Ocean water expands as it soaks up heat from a warming atmosphere. Add in water from melting glaciers and ice sheets, and the global mean sea level will most likely rise by anywhere from 1.4 to 2.8 feet (43 to 84 centimeters) by 2100, according to the Intergovernmental Panel on Climate Change.

This gradual swelling will stress coastal cities, which are already seeing more sunny-day “nuisance flooding” at high tide. But the bigger threat is from waves and storm surges, which are amplified by higher sea levels. If greenhouse gas emissions go unchecked, by 2100 this combination will produce peak sea levels that are, on average, 1.9 to 5.6 feet higher than today’s mean sea level. As soon as 2050, the kind of extreme coastal flooding we currently expect every 100 years will occur every year at tropical latitudes and every 10 years in many U.S. coastal cities. By 2100 annual flood damage could amount to 9.3 percent of the global gross domestic product, or tens of trillions of dollars a year.

Most nations are not living up to their Paris Agreement commitments to curb greenhouse gases, but even if they were, some sea-level rise would be inevitable. So there is really no choice but to try to defend our coasts.

The question is, How? Would it be smarter to build big, expensive surge barriers that protect entire harbors or to implement smaller-scale changes along the shoreline?

Not surprisingly, many city planners are attracted to the second, less costly option. In my hometown of Boston, which has a 47-mile shoreline, Mayor Marty Walsh’s “Resilient Boston Harbor” plan envisions a city buffered by restored marshes and by elevated parks, walkways and roads. Researchers at the University of Massachusetts Boston endorsed that approach in a 2018 preliminary study, concluding that such land-based resiliency measures would be more cost-effective than a barrier across the harbor’s mouth.

But the reality, I suspect, is that we will have to do both. Let’s say Boston elevated its frequently flooded Long Wharf and Seaport districts by three feet or so. That would fend off extra-high king tides, which occur when Earth, the moon and the sun align. But it would not help much against storm surges.

“Even though sea-level rise and storm surge are related, they are separate, distinct phenomena, and it’s important to address them with separate engineering and technology responses,” says William Golden, who filed the 1982 lawsuit that led to the cleanup of Boston Harbor and who later founded the National Institute for Coastal and Harbor Infrastructure, a Boston-based nonprofit. “What we feel is often possible and justifiable in urbanized areas is to focus on the concept of a layered defense: a land-based system on the perimeter to address sea-level rise integrated with a regional system of sea gates designed to prevent inundation from storm surge.”

The UMass researchers estimated the cost of Boston’s proposed Outer Harbor Barrier at $8 billion to $12 billion. Two huge “floating sector gates,” modeled on the mammoth Maeslantkering storm-surge barrier in the Netherlands, accounted for two thirds of that price tag. But there are cheaper options for sea gates, such as the $550-million floating barge that would close off the “Ike Dike” proposed for Texas’s Galveston Harbor. And even at $12 billion, a barrier might be a good investment. According to the U.K.’s Tyndall Center for Climate Change Research, a 100-year storm coming on top of a hypothetical 1.6-foot rise in sea level would threaten $460 billion in assets in the Boston area alone.

At his nonprofit, Golden is working to gather Boston community leaders in a push for a more thorough study of the Outer Harbor Barrier that could help qualify the project for federal funding. “What we need now is to have an in-depth additional cost-benefit analysis, so that we make sure our public policy isn’t based on a preliminary study,” Golden says. “This is going to affect the city forever.”