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Moon ‘wobble’ and climate change could mean ‘double whammy’ of flooding in 2030s, NASA warns

A full moon rises over Philadelphia on June 24.Matt Rourke/Associated Press

In the coming decades, higher tides coupled with sea-level rise will cause U.S. coastlines to experience a “dramatic” uptick in flooding, a new NASA study finds.

By the mid-2030s, scientists project there could be a "rapid" increase in the frequency of high-tide flooding in several parts of the country, according to the report published last month in the Nature Climate Change journal by the NASA Sea Level Change Team at the University of Hawaii.

"We're going to have sort of a double whammy," William Sweet, an oceanographer at the National Oceanic and Atmospheric Administration (NOAA) and author of the study, said Wednesday. "It means that coastal communities - unless they adapt and fortify - are likely to expect even greater flooding than they might otherwise."


The increase in flooding is caused in part by the moon's "wobble." As it orbits Earth, the satellite's angle relative to the equator changes over time. This phenomenon - spanning an 18.6-year period - influences how tides ebb and flow.

One half of this cycle subdues tides, making the high tides lower and the low tides higher. The second half amplifies them.

This latter part, combined with sea-level rise - a product of climate change - is what will lead to greater flooding in the next two decades, the study concludes.

To explain this phenomenon, one of the report's authors, Gary Mitchum - associate dean at the University of South Florida's College of Marine Science - likened it to a person flapping at water inside of a bathtub. While the liquid may not immediately overflow, the probability rises when more water is added.

"The little wave is the tide and the amount of water in the bathtub is sea-level rise," Mitchum said. "The two are completely unrelated. But the combination of the two means that you're liable to get sloshing out of your bathtub more often when you have sea-level rise and you have a bigger wave."


According to the study, the most pronounced effects of this combination will be seen during the mid-2030s and into the 2040s, when the lunar cycle shifts again. However, some of the impacts are already being felt.

"It's already occurring now, and they're likely to continue to worsen as time goes on," Sweet said.

Water spilling into communities from tidal effects is often thought of as a nuisance and happens regularly in coastal cities. Yet, a NOAA report published Wednesday notes tidal floods are increasing.

From May 2020 to April 2021, some cities along the Southeast Atlantic and Gulf coasts have either set or tied records for the amount of high-tide flooding - with Corpus Christi and Galveston in Texas and Bay St. Louis-Waveland, Miss., experiencing more than 20 days of flooding, according to NOAA.

The uptick in flooding caused by high tides is a trend that is expected to continue, according to the NOAA report. By 2030, seven to 15 days of such flooding is expected nationally. Two decades later, estimates indicate that there could be an average of 25 to 75 days a year.

These floods are much more than a minor inconvenience, said Phil Thompson, director of the University of Hawaii Sea Level Center, who was also part of the study.

"You might have a situation where a business might have its parking lot flooded 10 or 15 times in a month - that's not a nuisance any longer. That's a significant economic impact on that person," Thompson said. "It's really the accumulated impact of many small, seemingly minor issues. But when a minor issue occurs chronically, it becomes a bigger problem."


While the predictions appear grim - with researchers saying not much can be done to change their estimates for the next few years - there are some measures that can be taken, said Ben Hamlington, NASA Sea Level Change Team's lead.

In the short term, local communities have to adapt to changing environmental conditions and limit the impacts of sea-level rise. In the long-term, he said the global community must find ways to mitigate climate change.

“The goal needs to be to try to limit global warming and keep the sea-level rise that we see as low-level as possible - for instance, by reducing our emissions - to reduce some of these impacts,” Hamlington said. “So there are these bigger global discussions that play a very important role here.”