Kenny Louie

By 2021, lower Manhattan will start to be frequently flooded thanks to rising sea levels, according to a new study from the National Oceanic and Atmospheric Administration.

The agency says that high tide events are getting higher in most major U.S. coastal cities. After averaging out projected further increases in sea levels, and looking at where and when tidal flooding is occurring now, they found that these cities can now expect 30 days of "nuisance flooding" a year, the impact of which will include:

  • Deterioration and corrosion of infrastructure not designed to withstand frequent inundation or saltwater exposure
  • Frequent road closures
  • Overwhelmed storm water drainage capacity at high tide

"Coastal communities are beginning to experience sunny-day nuisance or urban flooding, much more so than in decades past,‚ÄĚ said study co-author William Sweet. ‚ÄúThis is due to sea level rise. Unfortunately, once impacts are noticed, they will become commonplace rather quickly."

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For lower Manhattan, regular nuisance flooding could begin as soon as 2021. Here's what sea level rise has looked like at the Battery, Manhattan's southern tip.

The phenomenon has already begun in places like Annapolis, Md, which now already sees an average of¬†40 days of such floods a year ‚ÄĒ a 10x increase from a half-century ago, according to the Baltimore Sun.¬†Here's a photo of an extreme tide surge in downtown Annapolis¬†from 2012 ‚ÄĒ it's what Lower Manhattan can start to expect within a decade:

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We will also start to see smaller storm surges more frequently turn into substantial flooding, NOAA says, and rising sea levels will increase the probability of a previously once-in-a-century storm surge into a once-a-decade occurrence.

Indeed, the damage from¬†Superstorm Sandy¬†came not from pounding winds or¬†driving rains, but from a 14-foot pulse of seawater. In case you forgot, here's what happened to the subway station in Hoboken, New Jersey ‚ÄĒ whose elevation is the same as the Battery's ‚ÄĒ as a result:

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Even a best-case scenario global sea level increase of 1.5 feet by 2100 will increase instances of daily high tide flooding to a point requiring an active and potentially costly response, NOAA says.

Here's the full table of projections for U.S. cities, with lower Manhattan highlighted. Also of note are major West Coast cities like San Francisco and Seattle, which often don't get as much attention but are nonetheless situated in low-lying areas.

The middle column shows the point above usual tide cresting at which nuisance flooding starts to occur, and the right column shows when 30-days' worth of annual average nuisance flooding will begin.

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Rob covers business, economics and the environment for Fusion. He previously worked at Business Insider. He grew up in Chicago.