Coastal neighborhoods in several Bay Area cities are likely to face such frequent flooding from rising sea levels over the next century that residents will simply pack up and leave, according to a new study of the effects of climate change.

Every local county will be dealing with frequent inundation of its bay shoreline by 2100, according to a report issued Wednesday by the Union of Concerned Scientists. The group said its report and accompanying maps, published in the peer-reviewed journal Elementa, are the first nationwide effort to identify the point at which coastal communities face the no-win decision of having to flee or fight sea level rise.

“There have been a lot of great sea level-rise maps, but what’s been missing from these is a time frame,” said Kristina Dahl, one of the report’s authors and a climate scientist in San Francisco. “We want communities to understand how long they have not just until permanent inundation sets in, but when flooding gets so bad that it makes a place really unlivable.”

Already, more than 90 communities across the nation have hit a point of disruption that’s driving people away, the report said. Eighty more are expected to reach that threshold within 20 years if global warming continues at a moderate rate.

Seventy percent of these cities will be in Louisiana and Maryland, with none on the West Coast in the next two decades, the report said. Ocean levels are generally thought to be rising more slowly along the Pacific coastline.

The report’s authors put the point of no return at the stage when at least 10 percent of a community experiences flooding 26 days a year, or one day every two weeks. When a town is “chronically inundated,” the report’s authors say, the slow creep of sea level rise causes road closures and sends water into basements, pushing down real estate values. Rather than deal with the constant annoyance and mess, residents head for higher ground.

California will increasingly see these problems as the century wears on, the report says. The island city of Alameda, neighboring Oakland and landfill-laden San Mateo, San Rafael and South San Francisco will be chronically inundated even under a scenario of merely moderate global warming by 2100, the report predicts.

Under a rapid warming scenario, many more cities will see neighborhoods depopulated within the lifetime of people who are alive now, the report says. By 2065, San Francisco, Corte Madera, Larkspur, Burlingame, East Palo Alto and Palo Alto could see chronic flooding in areas near the waterfront, along with expanded areas of the other flood-prone cities.

This scenario is increasingly likely, given accelerating ice-sheet melt in the Arctic and Antarctica, the report said.

The Union of Concerned Scientists is a national coalition of researchers that uses its work to advocate for certain policies. The authors of the latest report urge that the output of heat-trapping gases be reduced worldwide and that communities take greater steps to adapt to rising seas.

The report uses rates of warming from widely cited federal figures by the U.S. Global Change Research Program to estimate the amount of sea level rise that communities might experience. The actual amount of rise, the authors say, will depend on both the level of global greenhouse gas emissions over the next several decades and how communities respond.

Many Bay Area cities are already taking action to combat sea level rise, creating seawalls, implementing new building codes that require more resilient homes and businesses, and expanding wetlands to absorb high water.

Last year voters in the nine Bay Area counties passed a 20-year parcel tax that is projected to raise $500 million for marsh and habitat restoration.

Larry Goldzband, executive director of the San Francisco Bay Conservation and Development Commission, which is spearheading a region-wide plan to adapt to rising seas, said there is still much to do.

“We’re not as prepared as we should be,” he said. “We’re in the midst of getting better prepared, and counties and cities are working hard on this.”

Goldzband said California has the advantage of time, with several decades before catastrophic levels of damage occur.

“Fortunately,” he said, “San Francisco doesn’t have to build a 20-foot wall on the Embarcadero tomorrow.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander

Read the report

The report can be read at http://bit.ly/2vacc5j