The lunar cycle and climate change will deliver a one-two punch to the entire US coastline.

A map of 2021 sea surface height anomalies, with red and orange regions representing sea levels 10 to 15 cm higher than normal. (Image credit: NASA Earth Observatory/ Joshua Stevens)

Climate change has already increased the frequency and severity of hurricanes and other extreme weather events around the world. — However, there is a lesser, less apparent menace on the horizon that might destroy America’s coasts.

High-tide floods, often called as “nuisance floods,” occur in coastal areas when tides go over the daily average high tide by about 2 feet (0.6 meters) and begin to flood streets or seep through storm drains. These floods, true to their name, are more of a nuisance than a disaster, inundating streets and homes, forcing businesses to close, and causing cesspools to overflow — but the longer they last, the more damage they can do.

The U.S. experienced more than 600 of these floods in 2019, according to the National Oceanic and Atmospheric Administration (NOAA). But now, a new study led by NASA warns that nuisance floods will become a much more frequent occurrence in the U.S. as soon as the 2030s, with a majority of the U.S. coastline expected to see three to four times as many high-tide flood days each year for at least a decade.

The study, published in the journal Nature Climate Change, warns that these extra flood days will not be spread out evenly over the year, but will likely cluster together over a few months; coastal areas that now face two or three floods per month may soon face a dozen or more.

These prolonged coastal flood seasons will cause major disruptions to lives and livelihoods if communities don’t start planning for them now, the researchers cautioned.

“It’s the accumulated effect over time that will have an impact,” lead study author Phil Thompson, an assistant professor at the University of Hawaii, said in a statement. “If it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue.”

Several factors drive this predicted increase in flood days.

For one, there’s sea level rise. As global warming heats up the atmosphere, glacial ice is melting at a record pace, dumping enormous amounts of meltwater into the ocean. According to NOAA, global average sea levels have risen around 8 to 9 inches (21 to 24 centimeters) since 1880, with about a third of it occurring in the previous 25 years. Depending on how efficiently humans control greenhouse gas emissions in the following decades, sea levels could rise anywhere from 12 inches (0.3 m) to 8.2 feet (2.5 m) above where they were in 2000 by the year 2100.

While increasing sea levels alone will increase the frequency of high-tide floods, they will have a little help from the cosmos – especially, the moon.

The moon influences tides, but the intensity of the moon’s pull varies from year to year; the moon’s orbit has a “wobble,” slightly adjusting its position relative to Earth on a rhythmic 18.6-year cycle. The moon suppresses tides on Earth for half of the cycle, resulting in lower high tides and higher low tides. According to NASA, tides are amplified during the opposite half of the cycle, with higher high tides and lower low tides.

We are currently in the tide-amplifying part of the cycle; the next tide-amplifying cycle begins in the mid-2030s; — and, by then, global sea levels will have risen enough to make those higher-than-normal high tides particularly troublesome, the researchers found.

High-tide flooding would grow rapidly across the entire US coast as a result of the combined effect of sea-level rise and the lunar cycle, according to the team. High-tide flooding will “transition from a regional issue to a national issue with a majority of U.S. coastlines affected” in less than a decade, according to the authors. Other elements of the climate cycle, like El Niño events, will cause these flood days to cluster in certain parts of the year, resulting in entire months of unrelenting coastal flooding.

Scary as this pattern sounds, it is also important to understand for planning purposes, the authors wrote.

“Understanding that all your events are clustered in a particular month, or you might have more severe flooding in the second half of a year than the first — that’s useful information,” study co-author Ben Hamlington of NASA’s Jet Propulsion Laboratory said in the statement.

Extreme weather disasters may get all of the national media attention as they batter America’s coasts, but high-tide flooding will become impossible to ignore in the near future. The authors concluded that it is best to start planning for it now, before it is too late.