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By the mid-2030s, every U.S. coast will experience a sharp rise in high-tide floods, as a lunar cycle aligns with rising sea levels caused by climate change.
High-tide floods—also known as nuisance or sunny day floods—are already a common issue in many cities along the U.S. Atlantic and Gulf coasts. The National Oceanic and Atmospheric Administration (NOAA) reported more than 600 of these floods in 2019. However, starting in the mid-2030s, the combination of rising sea levels and the alignment of the lunar cycle will trigger a dramatic increase in flood events across U.S. coastal cities. This shift is detailed in the first study to account for all known oceanic and astronomical factors contributing to flooding.
Led by members of the NASA Sea Level Change Science Team from the University of Hawaii, the study reveals that high tides will regularly exceed existing flood thresholds, with some cities experiencing “flood clusters” that could last for weeks. These floods will be more frequent and intense, depending on the alignment of the Moon, Earth, and Sun. When these celestial bodies align in specific ways, their combined gravitational pull will cause sea levels to rise significantly, leading to floods every day or two.
“Low-lying areas near sea level are increasingly at risk from flooding, and the situation will only worsen,” said NASA Administrator Bill Nelson. “The combined effects of the Moon’s gravitational pull, rising sea levels, and climate change will continue to intensify coastal flooding across our coastlines and around the world. NASA’s Sea Level Change Team is providing vital data to help us plan, protect, and mitigate the damage to both the environment and people’s livelihoods.”
“It’s the cumulative effect over time that will truly have an impact,” said Phil Thompson, an assistant professor at the University of Hawaii and lead author of the new study published this month in Nature Climate Change. Thompson noted that because high-tide floods involve a relatively small amount of water compared to the massive surges from hurricanes, they are often dismissed as a less significant problem. “But when you’re facing flooding 10 or 15 times a month, businesses can’t continue operating with parking lots submerged. People lose their jobs because they can’t make it to work. And contaminated cesspools become a public health threat.”
So why will cities on such widely scattered coastlines experience these increased flooding events at nearly the same time? The main factor is a regular wobble in the Moon’s orbit that takes 18.6 years to complete. While this wobble is nothing new—it was first documented in 1728—what’s changing is how its effect on the Moon’s gravitational pull, which drives Earth’s tides, will interact with rising sea levels caused by global warming.
In half of the Moon’s 18.6-year cycle, Earth’s normal daily tides are suppressed: High tides are lower than usual, and low tides are higher. In the other half, tides are amplified: High tides rise higher, and low tides sink lower. Meanwhile, global sea level rise is pushing high tides in one direction—higher. For half of the lunar cycle, this effect is counterbalanced, but during the other half, it’s amplified.
Currently, the Moon is in the tide-amplifying phase of its cycle. However, along much of the U.S. coastline, sea levels have not yet risen enough for high tides to regularly surpass flooding thresholds, even with the added lunar boost. That will change when the cycle returns to its amplifying phase in the mid-2030s. By then, global sea levels will have risen further, and the combination of higher seas and lunar amplification will lead to a sharp increase in flood events across almost all U.S. mainland coastlines, Hawaii, and Guam. Only the far northern coastlines, including Alaska’s, will be spared for another decade or more, as these areas are rising due to long-term geological processes.
The researchers identified key tipping points in flood frequency by analyzing data from 89 tide gauge locations across every U.S. state and territory, except Alaska. They developed a new statistical framework that integrated NOAA’s widely used sea level rise scenarios, flooding thresholds, the frequency of annual flood events, astronomical cycles, and other known factors—such as El Niño—that affect tides. Using this framework, they projected future flood trends through 2080.
Ben Hamlington, a co-author of the study and leader of NASA’s Sea Level Change Team at the Jet Propulsion Laboratory in Southern California, emphasized that the study’s results provide critical insights for coastal urban planners, who may be focused on preparing for extreme events but need to account for more frequent high-tide floods as well.
“From a planning perspective, it’s crucial to know when these increases will occur,” Hamlington said. “Understanding patterns, such as clustering of flood events in specific months or more severe flooding later in the year, is valuable information.” A high-tide flood tool, developed by Thompson and available on NASA’s Sea Level Change portal, offers decision-makers and the public access to this data. The tool will soon be updated with the findings from this study.
Written by Carol Rasmussen
Jet Propulsion Laboratory, Pasadena, Calif.
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