Beware the Moon's Wobble

Few things seem as stable and predictable as the moon. It goes from new to full and back again every month. It moves the tides. The same side always faces Earth. And every so often, it temporarily blocks the sun.

But here’s something most people probably don’t know about the moon: It “wobbles.” Some years, the wobble lowers tides; in others, it makes them higher. This has been happening for millennia. But combine it with rising sea levels due to climate change, and we have a perfect storm for a rapid increase in the frequency of tidal flooding across coastlines.

This is no hypothetical warning. It will happen in the coming decades. The time to prepare is now.

The term “wobble” makes the phenomenon sound unpredictable, but it is actually part of the moon’s regular motions, as a recent report from NASA and researchers at the University of Hawaii documents. The lead author of the paper, Philip R. Thompson, explains that the moon doesn’t just revolve around the Earth on a flat plane; its orbit is tilted, so it oscillates along a path that’s similar to a coin that’s about to stop spinning.

The result is a reoccurring cycle lasting 18.6 years that has a huge effect on tides. For half that cycle, the moon suppresses the tide. But for the other half, it amplifies it. It’s that second half, combined with the rise in sea levels, that will cause the frequency of coastal flooding to skyrocket in the 2030s. When exactly this will happen depends on the location. But it will happen in every coastal community eventually.

Take, for example, St. Petersburg, Florida. The city is currently experiencing the amplifying part of the moon wobble, which will end by the mid-2020s. In the decade to follow, the moon will start to suppress the tides there. That means the city likely won’t see a huge increase in tidal flooding, even though sea levels will continue to rise due to climate change.

But once the moon begins wobbling back in the early 2030s, St. Petersburg will be in for a rude awakening. Whereas the city currently experiences only a handful of days a year with high-tide flooding, by 2043, Thompson’s team projects upward of 80 days of flooding or more annually.

Local, state, and federal lawmakers must not wait to plan for this. They must build partnerships, expand resources, and create policies that prioritize resiliency in coastal areas. Citizens must let elected leaders know that it’s not acceptable to push aside new warnings about climate change. And they must make clear that they will not tolerate frequent floodwaters lapping at their homes, businesses, and community infrastructure.

Some states are heeding the call, but more will have to follow. Virginia is creating a coastal resiliency plan that will address rising seas and stronger storms. And to make sure that diverse views are heard, the commonwealth is consulting with marginalized communities that historically have not been represented in planning, even when those communities are disproportionately affected by flooding.

Maryland and New Jersey have developed policies to promote nature-based solutions to combat future flooding problems. These solutions include open green space, restoration of wetlands, and other projects that maximize nature’s ability to absorb floodwaters.

South Carolina’s new statewide resilience program has first-ever funding to help communities rebuild in a better way and to support local governments with voluntary buyouts of flood-prone properties to restore those lands to naturally functioning flood plains. Other states can do the same.

These activities and projects—which should start now to reduce future risk to homes, buildings, and community infrastructure—are a sound investment for taxpayers. Research shows that every dollar invested in flood mitigation saves $6 in recovery costs.

Congress and the president also play pivotal roles. We need leadership that makes flood resilience a top priority, and not just for our coastal communities. Inland communities experience flooding year after year. In 2019, numerous towns along the Mississippi and Missouri rivers grappled with historic levels of floodwaters. Over the past decade, more flood-related presidential disaster declarations have been issued for inland states than coastal states.

We’ve seen in both the United States and Europe the terrible toll in lives and property that flooding can cause. Our country alone spends billions annually responding to floods. And the worst is yet to come. The moon wobble may be a decade away, but the time to act against rising sea levels and flooding is now.

Laura Lightbody is the director of The Pew Charitable Trusts’ flood-prepared communities project. 

This piece was originally published in The Washington Post on August 2, 2021.