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Sea Level Is Never Level: How Does Geology Inform Coastal Planning?

Emlyn Koster, PhD

July 2022

With his characteristic eloquence, a 1962 speech by President John F. Kennedy to America’s Cup crews in Rhode Island made a scientific fact relevant to everyone. Referring to the part humans play in the relentless motion of the Earth’s primordial water, he explained that “… all of us have in our veins the exact same percentage of salt in our blood that exists in the ocean, and therefore we have salt in our blood, in our sweat, in our tears. We are tied to the ocean. And when we go back to the sea – whether it is to sail or to watch it – we are going back to whence we came”.  

Always on the Move

Today’s ocean occupies 71% of the Earth’s surface. I say today because geologists know that the polar icecaps have repeatedly expanded and shrunk and that remelting of the Antarctic and Greenland icesheets would raise the ocean by about 220 feet. When the last Ice Age was at its peak 20-25,000 years ago, with ice covering the upper latitudes of North America and Eurasia, the ocean was almost 400 feet lower than now. And because the weight of those icesheets depressed the underlying land, today’s Hudson Bay and Gulf of Bothnia regions are rimmed by former shorelines due to the land rising faster than the ocean surface is rising.

Geologists label the continuous movement of water on, above and below the surface of the Earth – including through every animal and plant – the hydrologic cycle. Although inadequately considered as climates warm and glaciers retreat, it is an apt launch point for delving into today’s conundrum of coastal planning. The Dutch are the world’s experts at what must be done to keep rising and stormier seas at bay. Their Katrina-like lesson was in 1953 when a North Sea hurricane topped coastal dunes and drowned 1,800 people and 30,000 farm animals in below sea-level areas. 

Bostonians recently had this news: “New waterfront developments have been packaged with projects designed to help them weather sea-level rise... Climate-focused advocates are glad developers taking the matter seriously. But they worry that a project-by-project approach, driven by new development, may not be enough to ward off disaster.” Boston’s 2030 plan notes that the “… waterfront provides a unique nexus of historic, civic, economic and ecological resources and accommodates a variety of residential, commercial, and recreational uses”. The city’s Planning & Development Agency is “endeavoring to balance these interests and uses to promote an active, climate resilient, and accessible Harbor that sustains vibrant waterfront neighborhoods and water dependent businesses”.

A recently revised federal estimate noted: “Parts of Massachusetts, including Boston and Cape Cod, have become accustomed to experiencing a few minor ‘nuisance’ floods a year during high tides, but those will be replaced by several ‘moderate’ floods a year by mid-century, ones that cause property damage… By mid-century, the Northeast coast should get 10 inches to a foot of sea-level rise [and] The report is projecting an average of about two feet of sea-level rise in the United States — more in the East, less in the West — by the end of the century”. 

Weather or Not

Two record-setting storms – The Great New England Hurricane in 1938 and Superstorm Sandy in 2012 – are instructively reviewed in the context of the 2007 Worldwide Reinsurance Lifetime Achievement award to Boston’s Karen Clark. The insurance industry used to extrapolate property risk from past disasters. After a realization that “the wealth and mobility of Americans had allowed them to relocate in dangerously high numbers to scenic shoreline developments”, it began to hire scientists and engineers and rating agencies responded by using “catastrophe models” to calculate potential losses. These statements were in a 2012 article in BU Today. It was subtitled with Clark’s caution about “ignoring the elephant in the room” which is the combination of rising property values and global warming. Evidently, a recurrence of the 1938 hurricane “would cause losses three times higher than Sandy”.     

The Category 3 1938 event, which generated a 17-foot surge above high tide and waves as high as 50 feet, destroyed some 9,000 Boston buildings, left over 60,000 people homeless, and felled about two billion trees. Sandy trampled the New York City area with a coincidence of storm surge and high tide causing some $20 billion of property damage. As an example, New Jersey Transit had not safely relocated a quarter of its passenger rail fleet: 62 locomotives and 261 cars were damaged in a yard considered to be above the highest inundation. “You can prepare for a worst-case scenario, but the standard of preparedness was definitely raised by this storm… this was a hit of historic proportions”, noted Governor Christie’s spokesman.

Never Level

On days when the sea is calm, it is easy to forget the last destructive storm and be unaware that the ocean surface is never really level. Twice-daily tides are the rotating bulge of the ocean due to the Moon’s gravitational pull and vary locally with coastline geography. Just two feet in the open ocean, the tidal range across the Boston area is up to 10 feet: in Nova Scotia’s narrowing Bay of Fundy, it reaches almost 50 feet. Hurricane-force winds temporarily raise the ocean surface ahead of storm centers: the Category 3 Hurricane Bob in 1991 caused a 10-15 foot surge in Buzzard’s Bay and the most vulnerable sections of Martha's Vineyard and Nantucket lost as much as 50 feet of shoreline.

Tsunamis are the biggest oceanfront danger. As fast as a jet plane, these waves result from undersea earthquakes and slope failures. Although Pacific rim countries are at a massively greater risk, there are records of disastrous Atlantic Ocean tsunamis. Lisbon, Portugal was struck by earthquakes in 1321, 1531 and 1755. The last one, with magnitude estimates as high as 7.7, had a death toll in the tens of thousands. Its tsunami was as tall as 65 feet in nearby Spain and 13 feet in Martinique, 3,800 miles away. In 1929 a 7.2 earthquake in the Grand Banks region near Newfoundland severed trans-Atlantic cables and caused tsunami waves as far as Bermuda.

Some relieving news came last year from the US Geological Survey. Its updated analysis of the undersea flanks of La Palma in the Canary Islands, the volcano that erupted for several months in 2021, challenges a 2001 view that a giant tsunami could happen. “These new simulations suggest that the maximum wave height along the east coast of the Americas from a ‘worst-case scenario’ collapse of La Palma would be on the order of 3-7 feet—still hazardous, but similar to common storm surge.”

Citizen Science

The more that scientists decipher the past and delve into the future, the more complicated planning becomes. As my late colleague Ursula Franklin urged in the 1990 book Planet under Stress, “The task of the future is to build knowledge and understanding among and between citizens and scientists, so that the distinction between the two groups vanishes – so that both become citizen scientists, potentially able to solve our problems together”. Densely populated coasts are a candidate for this ideal approach with geological and archaeological evidence, as well as Indigenous knowledge, as baseline information.