The US will see a dramatic increase in high-tide coastal flooding in the coming decade, according to NASA. This increase will be powered by the interaction between the moon’s gravitational influence (which causes tides) and higher sea levels caused by climate change.

High-tide floods are an already familiar problem in several areas of the US, mainly in cities on the Atlantic coast and a few of those dotting the Gulf coast. According to the National Oceanic and Atmospheric Administration (NOAA), more than 600 such floods occurred in the States in 2019.

But such events are likely to pick up in frequency around the mid-2030s, a new paper reports, mainly due to changes in climate — although astronomy does play a part, as well. The research was led by researchers at the NASA Sea Level Change Science Team from the University of Hawaii. Apart from warning of an increased frequency of such events in the future, the team also explains that the floods will likely occur in clusters of about a month or so at a time, depending on the relative positions of the Earth, Moon, and Sun. When our planet, the Sun, and the Moon line up in specific ways, they note, the resulting gravitational pull is likely to cause floods, especially considering the higher sea levels of the future.

Floods-to-be

“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.”

The paper’s lead author Phil Thompson, an assistant professor at the University of Hawaii, explains that this process won’t be devastating due to the severity of the floods themselves, but rather, through their sheer number. Taken individually, high-tide floods involve relatively small amounts of water. “But if it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue.”

The root of the issue is the Moon’s wobble. Our planet’s satellite has a regular wobble in its orbit caused by the specifics of how it rotates, how the Earth rotates, and how the two move relative to one another. This wobbling cycles every 18.6 years, and it’s a phenomenon known since the early 1700s. What changed, however, is sea level — rising seas interact with this wobble to make flooding way more likely, and way more common.

Throughout half of the cycle, the daily tide on our planet is suppressed. High tides aren’t as high as they should be, and low tides are not as low. In the other half, however, they’re amplified — higher at their highest, lower at their lowest. But since climate change is pushing sea levels up, it’s also lifting the tide higher at all times. Overall, this means that one half of the 18.6-month cycle will be counteracted, while the other will see higher high tides.

Right now, the Moon is in the tide-amplifying part of the cycle, but sea level rise has not been significant enough for it to translate into flooding across U.S. coastlines. However, by the next time this phase comes around, in the mid 2030s, the mean sea level will be higher than today. Under the Moon’s gravitational effect, we’re very likely to see frequent flooding on almost all U.S. mainland coastlines, Hawaii, and Guam, the team explains. Northern coastlines such as Alaska’s will likely not see frequent flooding in the mid-2030s, as they are rising generally due to geological processes, but they will likely suffer the same by the mid-2040s.

For the estimations, the researchers studied recordings from 89 tide gauge locations in every coastal U.S. state and territory except Alaska. Factoring in NOAA’s sea-level rise scenarios and flooding thresholds, they created a statistical model that estimates the number of times these thresholds have been exceeded annually, and how this related to the lunar wobble cycle. Other processes known to affect tides, such as El Niño events, were also factored in. Then, this model was used to project changes in flooding (defined as sea levels exceeding flooding thresholds) up to 2080.

“From a planning perspective, it’s important to know when we’ll see an increase,” said co-author Ben Hamlington from NASA’s Jet Propulsion Laboratory in Southern California, leader of NASA’s Sea Level Change Team. “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.