Flooding in Annapolis's historic City Dock. (Jonathan Newton/The Washington Post)

When does the threat of climate change become reality? When does a forecast of sea-level rise become a here-and-now problem?

In a new study, researchers at the National Oceanic and Atmospheric Administration, better known as NOAA, grappled with that question, and defined the "tipping point" as the moment when coastal locations routinely experience at least 30 days of flooding each year.

By that measure, Annapolis; Wilmington, N.C.; and Washington, D.C., are already in trouble. Seven more cities, including Baltimore, Atlantic City and Port Isabel, Tex., will be there by 2020. And within the next 35 years, the study says, most cities along the Mid-Atlantic, Gulf and Pacific coasts will be dealing with routine flooding.

NOAA projections for when selected cities will pass a flooding tipping point of 30 days per year.
NOAA projections for when selected cities will pass a flooding tipping point of 30 days per year.

It's important to note that this is not storm-related flooding. That will no doubt happen more frequently, too. The NOAA analysis looked at "nuisance flooding" -- the inundation of city streets and landmarks by routine high tides  -- as measured at and projected for  NOAA tidal stations with a record stretching back at least 50 years.

Such flooding is already much more common than in decades past, the report’s co-author, William Sweet, an oceanographer at NOAA’s Center for Operational Oceanographic Products and Services, said in a written statement, adding: "This is due to sea level rise."

"Unfortunately, once impacts are noticed, they will become commonplace rather quickly," Sweet said. "We find that in 30 to 40 years, even modest projections of global sea level rise -- 1½ feet by the year 2100 -- will increase instances of daily high tide flooding to a point requiring an active, and potentially costly response. And by the end of this century, our projections show that there will be near-daily nuisance flooding in most of the locations that we reviewed."

The analysis was published Thursday in the American Geophysical Union’s online journal, "Earth’s Future." Editor Michael Ellis said the paper's importance stems from its focus on "the largely neglected" issue of flooding frequency.

Nuisance floods are "occasionally costly to clean up, but never catastrophic or perhaps newsworthy,” Ellis said. The new study raises the question, he said, of when the nuisance becomes "a serious hazard."

The study does not include Miami, arguable the U.S. city most vulnerable to sea-level rise, because NOAA tide stations there were destroyed by Hurricane Andrew in 1992.