Sea level rise caused by global warming is usually cast as a doomsday scenario that will play out so far into the future, it’s easy to ignore. Just ask anyone in South Florida, where new construction proceeds apace. Yet already, more than 90 coastal communities in the United States are battling chronic flooding, meaning the kind of flooding that’s so unmanageable it prompts people to move away.

That number is expected to roughly double to more than 170 communities in less than 20 years.

Those new statistics, compiled in the first comprehensive mapping of the entire coastline of the Lower 48 states, paint a troubling picture, especially for the East and Gulf coasts, which are home to some of the nation’s most populated areas.

By the end of the century, chronic flooding will be occurring from Maine to Texas and along parts of the West Coast. It will affect as many as 670 coastal communities, including Cambridge, Massachusetts; Oakland, California; Miami and St. Petersburg, Florida; and four of the five boroughs of New York City. The magnitude of the coming calamity is so great, the ripple effects will reach far into the interior.

The Union of Concerned Scientists, in a new report published today in the peer-reviewed journal Elementa, mapped the rate of sea-level rise for the first time for hundreds of coastal communities—in an online feature that allows viewers to zoom in from a bird’s eye view all the way down to street level. The report also creates a timeline when sea-level rise is projected to inundate various percentages of coastal communities—and then delves into the politically and economically sensitive arena of what to do about it.

Options are limited. All are costly, whether adapting to a watery future with seawalls and other barriers, or retreating and finding a new place to call home.

When does flooding become intolerable?

The study is the latest in a series of reports the science organization, based in Cambridge, Mass., has published on rising seas. The previous report, published last year, catalogued the risks rising seas pose for the nation’s military bases.

“We knew we were going to be going out ahead of where anyone has gone before, but this is where the science took us,” says Erika Spanger-Siegfried, the report’s co-lead author. “We recognized we were going to have to offer something useful to communities in the way of solutions. We wanted people to see this coming, to give them a sense of the time they have before this becomes untenable, and outline things they can do to respond.”

The new coastal project, Spanger-Siegfried says, began with several practical questions:

How many times a year would residents tolerate chronic flooding that overwhelmed their neighborhood? If saltwater flooding regularly soaked homeowners’ first floor or damaged their cars, how often would it have to occur before residents began to look for a new place to live? How long would it be before they could no longer afford to insure or sell their home?

“We are not mapping the so-called ‘bathtub approach,’ which is just measuring sea-level rise over time. In other words, where will high tide be every day with a foot of sea-level rise?” says Spanger-Siegfried. “That’s not how people plan and live their lives. They will make decisions based on what is chronically inundated decades before things are permanently inundated.”

Chronic, disruptive flooding was defined as 10 percent or more of a community’s usable land flooding 26 times a year, or every other week. Most of the 90 communities that experience such flooding already are in Louisiana and Maryland, where land subsidence has intensified the effects of sea-level rise. Life has already been altered in those places, where flood advisories are normal and residents have learned to avoid low-lying streets. (Watch an island disappear from the Gulf.)

For perspective, the report’s authors note that Miami Beach, considered Ground Zero for sea-level rise, has not reached the 10 percent threshold, even as it experiences high-tide flooding and has invested more than $400 million to rebuild the city’s storm sewers. Likewise, flooding in Annapolis, Maryland, home to the U.S. Naval Academy, is not expected to reach the 10 percent threshold, although key parts of the city, including the academy campus and downtown, now flood 40 times a year.

But over time, disruptive flooding will spread, engulfing the Jersey shore, the mainland side of North Carolina’s Pamlico Sound, and throughout South Carolina’s Low Country.

Rising seas more widespread

The study examined three scenarios: a “low” scenario that assumes carbon emissions decline dramatically and global warming is limited to 2 degree Celsius; an “intermediate” scenario, which projects carbon emissions peaking at mid-century, resulting in four feet of sea-level rise globally; and “high” scenario, which occurs toward 2100, with polar ice melting fast enough to produce about 6.5 feet of sea-level rise. Scientists consider the “high” scenario to be increasingly plausible, the study notes, as the melting of ice sheets accelerates.

“This is a much nearer-term and more widespread problem,” Spanger-Siegfried says. “We are going to have to appreciate that we are at the front end of a climate change adaptation century. Our coasts face a certain amount of transformation with sea-level rise and a huge share of our population lives, and GDP is generated, on the coasts. And as tens of communities turn into hundreds of communities, we are going to have to marshal federal resources to enable this change to unfold in a manageable way.”

On the West Coast, which lacks the East Coast’s long, shallow offshore shelf, rising seas will be less serious, but cities in the San Francisco Bay area will flood by 2060, the reports says.

There is time enough to prevent some of the flooding, the report’s authors contend, if the world’s nations can successfully cut greenhouse gas emissions as outlined in the Paris climate accords. But for hundreds of other communities, including the small towns that dot Maryland’s Eastern Shore, as well as Savannah, Georgia, New Orleans and Miami, it may already be too late. Chronic flooding can be avoided only by adaptation measures—seawalls, levees, and other barriers—or by moving away.

It’s the inclusion of that last option, and others, such as halting development along coastal areas at risk to flooding, that sets this study apart from others. Community groups in Miami, Norfolk, Virginia, and other vulnerable places along the coast are already planning, but are cautious about sounding the note of “retreat” too loudly, lest they crash the local economy decades prematurely.

Spanger-Siegfried says she understands the dilemma. But her team decided to include a discussion of options in the study after a climate change expert who works with communities advised them to. “If you want to do anything useful for coastal communities,” she said,”Tell them how much time they have to act.”