Every US coast is expected to experience a rapid rise in high tide floods in the mid-2030s. This is because the lunar cycle will be at its tidal amplification point, which will magnify the sea-level rise caused by climate change.
High-tide floods (aka sunny day floods or nuisance floods) are already a familiar problem in many cities along the Atlantic and Gulf coasts. In 2019, over 600 such floods were reported by the National Oceanic and Atmospheric Administration (NOAA). This period of increased flooding was brought on by a regular wobble in the Moon’s orbit that takes 18.6 years to complete.
Bill Nelson, the NASA Administrator, said:
“Low-lying areas near sea level are increasingly at risk and suffering due to the increased flooding, and it will only get worse. Combining 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.”
This new NASA study, led by the NASA Sea Level Change Science Team from the University of Hawaii, is the first to consider all known oceanic and astronomical causes for floods, like this wobble. It found that when the cycle comes back around to its peak, all US coastal cities will dive into a decade of dramatic increases in flood numbers because climate change will have exasperated sea level rise too.
The wobble’s effects on the gravitational pull of the Moon works like this:
- Earth’s normal daily tides are suppressed during one half of the 18.6-year cycle, so low tides are higher than usual, and high tides are lower than usual.
- During the other half of the cycle, the tides are amplified, so low tides get lower and high tides get higher.
- Meanwhile, global sea-level rise continues to push tides only higher. Therefore, the suppressed half of the 18.6-year lunar cycle counteracts sea-level rise while the amplified half increases the effect.
The study shows that floods will occur more frequently as high tides exceed known flooding thresholds nationwide. The events will occur in clusters sometimes, perhaps lasting over a month depending on the positions of the Earth, Moon, and Sun. When the Moon, Earth, and Sun align in a specific way, the ocean’s corresponding response to the resulting gravitational pull may leave coastal city dwellers coping with floods every day or two for weeks.
Lead author Phil Thompson, an assistant professor at the University of Hawaii, said:
“It’s the accumulated effect over time that will have an impact. Since high tide floods involve a small amount of water compared to hurricane storm surges, there’s a tendency to view them as a less significant problem overall. 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.”
In the mid-2030s, when the tide-amplifying part of the Moon’s cycle swings back around and global sea-level rise will have been in process for over a decade, the situation will be much more complicated than now. Only far northern coastlines like Alaska’s will be spared until perhaps the next cycle because these land areas are rising by cause of long-term geological processes.
Co-author Ben Hamlington from NASA’s Jet Propulsion Laboratory in California and NASA’s Sea Level Change Team leader pointed out how the findings are vital for urban planners who might primarily focus on preparing coastal developments for extreme weather events. This study highlights the importance of also preparing for more high tide flooding.
Hamlington said:
“From a planning perspective, it’s important to know when we’ll see an increase. So, for example, 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.”
Thompson developed a high-tide flood tool that’s available on the NASA team’s sea level portal, a resource for the general public and decision-makers. The researchers plan to update it with the findings from this study.
