King tides, boosted by a rare supermoon and sea level rise, flood the Atlantic coast
The highest normally occurring tides of the year, known as king tides, combined with sea level rise and a rare supermoon brought coastal flooding to the Atlantic Coast in mid-November.
King tides are a seasonal event occurring a few times a year when the orbits and alignment of the Earth, moon, and sun combine to produce the greatest tidal effect. The tides get even higher during supermoons, which is the name given to moons that orbit closest to the Earth.
Coastal sections of South Florida and New England were among those immersed in ocean water on November 14 and 15 during high tide cycles. The National Weather Service issued coastal flood advisories from Palm Beach to Miami cautioning that tidal levels were running about a foot above normal. Further up the coast, Boston experienced the peak of the king tide on November 15, with the tide reaching about 12.5 feet, and Portland experienced water levels up to 12 feet, or more than 2 feet higher than a typical high tide.
In the context of high tides that are 1 to 2 feet above normal, the sea level rise to date of 8 inches makes for a significant increase in tide height, which in turn reaches much further inland in low-lying areas. The National Oceanic and Atmospheric Administration (NOAA) reports that nuisance, or 'sunny-day flooding,' brought on by high tides has already increased 300 to 925 percent due to sea level rise to date. Such flooding inundates areas that previously flooded only during big storms. It makes roads impassable, overwhelms storm drains and seeps into structures.
Sea level rise and high tides
Climate change has boosted king tides—the highest of high tides—even higher. Due to the melting of land ice and the expansion of sea water as it warms, global sea level has risen 8 inches since 1880.
Sea level rise boosts high tides so that they reach higher and extend further inland than in the past, increasing the risk of coastal flooding during high tide events. High tides are a problem even on storm free days—a phenomenon known as “nuisance flooding.”
Since 2006, flooding in Miami Beach has soared—400 percent from high tides and 33 percent from rain. The increased flooding, calculated from insurance claims, media reports and tidal gauges, stems from a regional rise in sea levels well above global increases. From 1998 to 2006, tidal gauges in South Florida show sea level rise keeping up with global averages of about .04 to .20 inches a year. But in 2006, local sea level rise suddenly underwent a rapid acceleration, averaging about .20 inches to a half inch per year.
Across the US, NOAA reports that nuisance flooding has already increased 300 to 925 percent due to sea level rise to date. NOAA water level gauges document an increasing frequency of tidal flooding around much of the US, which is driven primarily by local relative sea level rise. Since 1950, about two-thirds of US coastal flood days in 27 different locations were linked to human-caused sea level rise.
While sea level rise may be modest relative to the total height of high tides, it can be responsible for a disproportionate amount of damage. This is because human infrastructure and natural systems have developed to cope with a range of historical extremes, and climate change signals like accelerated sea level rise can overwhelm existing human systems and structures.