sea level rise by 2040, and added 21 inches of storm surge. The researchers called both numbers conservative.

By comparison, the storm surge for Hurricane Florence last year ranged from 9 feet to 13 feet. Wilbert said the city's strategy plans for 2 to 3 feet of sea rise over the next 50 years.

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Storm surge is the height of the water pushed up beyond tide by a hurricane. 

"These costs reflect the bare minimum coastal defenses that communities need to build to hold back rising seas and prevent chronic flooding and inundation over the next 20 years," the study said.

"They represent a small portion, perhaps 10 to 15 percent, of the total adaptation costs these local and state governments will be forced to finance during that time and into the future," the study said.

Richard Wiles, director of the Center for Climate Integrity, said most communities won't be able to afford the sea rise costs, and compared the advocacy to compelling tobacco companies to pay for health damages.

"As things stand, taxpayers are on the hook for all of the cost. Polluters pay nothing," he said.

The study results are not the worst-case scenario and not that far out in the future, he said.

The study noted that South Carolina is one of a few states where discussions are underway on how to deal with sea rise.

Robert Behre contributed to this report.

Reach Bo Petersen at @bopete on Twitter or 843-937-5744.

Science and environment reporter. Author of Washing Our Hands in the Clouds.