In a NASA-led study published in June in the journal Nature Climate Change, researchers said that the planet's lunar cycle would exacerbate sea level rise (SLR) spurred by climate change in coming years.
The study authors used scenarios and "flooding thresholds" to show how the effects of SLR and "nodal cycle modulations of tidal amplitude" lead to severe inflections in projections of future HTF.
In addition, the research – which the agency said takes all known oceanic and astronomical causes for floods into account – said annual cycles and sea-level anomalies could lead to seasons or monthslong periods during which days of HTF will cluster, resulting in "critical frequencies" of HTF occurring during monthly or seasonal periods one to two decades prior to being expected on an annual basis.
"When the moon and Earth line up in specific ways with each other and the sun, the resulting gravitational pull and the ocean’s corresponding response may leave city-dwellers coping with floods every day or two," NASA said in a release.
While HTF or "nuisance" flooding is already an issue for coastal cities around the country – especially bordering the Atlantic Ocean – the impact of risings seas has been comparatively minor in this decade.
Nevertheless, the National Oceanic and Atmospheric Administration (NOAA) reported a total of more than 600 nuisance floods in 2019 and noted that the national trend in such flooding is accelerating and is "more than twice as likely now as it was in 2000" due to relative SLR.
The agency said that by 2050, high-tide flooding is likely to occur between 25 and 75 days per year, depending on location.
"It’s the accumulated effect over time that will have an impact," said Phil Thompson, an assistant professor at the University of Hawaii and the lead author of the new study. "But if it floods 10 or 15 times a month, a business can’t keep operating with its parking lot underwater. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue."
The lunar "wobble" takes 18.6 years to complete and has been observed for generations, but how that wobble impact's the moon's gravitational pull combines with rising sea levels is "what's new."
While Earth's regular tides are suppressed in half of the 18.6-year lunar cycle, SLR pushes tides higher during the other half.
NASA said that the next time the moon is in the tide-amplifying part of its cycle, higher seas will cause a "leap" in flood numbers for almost all of U.S. mainland coastlines, Hawaii and Guam.
The scientists projected results to 2080, revealing tipping points by studying 89 tide gauge locations in every coastal U.S. state and territory but Alaska and using the flooding thresholds, astronomical cycles and statistical representations of other tide-affecting processes like the Pacific Ocean's El Niño climate pattern.
The global sea level has been rising at an increased rate in recent decades and continues to rise at a rate of about one-eighth of an inch per year.
"Low-lying areas near sea level are increasingly at risk and suffering due to the increased flooding, and it will only get worse," warned NASA Administrator Bill Nelson. "The combination of the Moon’s gravitational pull, rising sea levels, and climate change will continue to exacerbate coastal flooding on our coastlines and across the world. NASA’s Sea Level Change Team is providing crucial information so that we can plan, protect, and prevent damage to the environment and people’s livelihoods affected by flooding."