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.

Take, for example, St. Petersburg, Fla. 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.