High-tide flooding and how a moon wobble may put US coastline at risk in the mid-2030s

A new study led by members of the NASA Sea Level Change Science Team from the University of Hawaii shows that high tides will exceed known flooding thresholds around the US more often. In a warning, the study says that in the mid-2030s, every US coast will experience rapidly increasing high tide floods, when a lunar cycle will amplify rising sea levels caused by climate change. What are high-tide floods? High tide flooding — flooding that leads to public inconveniences such as road closures — is increasingly common as coastal sea levels rise. As the relative sea-level increases, it no longer takes a strong storm or a hurricane to cause coastal flooding. Flooding now occurs with high tides in many locations due to climate-related sea-level rise, land subsidence, and the loss of natural barriers. High-tide floods – also called nuisance floods or sunny day floods – are already a familiar problem in many cities on the US Atlantic and Gulf coasts. The National Oceanic and Atmospheric Administration (NOAA) reported a total of more than 600 such floods in 2019. According to NOAA, the effects of rising sea levels along most of the continental US coastline are expected to become more noticeable and much more severe in the coming decades, likely more so than any other climate-change-related factor. Any acceleration in sea level rise that is predicted to occur this century will further intensify high tide flooding impacts over time, and will further reduce the time between flood events. How things might change in the mid-2030s Starting in the mid-2030s, however, the alignment of rising sea levels with a lunar cycle will cause coastal cities all around the US to begin a decade of dramatic increases in flood numbers, according to the first study that takes into account all known oceanic and astronomical causes for floods. The study further says that the floods will sometimes occur in clusters lasting a month or longer, depending on the positions of the moon, earth, and the sun. 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. “Low-lying areas near sea level are increasingly at risk and suffering due to the increased flooding, and it will only get worse,” the study quoted NASA Administrator Bill Nelson as saying. “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.” “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, published this month in Nature Climate Change. Although high tide floods involve a small amount of water compared to hurricane storm surges, it cannot be ignored as 10-15 floods a month will affect businesses and health. Despite the moon being in the tide-amplifying part of its cycle now, along most US coastlines, sea levels have not risen so much that even with this lunar assist, high tides cause regular flooding. There is a fear that it will be a different story altogether the next time the cycle comes around to amplify tides again in the mid-2030s. There is a concern that a leap in flood numbers on almost all US mainland coastlines, Hawaii and Guam. Perhaps the far northern coastlines, including Alaska’s, will be spared for another decade or longer. What is moon wobble? NASA calls the moon “wobble” is a cyclical shift in the moon’s orbit that can lead to coastal flooding in the 2030s. It was first reported in 1728 and happens every 18.6 years. This slow fluctuation either suppresses or amplifies tides on our planet. “In half of the Moon’s 18.6-year cycle, earth’s regular daily tides are suppressed: High tides are lower than normal, and low tides are higher than normal,”  NASA said. “In the other half of the cycle, tides are amplified: High tides get higher, and low tides get lower. Global sea level rise pushes high tides in only one direction – higher. So half of the 18.6-year lunar cycle counteracts the effect of sea level rise on high tides, and the other half increases the effect.” While in half of the Moon’s 18.6-year cycle, the earth’s regular daily tides are suppressed: High tides are lower than normal, and low tides are higher than normal, in the other half of the cycle, tides are amplified: High tides get higher, and low tides get lower. The global sea-level rise pushes high tides in only one direction – higher. So half of the 18.6-year lunar cycle counteracts the effect of sea-level rise on high tides, and the other half increases the effect, the study said. What is the methodology of the research? The researchers reached the conclusion by studying 89 tide gauge locations in every coastal US state and territory but Alaska. The team created a new statistical framework that mapped NOAA’s widely used sea-level rise scenarios and flooding thresholds, the number of times those thresholds have been exceeded annually, astronomical cycles, and statistical representations of other processes, such as El Niño events, that are known to affect tides, a NASA report said. They projected results up to 2080.