Coastal communities should expect much more frequent flooding in coming decades as sea levels rise, according to a new federal report. Many places that are dry now could flood every day by the end of the century.

The report, published by the National Oceanic and Atmospheric Administration, projects the impact of sea level rise on coastal flooding along the nation's shorelines and says it's already having an effect, particularly on the East Coast. In the Southeast, the average number of days with high-tide floods has more than doubled since 2000, to three per year, while the number in the Northeast has increased by about 75 percent, to six per year.

"We're seeing an accelerated increase up and down most of the Atlantic Seaboard," said William V. Sweet, an oceanographer at NOAA and the lead author of the report. "That's not a good place to be, because impacts are going to become chronic rather quickly."

While Miami currently experiences only a few days of high-tide flooding per year, for example, it should expect 10 days each year by the early 2030s under an intermediate scenario for sea level rise. Just a decade later, that number could triple. And flooding would likely occur every other day by 2060.

Flood Risk Varies Region to Region

It doesn't take a scientist to tell you that rising seas will worsen coastal flooding, but the new report shows how the effects will vary greatly across different regions.

The Northeast currently experiences the most frequent flooding, largely because of regular winter storms—including a recent series of storms that has caused flood damage across the region.

In places where the weather is relatively calm most of the year and the difference between low and high tides is smaller, such as Southeast, coastal flooding is not yet as frequent. But those same factors that create a relatively constant water level mean that once flooding begins, it will worsen more quickly. This is what we're seeing now in places like Miami and Charleston, South Carolina, where tidal flooding is quickly becoming more than just a nuisance.

By mid-century, the Western Gulf of Mexico should expect to have 80 to 185 days of flooding per year, and the coastal Northeast should expect 45 to 130 days. The Southeast and the Eastern Gulf of Mexico will likely experience between 25 and 85 days per year, and the West Coast fewer still.

By the end of the century, though, the gap narrows or disappears, with most of the East and Gulf coasts experiencing flooding at least every other day under a lower estimate of rising seas, and every day under a higher one.

Sinking San Francisco

The report uses two scenarios—an "intermediate low" of about 1.5 feet by 2100 and an intermediate of about 3 feet. The two represent the lower and upper bounds of what's likely to occur, Sweet said, though the actual rise could be far greater if greenhouse gas emissions don't fall later this century or if Antarctic ice sheets begin to collapse.

Even under the more moderate scenarios, however, flooding could still be worse than NOAA projects in some places. Land is sinking across many coastal areas, and while broader regional rates are generally well known and incorporated into sea level rise estimates—it's part of why the Northeast is experiencing higher relative sea level rise—subsidence can vary greatly on a more local level.

A separate study, published Wednesday in Science Advances, uses satellite data to examine subsidence across the San Francisco Bay Area. It found that most places are sinking at a rate of less than 2 millimeters per year, but that certain spots, including San Francisco International Airport, are sinking at up to 10 millimeters per year. Add this all up, the authors write, and rising seas could actually inundate perhaps twice as much land as expected in the Bay Area. Many other coastal cities, including Tokyo, Jakarta, and the Hampton Roads region of Virginia, have similar problems with subsidence.

What Can Cities Do?

Of course, the actual impact of flooding will depend on how cities adapt, such as by building seawalls, flood gates or abandoning some low-lying spots. New York, Miami, Norfolk, Virginia, and other coastal cities have already begun to implement some measures, such as requiring that new buildings be elevated a certain amount—called freeboard—above the flood level, generally between 1 and 3 feet.

Sweet said the NOAA report shows how vulnerable most places are to rising seas. He found that minor coastal flooding generally occurs when waters rise about 1.5 feet above normal, and damaging flooding occurs with less than 3 feet of water.

"It's kind of laid bare America's infrastructure," he said. "There's really not that much freeboard separating our infrastructure from sea levels."