NASA warns of more coastal flooding in the 2030s: How the Moon’s 18.6-year cycle could bring floods on clear, calm days

A clear sky, calm water and no storm could still bring flooding to some coastal areas in the coming decade. A 2021 NASA-led study warned that high-tide flooding could become far more frequent in the 2030s, as a natural cycle in the Moon’s orbit combines with rising sea levels.

The study does not suggest that the Moon itself is causing sea levels to rise. Instead, its gravitational influence can amplify tides at certain points in its 18.6-year cycle. With oceans already higher because of climate-driven sea-level rise, those stronger tides could push seawater onto roads and other low-lying areas more often.

How the Moon’s 18.6-Year Cycle Affects Tides

The Moon’s gravity creates tidal bulges in Earth’s oceans. As Earth rotates, coastlines experience the regular rise and fall of tides.


However, the Moon’s orbit is tilted relative to Earth’s equator, and that tilt slowly changes over an 18.6-year cycle.

During part of the cycle, this movement can reduce the height of high tides. During another phase, it can amplify them. The next amplifying phase is expected to become significant in the middle of the next decade.

The important difference now is that the baseline sea level is higher than it was during the previous cycle.

Rising Sea Levels Could Make High Tides More Problematic

The lunar cycle has occurred throughout Earth's history without creating this kind of widespread concern. The difference today is sea-level rise caused by warming oceans and melting ice.

When a stronger-than-usual tide arrives on top of a higher sea level, water that previously remained within coastal channels can reach roads, parking lots and other low-lying areas.

Ben Hamlington, who leads NASA’s Sea Level Change Team at the Jet Propulsion Laboratory, explained the concern when the study was released: “We’re getting closer and closer to the flooding thresholds or tipping point in these coastal locations. The same variability in the past that didn’t cause flooding is now going to cause flooding.”

High-Tide Flooding Could Become Much More Frequent

According to the NASA-led study, a majority of the U.S. coastline could experience three to four times as many high-tide flooding days annually during the 2030s as the lunar cycle and rising sea levels combine.

These events may also occur in clusters, with several flooding days occurring within a short period rather than being spread evenly throughout the year.

Why ‘Nuisance Flooding’ Can Become a Serious Problem

High-tide or nuisance flooding is generally much less dramatic than a hurricane or major storm. It can involve relatively small amounts of water covering roads, parking areas or low-lying properties.

NOAA describes minor tidal flooding as flooding that can overtop older sea walls or cause water to accumulate on low-lying roads and is “not necessarily widespread or damaging, but just… relatively easy to navigate around.”

The problem is that frequency can turn minor flooding into a major infrastructure challenge.

Repeated flooding can put additional pressure on roads, drainage networks, basements and other infrastructure. A location that floods several times a year may face a very different situation if those events begin occurring several times a month.

The Impact Will Vary From Place to Place

The increase will not be identical across every coastal community. Local geography, land subsidence, shoreline conditions and the shape of individual harbours can all influence how much water reaches a particular location.

NOAA modelling has indicated that some East Coast tide stations could see nuisance-flooding days increase by 33% to 125%, depending on the location.

That means coastal communities will need to assess their own local flood risks rather than rely on a single nationwide prediction.

What Makes the Coming Flooding Different?

The biggest concern is not necessarily a single catastrophic flood. Instead, it is the possibility of repeated flooding during otherwise calm weather.

A parking lot with an inch of water on a sunny day may not appear particularly dangerous. But if the same area floods repeatedly over many years, the cumulative impact can become costly for infrastructure and property owners.

The Moon's tidal cycle is a natural phenomenon that has existed for billions of years. What has changed is the higher sea-level baseline on which that cycle now operates.