The Seven-Year Switch

How to understand the changing shape of our fragile coastal landscape.

Photo by Terry Pommett

Erosion has been unkind to Cape Cod and the Islands. Over the last seven years, we have witnessed houses swept off beaches or left teetering on the edge of cliffs. On Nantucket, Sankaty Head Lighthouse and the old ‘Sconset summer homes had to be moved back from the edge of eroding hundred-foot bluffs. By 2008, Chatham was losing 10 feet off the end of North Beach every day when the inlet migrated north. North Beach Island eroded at the rate of 80 feet a year, threatening a dozen camps owned by private homeowners and the Cape Cod National Seashore. The east side of Martha’s Vineyard lost about a foot of beach every day and a troublesome new break opened into Katama Bay, disrupting the Chappaquiddick ferry on the Vineyard.

Then, in November of last year, it seemed as if someone had flipped a switch. The weather turned unseasonably mild, temperatures rose above 60 degrees, and lilacs and forsythia started blooming in December. Snowy owls started appearing frequently on Cape Cod. There was no snow, few storms, and, compared to previous years, little erosion.

And here is the irony: Scientists can predict that Cape Cod is going to wash away in about 5,000 years, that the Cape will lose about a thousand acres to erosion in 200 years, and when we should expect a hundred-year storm, but they can’t tell a homeowner if he can stay in his barrier beach camp for another season or if it makes sense to build a seawall to protect his oceanfront house for another 20 years.

This is because erosion doesn’t proceed in a nice linear fashion according to sea level rise. The pattern is different: seven-year stretches of severe erosion follow seven-year stretches of mild erosion. And we saw this global process shift dramatically at the end of 2011.

Photo by Ron Wilson

Before 2011, the North American climate had slipped into an intense La Niña period. La Niñas occur when cool waters spread across the Pacific Ocean, changing the way weather systems travel across the American continent. During normal years and El Niño years, cold air bulges out of Canada, causing winter storms to swoop south and gather up humid air from the Gulf of Mexico. Then the storms careen up the East Coast, glance off Cape Hatteras and slam into Cape Cod as raging Northeasters, which hang around through several erosion-causing cycles of high tides.

However, the tides are the main factor causing the seven-year stretches of erosion. Tides are caused by the position of the moon, the sun, and earth and the distance between the planets. First, there are “spring” tides that occur every 14 and a half days, when the sun the moon and the earth are all lined up in a row as they are during a new or full moon. (The term spring tide comes for the Dutch word “springen” to jump and has nothing to do with the season.) Second, there are perigean tides that occur every 28 days, when the sun, moon, and the earth are lined up and the moon is closest to the earth. Third, there are proxigean tides that occur about every 16 months when the sun, the moon, and earth are all lined up with the earth closest to the sun. Finally, you have extreme proxigean spring tides every 31 years, when all of the elements are combined: the moon is closest to the earth and the earth is closest to the sun, and all are in a line.

In 1978, Fergus Wood, author of the book Tidal Dynamics, researched 300 years of erosion events extending back to Mayflower pilgrim William Bradford’s description of a hurricane in the 1600s. Wood found that seven-year stretches of erosion occur during the peaks of the proxigean tides. But his discovery of this phenomenon was largely ignored because Fergus overplayed his hand: Although he had pointed out that storms were needed to trigger these erosion events, his book left the impression that proxigean tides were the primary cause of erosion events.

Today, we know that proxigean tides do not cause erosion, but they can be the best predictors of erosion events.

A storm that occurs during a full moon proxigean tide can cause a great deal of damage, but if that same storm occurs only seven days later during a non-proxigean half moon, it causes considerably less erosion because the high tide might be lower by a foot or more.

Photo by Greg Hinson

During a month without proxigean tides, there might only be five days out of a single month when the tides go over 11 feet. But if a proxigean tide occurs during the same month a few years later, there might be as many as 20 days when the tides go over 11 feet—four times as many days when a storm can cause extreme erosion. During an average New England winter you might be able to squeeze by without having a storm during a five-day period, but you are almost guaranteed to have one during a 20-day period.

Of course, it is always possible that a single storm can occur during a seven-year stretch of lower high tides and still cause considerable damage. (This happened during “The Perfect Storm” in 1991.) But what we are interested in here is probability: You are about four times more likely to have significant erosion during a proxigean month, when even a run-of-the-mill storm can ride in on tides that are several feet higher than normal.

There is another intriguing possibility: Some researchers think that the reason proxigean tides and La Niñas track so closely together is that the proxigean tides actually cause La Niñas by initiating the upwelling of cold water in the Pacific Ocean.

Some of this might sound esoteric, if not pretty darn complicated. However, if you are a homeowner, the knowledge of proxigean tides can be crucial, and it can help you decide whether your home will be safe for another five, 10, or 15 years. [See sidebar]. Proxigean tide tables can help town officials decide whether to give homeowners permission to build anti-erosion devices like seawalls or gabions to slow down the effects of sea level rise. Marine contractors can use knowledge of proxigean tides to forecast the years when business will be brisk and when it may be lean.

For now, we can expect mercy. We have just entered a stretch of mild erosion that will last for another seven years when we will be in the low part of the proxigean tidal cycle, and when the proxigean tides will coincide with the less stormy summer months. But this will only be a temporary reprieve. The tides will rise again.

William Sargent has been using Chatham as a case study for sea level rise for the past 25 years. He was a consultant for the NOVA film The Sea Behind the Dunes, and the author of Storm Surge: A Coastal Village Battles the Rising Atlantic and Sea Level Rise: The Chatham Story. His newest book, Nauset, Ten Thousand Years on a Barrier Beach, will be released by Strawberry Hill Press in 2012. He can be reached on his daily blog about coastal events at coastlinesproject.wordpress.com.