Moon ‘wobble’ will exacerbate tidal flooding on MV

More extreme tides could compound with sea level rise as lunar cycle shifts in the coming decades.

Five Corners in Vineyard Haven is one of the most heavily affected areas on the Island when it comes to tidal flooding. — Rich Saltzberg

Within the next two decades, the combined effects of sea level rise and a tidal shift brought on by a change in the lunar cycle will create higher high tides and lower low tides. 

For folks on Martha’s Vineyard and in other coastal communities, this could mean more tidal flooding in low-lying areas like Five Corners in Tisbury and the Memorial Wharf in Edgartown.

Although the term “nuisance flooding” is used to describe these kinds of predictable tidal flooding events, an increased frequency and scope of inundation of the many floodplains on-Island would be disastrous for infrastructure and public and private property.

According to Islander and director of global forecasting sciences and technologies for the Weather Co. Peter Neilley, the topic arises from a paper recently published in the science journal Nature Climate Change that looks at the frequency of high-tide flooding events.

These events occur when the normal tides exceed some important level such that the tide floods some areas and becomes disruptive. “This is not looking at the ‘worst case scenario’ flooding events that might occur in 50 or 100 years, but more at the disruptive yet not catastrophic flooding events,” Neilley said.

He noted that the issue reminds him of a similar study conducted in Chilmark to assess the frequency of flooding events for the Menemsha docks, Basin Road, and Hariph’s Creek Bridge. 

However, in the Nature Climate Change study, Neilley said, the authors added into the assessments the natural variations in the amplitude of tides that occur on roughly 18-year cycles related to changes in the declination of the moon’s orbit.

A flood of similar reports from major publications on the NASA-led study conclude that unless coastal communities can adapt and fortify in preparation for this change, they are likely to experience more flooding on a more regular basis than before.

This is what is referred to as the “wobble.”

By combining the steady rise in sea levels associated with climate change with the changes in the amplitude of the tides associated with the moon’s wobble, the authors then estimated when in the coming decades we should expect to start seeing a rapid increase in the frequency of these disruptive high-tide flooding events. “Right now, we see these kinds of events on the Island a handful of times a year, but soon they will be monthly and then weekly events. When to expect the transition from rare to regular occurrences of these events is what this study was getting at,” Neilley said.

The answer to that question, according to Neilley, turns out to be location-specific, based on a number of factors.

In some locations, the onset of rapid increases in the flood events will start within the next decade, while in other places the onset might be delayed until 2040 or so. The authors in Nature Climate Change didn’t look at Martha’s Vineyard specifically, but did look at Boston, which should be a reasonable proxy for the Vineyard, according to Neilley.

For Neilley and other folks on the Island involved with looking at these trends and trying to address the issues, there is good news and bad news.

“The good news, if you can call it that, is that they concluded that the onset of the rapid increase in flooding events in our area will be delayed until the 2040s. This gives us more time to prepare,” he said. “The bad news, I suppose, is that that drives complacency in action, which could leave us more unprepared for that transition when it eventually occurs.”

Island environmentalist and member of the Island Climate Action Network Ben Robinson said although he hasn’t done a deep dive into the study, he is aware of the basics of how these changes could gradually affect the Island.

“If you think about Dock Street in Edgartown or near The MV Times building at high tide, or if you get a slow-moving nor’easter that holds a lot of water, you might see even higher levels of flooding with this ‘wobble’ that is going to increase the extremes of the tides,” Robinson said.

He added that there could be more coastal erosion, and more flooding during storms.

High tide will be significantly more pronounced, on top of whatever sea level rise will occur over the next decade or two.

“I would certainly conclude that science is always progressing, and we are always building our understanding over our past understanding — we realize this when we look back at how well the modeling science is responding to our actual observations,” Robinson said. “There is always going to be a need for reassessment, and unfortunately, the recent assessment is that it’s going to be worse than we were expecting, and it’s going to happen earlier.”