Recurrent high-tide floods are expected to worsen as sea levels continue to rise on account of climate change, but, as a new study warns, a regularly occurring 18.6-year cycle involving the Moon could trigger unprecedented flooding along U.S. coasts in the 2030s.
Nuisance flooding, sunny day flooding, or high-tide flooding—it’s all the same thing, and an annoying pain in the ass. In 2019, NOAA tracked more than 600 of these recurring high-tide flooding events, in which high tides extend 2 feet (0.6 meters) above the norm. These floods aren’t life threatening, but they can damage coastal infrastructure in affected areas and create annoyances like flooded parking lots. Needless to say, nuisance flooding is happening more frequently on account of human-induced climate change, and it’s poised to get even worse as sea levels continue to rise.
If that’s not bad enough, an 18.6-year lunar cycle is expected to amplify this effect even further, according to new research published in Nature Climate Change. The authors of the paper, led by Phil Thompson from the University of Hawaii, say the confluence of rising sea levels and a periodic wobble in the Moon’s orbit will increase the frequency and severity of high-tide floods along U.S. ocean coastlines. By the mid-2030s, tidal floods could occur in batches that last for a month or more and on a nearly daily basis, the scientists say. Members of NASA’s Sea Level Change Science Team from the University of Hawaii contributed to this research.
Scientists have known about this wobble in the Moon’s orbit since the early 18th century, as well as how alignments involving the Moon, Earth, and Sun can influence the tides. During the first half of this cycle, high tides are below the normal average and low tides are higher than normal. During the other half of the cycle, both the high and low tides are amplified, appearing both higher and lower than usual. The reason for this has to do with the Moon’s gravitational pull, which causes Earth’s ocean tides. We’re currently in the amplification phase of this cycle, but the Moon’s gravity is not affecting tides to the degree expected in the mid-2030s when the amplification phase renews.
This is all well known, but scientists are now having to predict the effect of this lunar cycle in the era of climate change and rising sea levels. Indeed, the situation looks bad, Moon wobble or no. Figures provided by NOAA paint a grim picture, with estimates suggesting global sea levels will rise by at least 12 inches (0.3 meters) by the turn of the century. Unfortunately, the world is currently on track for the worst-case sea level rise scenario that scientists have modeled and researchers have uncovered increasing worrisome signs about Antarctica’s ice. As of 2014, nearly 40% of the U.S. population inhabits coastal areas that could be vulnerable to rising sea levels.
“Low-lying areas near sea level are increasingly at risk and suffering due to the increased flooding, and it will only get worse,” said NASA Administrator Bill Nelson in a statement. “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.”
To build the new predictive model, Thompson and his colleagues studied tidal information gathered by 90 gauges distributed along U.S. coasts, statistics on high-tide flooding and meteorological events like El Niño events, astronomical cycles, among other data points. Recurrent high-tide floods are expected to happen more often along nearly all U.S. mainland coastlines, Hawaii, and Guam. Alaska won’t experience these problems for at least another decade or longer, because its land masses are actually rising on account of geological processes.
Thompson said that high-tide floods are not as bad as hurricane storm surges, but he warned of the cumulative effects and also the emergence of “seeping cesspools” as a public health issue. Urban planners should take notice of the new findings and act accordingly, the scientists conclude in the study.