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, according to data released by space agency NASA.

High-tide floods are already a familiar problem in many cities on the US Atlantic and Gulf coasts, with over 600 such floods reported in 2019. Starting in the mid-2030s, however, the alignment of rising sea levels with a lunar cycle will cause coastal cities all around the country to begin a decade of dramatic increases in flood numbers according to the study which takes into account all known oceanic and astronomical causes for floods.

Led by the members of the NASA sea level change science team from the University of Hawaii, the study shows that high tides will exceed known flooding thresholds around the country more often. In addition, 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,” said 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.”

NASA describes the upcoming lunar changes as a regular ‘wobble’ in the Moon’s orbit that takes 18.6 years to complete. The shift within the lunar cycle is not new and was first reported in 1728, but the agency points out that what is new is how one of the wobble’s effects on the Moon’s gravitational pull – the main cause of Earth’s tides – will combine with rising sea levels resulting from climate change.

In half of the Moon’s 18.6-year cycle, Earth’s regular daily tides are suppressed, meaning that high tides are lower than normal, and low tides are higher than normal. In the other half of the cycle, tides are amplified so that high tides get higher, and low tides get lower. NASA believes that global sea level rises – and the resulting higher tides – will combine with the ‘lunar assist’ in the 2030’s to create the heightened flooding risk.

Ben Hamlington of NASA’s jet propulsion laboratory in Southern California is a co-author of the paper and also the leader of NASA’s sea level change team. He said that the findings of the new study are a vital resource for coastal urban planners, who may be focused on preparing for extreme events rather than more high-tide floods: “From a planning perspective, it’s important to know when we’ll see an increase. 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.”

A high-tide flood tool developed by Thompson already exists on the NASA team’s sea level portal, a resource for decision-makers and the general public. The flood tool will be updated in the near future with the findings from this study.

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