Rising sea levels could flood the homes of 7,000 Long Islanders within 35 years, and within a century the ocean could displace nearly 165,000 residents and all but destroy Fire Island, a new report concludes.

Long Island and other coastal regions have begun girding for major storms, but they have failed to prepare for the looming — but less visible — threat of higher sea levels, said Robert Freudenberg, director of energy and environment for the Regional Plan Association, which released the report this month.

In communities hit by rising waters, “you start seeing high tides become a flood event,” Freudenberg said. “That’s happening in Mastic Beach, and it’s going to start happening in more and more places.”

The Manhattan-based nonprofit research and policy group examined the coasts of New York City, Connecticut and New Jersey as well as Long Island, using methodology developed by the National Oceanic and Atmospheric Administration for estimating the effects of rising sea levels.

Rising waters “pose an enormous threat to the safety, prosperity and quality of life of the tristate area,” the group wrote. However, the report found, “relatively little has been done” to protect buildings and infrastructure from permanent flooding.

Since 1900, sea levels have risen by 1 foot, and the pace has accelerated sharply since 1990, the report found. If those trends continue, waters could rise another foot by 2050 — or perhaps as early as the 2030s — flooding the homes of 19,000 people throughout the New York City region. A 6-foot rise could displace 619,000 residents by early in the next century, the report found.

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On Long Island, a 1-foot rise would swamp homes nearest the Great South Bay and South Oyster Bay. A 6-foot rise would inundate homes throughout the South Shore of Nassau and Suffolk counties, and in parts of Nassau’s North Shore.

In Mastic Beach, Mayor Maura Spery calls her community the Island’s “canary in the coal mine” when it comes to rising waters.

The flooding has gotten steadily worse over the past 10 years, and now the roads near her home are impassable up to 15 days each year, she said. “I park four blocks away, I put on my hip waders, I grab my stuff and I walk down the middle of the street to my house,” said Spery, who lives about 100 yards from the water. “That’s crazy, right?”

Eventually, the community might need to fund boardwalks so residents can access their homes, and special fire trucks that could navigate the boardwalks, she said.

Anyone buying low-lying, waterfront property should budget up to $150,000 to elevate their home, said Michael Raab, superstorm Sandy recovery liaison for Nassau County. “Long Island today should start to think about building smarter, building taller,” he said.

The effects will be felt throughout the Island, as residents of frequently flooded communities such as East Rockaway move inland, Raab predicted.

The Regional Plan Association’s report advised coastal communities to study which areas are most at risk; consider halting development in certain areas; elevate homes and protect infrastructure; and pay “special attention” to poor and elderly residents. Locally, regionally and nationally, carbon emissions should be minimized, the group advised.

Already, the region has taken certain steps to protect against storms. In 2013, Congress authorized a $60 billion Sandy aid package. The federal government funded dunes to protect Long Beach and nearby communities, and the state paid for bulkheads and other measures, said Jack Schnirman, Long Beach’s city manager. But, he said, “more needs to be done regionally so we’re thinking about not just how to protect Long Beach but how to protect the entire South Shore.”

For many coastal residents, the lure of living on the water is strong.

Owen Cumisky, 55, who works in sales for a medical distributor, said he does not know enough about the science to know if climate change is real. After his family’s waterfront home in Massapequa was destroyed by Sandy, he and his wife considered moving off the Island, but their two sons love the community too much to leave, he said. So the family borrowed $250,000 to build a new home 11 feet above sea level.

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“I’m more concerned about the roads,” he said. “From a taxpayer’s perspective, I’ve got to believe that Long Island is trying to figure out how to get the streets to be navigable so they can continue to have property that they can tax.”

Peter Chaplin, 55, said he and his wife, Lori, adored their waterfront home in Massapequa, with its glassed-in balcony and hand-carved woodwork. The water, he said, “soothes the soul.”

But after Sandy wrecked the house, they moved to Fishkill in upstate Dutchess County and started commuting to their business, Franklin Square Physical Therapy. During the week, they stay in a small rental apartment nearby.

After losing his home to Sandy and finding that it wouldn’t make financial sense to elevate it, “I never want to experience that again,” Chaplin said.