In the Florida Keys, residents are facing a question that may soon plague communities up and down the U.S. coastline: How much water are they willing to live with?

During a bad bout of high tides in 2015, a Key Largo neighborhood flooded for 34 days, stranding and infuriating the people who live there. County officials responded by agreeing to raise the roads, but keeping them dry year-round would require making the roads 28 inches higher. That would cost $7 million for less than a single mile; extrapolated across the Keys, zero days of flooding was a goal more ambitious than the county’s 75,000 full-time residents could afford.

So the county adopted a new standard across the Keys: Roads should be elevated enough so that they would flood, on average, no more than seven days a year. That meant roads in part of Key Largo would be raised six inches instead. “We have to make tough decisions,” said Rhonda Haag, Monroe County’s sustainability program manager. As climate change gets worse, “there may be sections of road that have to go under.”

Local governments around the coastal U.S. will face variations on that dilemma, perhaps sooner than they realize. In a report released Wednesday, the Union of Concerned Scientists used federal data to project the year communities will become what it calls “chronically inundated.” The group, which urges Congress to take action to address climate change, defined that threshold as any community that faced flooding an average of every two weeks across at least 10 percent of its area.

It’s not just small towns along the beach. By 2100, a third of Savannah, Georgia could be flooded twice a month, including more densely populated parts of the city’s historic downtown.

By 2100, parts of Connecticut, including Bridgeport, East Haven, Stratford and New Haven, could also suffer chronic flooding across at least 10 percent of their area.

The threat of worsening tidal flooding isn’t limited to the east coast. Around San Francisco Bay, one-quarter of Alameda could be chronically inundated by 2070. By 2100, more than 10 percent of Oakland, San Rafael and San Mateo could meet that threshold.

By 2060, the report projects that chronic inundation could also spread to Cambridge, Massachusetts; Wilmington, Delaware; and Hoboken, New Jersey. By 2080, it could reach Boston, and parts of Baltimore County. By 2100, chronic inundation could affect every borough of New York City except for the Bronx.

Erika Spanger-Siegfried, the Union of Concerned Scientists senior climate analyst who was a lead author of the report, says the aim was to create both a map and a timeline for what will happen beyond today’s tidal flooding, but before communities become permanently underwater. She called that time the “in-between” space, and said local officials should start considering which areas to defend against flooding—and which to retreat from.

“We are entering a century of adaptation,” said Spanger-Siegfried. “We hope that this new work shows people the response time that we have to work with, and that we can use it wisely.”