Under a worst-case scenario, some 130 communities in the Garden State could be inundated every other week

By the end of the century, up to 131 communities in New Jersey will face chronic flooding from high tides — once every other week — under a worst-case scenario for sea-level rise, according to a study released yesterday by the Union of Concerned Scientists.

The study, the first to look at the entire coastline of the lower 48 states, projects when communities will experience chronic inundation so extensive it will force them to make expensive investments to fortify against rising seas or retreat from their homes and businesses.

The analysis projects New Jersey will suffer more than any state other than Louisiana from the chronic flooding, a designation for when it becomes unmanageable in people’s daily lives. The threshold for that level is when 10 percent of a community’s usable land is flooded at least 26 times a year, according to the study

Sea-rise scenarios

For New Jersey, the projections under three different sea-rise scenarios — high, intermediate, and low — reflect problems that are forecast to occur along the Jersey Shore and parts of the Delaware Bay; in the state’s two largest cities, Newark and Jersey City; and on the Hackensack Meadowlands.

The study suggests there is not a lot of time to adapt, and even then, it will be at great cost and only for a limited time.

“A lot of coastal communities know they have to deal with sea-level rise; a lot don’t,’’ said Erika Spanger-Siegfried, a lead author of the report. “If they’re not planning already, the need to start right away.’’

By 2035, 21 communities will face chronic flooding under an intermediate sea-level rise scenario of four feet by midcentury. By 2100, the number of communities facing chronic flooding jumps to 103 — 131, if sea levels rise by 6.5 feet, the study said.

The study acknowledges many of the cities and towns are along the Jersey Shore, where coastal defenses against storm surges have been built or repaired since Hurricane Sandy nearly five years ago. But it also noted most of the chronic flooding encroaches from the bay side of the barrier islands.

Newark and Elizabeth

By the end of the century, Newark and Elizabeth would face chronic inundation over more than 30 percent of their land area. The projection underlines a warning that much of the chronic inundation will hit low-income areas hardest, since they have limited resources to move or adapt.

The study does not project costs to adapt to the rising seas, emphasizing curtailing global warming — the projection used under the low scenario — is the best way to avoid problems. That scenario, however, assumes the world achieves reductions proposed in the Paris climate agreement, which the United States pulled out of earlier this year.

“The longer we wait to make the investment, the longer we wait to make smart decisions, the more it is going to cost,’’ Spanger-Siegfried said. Local communities also are not going to be able to make these investments alone, she added.

The study shows the problem is not confined to New Jersey; 90 communities across the country already face chronic flooding from rising seas, a number that could jump to nearly 170 in less than two decades. By 2100, roughly 40 percent of oceanfront communities along the East and Gulf coasts face chronic flooding with intermediate sea-level rise.

What to do? The study recommends three general approaches: Halting or phasing out current policies that perpetuate risky coastal development; enhancing existing policies, such as pre-disaster investments; and creating bold new polices to reduce the scale of coastal risks.