After President Trump met with Mexican President Enrique Pena Nieto last Friday at the G20 summit in Germany, a reporter asked him if he still wants Mexico to pay for a wall along the U.S. southern border. “Absolutely,” Trump replied.

Regardless of who foots the bill, the wall—which could cost as much as $21 billion—would be a colossal waste of money, with or without the solar panels Trump says he now wants to add. The border is already well-defended, undocumented migration from Mexico has dropped dramatically since 2008, and undocumented immigrants don’t take jobs away from Americans.

That said, building a wall is actually a good idea. Several walls, in fact. But not to keep out undocumented immigrants. To keep out the sea.

Flooded Coastal Communities

Earlier this year, the National Oceanic and Atmospheric Administration (NOAA) released a report on how rising sea levels brought on by climate change could affect U.S. coastal communities, home to 40 percent of our population. In a worst-case scenario, the agency estimates that seas along the coasts in some places could rise nearly 2.5 meters — about 8 feet — by the year 2100. That’s 2 feet higher than what NOAA estimated just five years ago.

The year 2100, however, is a long way off, and sea level rise is a serious problem right now. More than 90 U.S. coastal communities are already experiencing chronic flooding, according to a new study by researchers at the Union of Concerned Scientists (UCS) published today in the journal Elementa. These high tide floods, which are often only a foot or two deep, can cover coastal roads for hours, trap residents in their homes, disrupt businesses, and cause structural damage.

The incidence of chronic flooding — which UCS defines as occurring at least 26 times a year and affecting 10 percent or more of a municipality’s usable land — will increase as time goes on due to climate change. The only question is how much. UCS researchers project that the number of chronically inundated cities and towns will double by 2035. By mid-century, the number of localities likely will jump to somewhere between 270 and 360, depending on whether carbon emissions continue to rise or decline.

A 2014 UCS sea level rise study, meanwhile, estimated that the number of high-tide floods in two-thirds of 52 cities along the Eastern and Gulf coasts, including Boston, Miami, Philadelphia and Savannah, could triple by 2030. Several New Jersey shore towns could see at least 80 tidal floods a year, while Annapolis, Maryland, and Washington, D.C., could average more than 150 tidal floods annually. Throw in some hurricanes and other storms, and this increased flooding along the two coasts will likely devastate local economies.

Let’s translate that into language our real-estate-developer-in-chief would understand.

If we continue to burn fossil fuels at present rates, “by 2050 between $66 billion and $106 billion worth of existing coastal property will likely be below sea level nationwide, with $238 billion to $507 billion worth of property below sea level by 2100,” according to a 2014 report commissioned by the Risky Business Project headed by former New York Mayor Michael Bloomberg, former Treasury Secretary Henry Paulson and hedge fund billionaire Tom Steyer.

That bill will come due well before 2050, however. “Within the next 15 years,” the Risky Business report projected, “higher sea levels combined with storm surge will likely increase the average annual cost of coastal storms along the Eastern Seaboard and the Gulf of Mexico by $2 billion to $3.5 billion. Adding in the potential changes in hurricane activity, the likely increase in average annual losses grows to up to $7.3 billion, bringing the total annual price tag for hurricanes and other coastal storms to $35 billion.”

So, if President Trump is keen on building a wall, his administration should provide federal support to coastal states, counties and cities that are already grappling with rising ocean levels. They will need not only walls, but also bulkheads, jetties and other hardened structures, as well as vegetated dunes, salt marshes and other natural “soft” shoreline defenses to hold back the sea. And all of that infrastructure may still not be enough. A good number of coastal residents will have to abandon their homes and businesses and move inland to higher ground.

Trump Properties at Risk

Several coastal cities are now considering sea walls and other barriers. City officials in Boston, for example, are exploring the possibility of building a 4-mile-long sea wall in an arc around Boston Harbor that would stand at least 20 feet above the water at low tide. They also are investigating other ways to protect city residents and $80 billion worth of real estate, including constructing berms around neighborhoods, redirecting flood waters into canals, and flood-proofing buildings. Meanwhile, more than 60 elected officials and business leaders in Texas sent a letter to President Trump in April requesting $15 billion in federal funds for a coastal barrier system to defend the Houston and Galveston bay areas from hurricane storm surges. The signatories, who include 20 mayors and eight state legislators, stressed the area’s economic importance — and its vulnerability. In 2008, Hurricane Ike caused more than $29 billion in damages on the state’s upper coast. If Ike had hit the port of Houston, the letter pointed out, it would have resulted in more than $100 billion in damages.

Given there are no Trump hotels or golf courses in Texas or Massachusetts, President Trump may not care much about Houston or Boston. But he — or at least someone in his far-flung empire — apparently does worry about the threat rising seas pose to Trump properties. His Irish firm, for instance, has been trying to get a permit to build a nearly 2-mile long, 13-foot-high wall to protect a Trump luxury golf resort in the village of Doonbeg from rising sea levels and increasingly severe storms.

As it turns out, there are a number of Trump properties here in the United States that also are in harm’s way.

New York City: Let’s start with Trump’s hometown, New York, where his family owns 13 buildings in Manhattan. Five years ago, Hurricane Sandy, which cost the region $60 billion, prompted local officials to look into ways to defend the city from floods and storm surge.

As writer Jeff Goodell pointed out in a July 2016 feature in Rolling Stone, “Can New York Be Saved in the Era of Global Warming?” a lot is at stake. Home to 8.5 million people, the city generates nearly 10 percent of the nation’s gross domestic product. Then there’s its vast network of subways, tunnels and other underground infrastructure, and — of course — row upon row of skyscrapers. By Goodell’s count, “71,500 buildings worth more than $100 billion stand in high-risk flood zones today, with thousands more buildings at risk with each foot of sea level rise.” The eight Trump buildings clustered around Central Park’s south end and the Upper East Side are relatively safe, but two of his properties — the 46-story Trump Soho Hotel Condominium and the 70-story Trump Building on Wall Street — are on the island’s southern tip, one of the most vulnerable areas in the city.

New York is currently planning to construct a massive barrier system, dubbed “the Big U,” that may eventually loop around the bottom of Manhattan, from 42nd Street on the East Side to 57th Street on the West Side. The barrier, more of a berm than a wall, will be covered by grass and trees, as well as benches and bike paths, and is expected to cost more than $3 billion. Will the Trump administration include it in its infrastructure plans — and will those plans ever get off the ground?

Florida: South Florida also is worthy of the president’s attention. After all, it’s home to his “Winter White House,” the $200-million, 123-room Mar-a-Lago resort in Palm Beach, as well as the Trump Towers and Trump Grande complex in Sunny Isles Beach, and Trump Hollywood in Hollywood, all which sit on narrow barrier islands between Florida’s Intercoastal Waterway and the Atlantic Ocean. There are also three Trump golf courses in the state, in Jupiter, Miami, and West Palm Beach. All of the properties, except the Jupiter golf course, are at risk.

Mar-a-Lago’s 20 acres stretch the width of a barrier island off the coast of Palm Beach, an area already plagued by chronic tidal flooding. A 3-foot sea level rise — expected by 2060 or 2080 depending on how fast the ocean rises — would inundate the resort’s western lawn and nearby roads that lead to the property. Likewise, a 3-foot sea level rise would flood much of the west side of the barrier island where the Trump Towers and Trump Grande complex are located, just east of North Miami Beach. Both properties would be spared in that scenario, but add another foot and major sections of the main road running south to Miami Beach would be permanently under water.

Before that happens, though, chronic flooding along the coast is expected to worsen significantly. Based on U.S. Army Corps of Engineers estimates and tide gauge data, a 2016 UCS report projected that tidal floods in Coral Gables, Miami, Miami Beach and other South Florida municipalities will jump from today’s six times per year to as many as 80 times per year by 2030 and more than 380 times per year by 2045 — more than one a day. But given that saltwater is already tainting regional drinking water supplies and tidal flooding is commonplace even when the sun is shining, government agencies are now beginning to respond to the threat.

Three years ago, Miami Beach initiated a $500-million pump project to keep water off the streets. Last year, Fort Lauderdale raised the required height for sea walls, but only for rehab projects and new construction. Delray Beach has installed valves in some sea walls that prevent saltwater from spilling into the city’s drainage system. And later this year, Miami will kick off a $100-million flood prevention program to raise roads, install pumps and water mains, and redo sewer connections in two neighborhoods, part of a citywide effort that is expected to cost as much as $500 million. But much more needs to be done to protect the 3.5 million state residents at risk of coastal flooding, and that will take millions, if not billions, of dollars.

Hawaii: Finally, the Trump family owns a hotel on Waikiki Beach in Honolulu. Like South Florida, tidal flooding is already wreaking havoc in the city, and rising sea levels will make things much worse. According to a March University of Hawaii study, if the sea level increases 3 feet, flooding that occurs when groundwater seeps above ground level would inundate much of Honolulu.

“The flooding will threaten $5 billion of taxable real estate; flood nearly 30 miles of roadway; and impact pedestrians, commercial and recreation activities, tourism, transportation and infrastructure,” said Shellie Habel, lead author of the study. “The flooding will occur regardless of seawall construction, and thus will require innovative planning and intensive engineering efforts to accommodate standing water in the streets.”

An Ounce of Prevention

Boston, Honolulu, Houston, Miami and New York are just a small sample of the cities and towns that will need federal assistance to protect their residents and real estate from rising seas. The cost of adaptation, including sea barriers, pump stations, and better road and bridge design, will not come cheap, but compared to the cost of everyday flooding, let alone hurricanes and storm surges, it’s a bargain.

Beyond adaptation, however, there’s an obvious, common-sense solution: prevention. How can the world avoid a 3-foot sea level rise by 2060, let alone an 8-foot rise by 2100? By dramatically reducing carbon emissions. A certain amount of sea level rise is already locked in, but slashing emissions would slow the rising sea rate and reduce the frequency and intensity of the resulting floods. Would it save Mar-a-Lago and other Trump coastal properties? Yes, it most certainly would. Will that stark reality stop Trump from trying to sabotage worldwide efforts to curb carbon emissions? One could only hope so.

Elliott Negin is a senior writer at the Union of Concerned Scientists (UCS). Research assistance was provided by climate scientist Kristina Dahl, a UCS consultant. Data on the impact of sea level rise on Trump properties are from the National Oceanic and Atmospheric Administration’s Sea Level Rise Viewer.