They call it “sunny day” or “nuisance” flooding: days when it doesn’t rain and there’s no extreme weather, but streets in coastal areas become impassable all the same because an extra high tide comes on top of an already rising ocean. Across the country, more and more cities are experiencing these high tidal events and–if nothing is done to avert climate change–hundreds more could join the ranks of Miami Beach, Charleston, and Annapolis in the coming years.

A newly published report from the Union of Concerned Scientists, which campaigns for action on global warming, calculates just how many. By 2035, it says 170 communities could see “chronic flooding” every two weeks, or more frequently, under an “intermediate” climate scenario. By 2060, it forecasts the same for 270 communities, with at least 40% of their land under water 26 or more times a year.

“The analysis shows the sheer number of communities up and down our coasts that will be coping with chronic inundation,” Shana Udvardy, one of the authors of the study, tells Fast Company. “It’s a clarion call for responses to sea level rise within local, state, and federal governments and particularly for a federal response to this ballooning challenge.”

The analysis, based on federal data, was published in a peer-reviewed journal. It defines chronic flooding as inundation across at least 10% of a community at least 26 times a year. It calculates affected communities according to a scenario where global carbon emissions crest in the middle of this century and seas rise about four feet. Some estimates for future climate impacts are more serious, though, with emissions continuing to climb through this century and sea levels rising 6.5 feet or more.

Under the less serious scenario, the affected areas include some of the country’s most popular holiday-home destinations, including the Jersey Shore, North Carolina’s Pamlico Sound, southern Louisiana, and Maryland’s Eastern Shore. By 2100, up to 490 communities—or 40% of all East and Gulf Coast oceanfront communities—will be chronically inundated, the study says. (See the detailed map here for other affected towns and cities.)

Miami Beach–often seen as ground zero for sunny-day flooding–doesn’t yet cross the threshold for chronic inundation. But it is a poster child for flood adaptation. It plans to spend at least $400 million on raising its roads and installing new pump infrastructure. Other parts of the state aren’t so proactive. Florida governor Rick Scott has denied climate science and outlawed the use of the term “climate change” in official communication. Udvardy also praises Charleston, South Carolina, which has developed a citywide sea-level rise strategy, and Annapolis, Maryland, where flooding already occurs 40 times a year and planners are thinking about how to provide local businesses with interruption and flood insurance.

Globally, climate change is expected to have a disproportionate effect on poorer countries. And it’s a similar story in the U.S. The report says: “Low-income communities, communities of color, and other traditionally underserved communities” tend to be less prepared and face greater risks than “wealthier, often whiter communities, especially in urban settings.”