For many coastal dwellers in the United States, high tide was once just a more preferable time to swim or surf, but now, it portends something more ominous.

In 2017, the National Oceanic and Atmospheric Administration (NOAA) found that U.S. cities experienced a record number of flooding events related to high tides. Miami, Charleston, Annapolis, Boston, Galveston, and Norfolk are just a few of the more recognizable places where a mere rainstorm or an unusually high tide now mean that roads are inundated, storm drains are overwhelmed, and infrastructure is damaged in many neighborhoods.

With seas rising across the globe due to melting ice (currently projected at about 3.4 millimeters per year and accelerating), tidal flooding events are growing more frequent, more extreme, and more widespread. A recent analysis published to Scientific Reports conducted by researchers at the University of Illinois-Chicago, the University of Hawaii, and the U.S. Geological Survey provides more precise and disconcerting details.

Examining historical data from more than 200 tidal gauges positioned on the U.S. west coast, east coast, and Hawaii, they examined the effects of increasing sea levels on tidal fluctuations and extrapolated those effects into the future. Their findings? By 2050, 70% of the sites will experience record high tides that, under current conditions happen on average once over fifty years, every year. By 2100, these '50-year-tides' will occur every day in 93% of sites.

The authors wanted to accentuate this conclusion.

"Sea-level rise will likely cause ‘once-in-a-lifetime’ coastal flooding events to occur nearly every day before 2100," they wrote.

This photo from the Office for Coastal Management's Sea Level Rise Viewer shows the affected areas of New Orleans under high tide flooding.

Estimates of sea level rise inform us that oceans could swell as much as three feet by 2100, radically reshaping coastlines around the world. The current paper is telling us that coastal areas could be flooded with that sort of severity (or worse) on a yearly basis decades beforehand. Tens or even hundreds of thousands of Americans will be forced to decide if living along the coast is worth their neighborhoods being inundated and their property getting damaged every year. It's a calculus that will likely prompt many to pack up and move.

"Our society has yet to fully comprehend the imminence of the projected regime shifts in coastal hazards and the consequences thereof," the researchers conclude.

Source: Taherkhani, M., Vitousek, S., Barnard, P.L. et al. "Sea-level rise exponentially increases coastal flood frequency." Sci Rep 10, 6466 (2020). https://doi.org/10.1038/s41598-020-62188-4