High tides, caused by the effects of the sun’s and moon’s gravitational pulls on the Earth, are no new phenomenon. But coastal flooding from such high tides is becoming more and more of a problem. As global warming worsens and sea levels rise, high-tide flooding is affecting coastal communities more than ever before.
A new website shows users, via multiple maps of the U.S., the estimated number of days of high-tide flooding that could occur per year up to 2100. As someone looks at the maps, they can drag a bar to change the year, in one-year increments, and observe the severity of flooding in 99 different coastal cities across the country. Different colors represent greater danger, and users can zoom in and click on a specific city to see the number of flooding days there.
Six maps represent six different scenarios, ranging from low to extreme, based on estimated mean sea-level rise (which could vary from 0.3 meters to 2.5 meters). Ultimately, how those scenarios will actually play out depends on how seriously we take climate change. “What this app is showing is what all the potential scenarios are, for either our lack of change or if we make drastic changes,” says Dan Pisut, environment content lead at ArcGIS Living Atlas of the World, a collection of maps and geographic information. Esri, which developed the site, makes the ArcGIS cloud-based mapping software that allows for data analysis through geographic and spatial visualizations.
High-tide flooding is also called “sunny-day flooding” or “nuisance flooding,” names that reflect the previous lack of serious attention paid to the problem. The highest tide levels occur once every 28 days, during perigee, the point when the moon is closest to earth and its gravitational pull is therefore strongest. When there’s a full or new moon during these perigean tides, it can cause tides to be more than a foot higher than normal. It used to take an extreme weather event, such as a hurricane, for those tides to manifest into floods. But now, with already high sea levels, flooding happens regardless of rain or storm events, and those flooding days are much more frequent.
The National Oceanic and Atmospheric Administration (NOAA) originally published the sea-level and flood modeling data that Esri uses for its site, which the company hopes is an easy way to visually showcase NOAA’s data. According to NOAA, high-tide flooding is now twice as frequent as it was in 2000. In the Southeast region, this type of flooding has increased by 300% since 2000, and in the western Gulf of Mexico region, by 500%.
Some coastal communities will be more vulnerable than others. According to Esri’s analysis, in the intermediate scenario (where the mean sea-level rise is at 1 meter), the number of flooding days in Atlantic City, New Jersey, will increase from 25 a year to 55 a year between 2021 and 2031; and in Sewell’s Point, a peninsula in Norfolk, Virginia, from 22 to 52. Galveston, Texas, is not yet among the top ten spots with the most flooding days, but annual flooding days are predicted to rise there from 16 to 58, a 263% increase; and in Grand Isle, Louisiana, from 7 to 54, a 671% increase. By 2050, Grand Isle would have 324 flooding days under the intermediate scenario—and in the extreme scenario, 365 days of flooding a year.
Had we dealt with global warming better earlier on, the flooding outlook would have appeared a lot better, Pisut says. “If we keep doing what we’re doing, we’re going to be on a very high impact,” he says. “We are personally contributing to raising that baseline of the mean high watermark of the ocean for any coastal area.” Mean global sea-level rise has been on an upward curve: in 2019, it was 8.8 cm above the 1993 average, and between 2018 and 2019, it rose another 0.6 cm. The rise rate could double in the next century.
City planners and municipal and state leaders already use Esri’s tools for making impact scenario assessments, and the company hopes they’ll be able to make strategic decisions about coastal management using these maps. Flood water is affecting roads, rail, and property near coastlines and causing stress to the stormwater pumping systems already set up to mitigate problems. Miami and Charleston have been able to build seawalls, but they may not be protective enough very soon if levels keep increasing at the same rate.
Esri hopes people will use the free app to see the alarming prospects of global warming and take it more seriously. “We have done a pretty good job at minimizing these impacts with engineering efforts,” says Keith VanGraafeiland, the ocean curator for the Living Atlas team. “But there’s going to be a certain point in time when those efforts are going to be outweighed by what Mother Nature is doing.”
This post has been updated to clarify that ArcGIS is the software made by the company Esri.