Within 30 years, the District and Annapolis can expect to experience tidal flooding nearly every day of the year. (Jonathan Newton/The Washington Post)

Daily flooding caused by high tides will occur in the District and Annapolis within three decades as sea levels continue to rise due to global warming, a new study says.

The study by the Union of Concerned Scientists predicts that by 2045 the nation’s capital and the capital of Maryland will experience about 400 floods per year, sometimes twice in a single day, and several other cities and towns on the Atlantic coast will have tidal flooding almost as bad.

Miami, Atlantic City, Cape May, N.J., and Lewisetta and Windmill Point, both on the Chesapeake Bay in Virginia, can expect at least 240 days of flooding by 2045.

High-tide floods along the Atlantic coast in Baltimore, Norfolk, Philadelphia, Charleston, S.C., Key West, Fla., and Sandy Hook, N.J., will happen less frequently, with about 180 events or more per year, according to the study released Wednesday.

Tidal flooding is sometimes called nuisance flooding. It’s considered minor when it is the result of the gravitational pull of the moon and sun. It’s considered moderate when heavy rains increase its size, and major when storms supersize it. Sea-level rise is considered an X-factor that could make each factor irreversibly worse.

Tidal flooding today, in 2030 and in 2045

Nuisance flooding can overwhelm utility pumping stations and spill water onto roads and into homes. In and of itself, it is an annoyance limited to a small area or community.

But it can be deadly when high tides mix with drenching rains from nor’easters and wind-driven water surge from tropical storms and hurricanes. Sandy Hook was one of more than a dozen cities that suffered catastrophic flooding when Tropical Storm Sandy struck in October 2012.

The study, “Encroaching Tides: How sea level rise and tidal flooding threaten U.S. East Coast and Gulf Coast communities over the next 30 years,” relied on data from the National Weather Service and the National Oceanic and Atmospheric Administration.

“We found places that are all flooding right now, and we think they have resonant stories,” said Melanie Fitzpatrick, a climate scientist for the Union of Concerned Scientists and one of the study’s three authors.

For example, she said, residents of the Broad channel section of Queens are routinely warned to move their cars during high tide. Flooding in Miami could become so bad that some are calling it a future Venice, where canals serve as roads, she added.

“Clearly . . . much of the U.S. is experiencing long-term sea level rise,” and high-tide flooding “is becoming more noticeable and widespread,” said a NOAA study released in July. Tidal flooding occurs more frequently today than at any other time in recorded history.

“In 1950, such events would have the probability of recurring every couple of years,” the NOAA study said of tidal flooding. “Whereas today, they occur so frequently due to decades of sea level rise” that they are normal, “no longer classified as an extreme event.”

William Sweet, a NOAA oceanographer who authored the July study, called the Concerned Scientists’ study a solid piece of work. He had one quibble: that its findings are based on a single scenario — a sea-level rise of four feet by the end of the century. Climate scientists usually offer four scenarios of varying sea-level rise based on assumptions on how the world’s governments limit greenhouse gas emissions that heat the atmosphere, cause ice to melt into the ocean, and water to warm and expand.

“It’s hard to tell what the ice sheets . . . will do,” Sweet said.

Sweet described the NOAA study as a 30-year look back at tidal flooding, compared with now, and the Concerned Scientists’ study as a 30-year look forward.

Fitzpatrick said the new study chose a single mid-range prediction of ice melt offered in one of several scenarios of climate change in the federal National Climate Assessment and explored how it would impact the Atlantic and Gulf coasts within a relatively short time frame. The authors realized that their choice could open the study to criticism that its findings are too narrow.

Analyst Erika Spanger-Siegfried and Kristina Dahl, a consultant, are the other authors. Wednesday’s release of the report coincided with the start of the “king tide,” an especially high seasonal tide that occurs yearly along the Atlantic and Gulf coasts between Oct. 8 and Oct. 10.

The District and Annapolis now have fewer than 50 days of tidal flooding. In 15 years, according to the study’s prediction, that will rise to 150. After another 15 years, flooding will more than double to 400, with multiple floods in one day.

Flooding will also be the norm for cities in New Hampshire, Delaware, North Carolina and Florida.

On the Atlantic coast, flooding is especially worrisome because land is sinking by a few millimeters each year, the result of ancient geological forces such as a meteor impact crater in the Tidewater area of Virginia.

The study’s recommendations to combat sea-level rise and flooding aren’t new. Neither are its proposed solutions, which include enacting government policies to trap and reduce gases from power plants and other facilities. But those steps face strong opposition from some conservatives who say global warming is a hoax or a natural phenomenon that is not as bad as scientists say.

Still, many cities are heeding the warnings of climate scientists. In Miami Beach, where water often bubbles up from under storm-drain covers, city leaders have invested $400 million in infrastructure such as pumping stations to keep sea water out of sewers. Norfolk has identified $1 billion in necessary storm and drainage improvements, and is seeking money to fund them.