Imagine if the waters of Boston Harbor rose up, ran through the streets and flooded the subways.  It has happened before, and it will happen a lot more as climate change pushes sea levels higher.

Boston's Long Wharf, from the Harbor. Credit: Wikimedia Commons

Today, the Atlantic Ocean laps eight feet below the stone wall of Boston’s Long Wharf. The wharf is one of many that jut into the gray waters of Boston Harbor, reaching like outstretched fingers. When it was built 300 years ago, this popular tourist spot was an active commercial dock, stretching 1.3 miles from Quincy Market, the longest wharf in Boston. Over time, landfill was added around the wharf to extend Boston seaward, creating a city built on low, flat land that sits precariously close to the ocean.

As that ocean rises over the next century, Long Wharf and the city beyond it could be periodically swallowed back up by the sea. Climate scientists with the Union of Concerned Scientists warn that rising sea levels from global warming will create more severe and frequent floods along the Northeast Coast. They predict that by 2050, sea levels around Boston could rise as much as two feet. If these predictions hold, 10-foot floods—which used to be a once-in-a-century event—could engulf New England’s shores every two to four years. That’s enough water to swamp every wharf in Boston and fill the streets of downtown.

The Peter Stuyvesant, once part of Anthony's Pier 4 restaurant, sank in the icy waters of Boston Harbor during the Blizzard of '78. Credit: Boston Globe

A series of floods this severe would cost Boston billions in repairs to flooded buildings and submerged streets. Such a flood happened only once last century, during the Blizzard of 1978, when four days of snow created a 16-foot tall swell that shattered homes along the coast and swamped Boston Harbor.   The blizzard alone cost the state $500 million in damages. Three or four major floods every decade could cost Boston alone anywhere from $20 billion to $94 billion over the next century according to Paul Kirshen, a professor of civil engineering at Tufts University. These figures represent only property damages—they don’t include the other financial and psychological costs associated with living in a city that returns to the sea every decade.

A storm rolls into Boston Harbor. Credit: Rachel Blumenthal, rachelblumenthal.net

Imagine what such a flood would look like, standing at the edge of Long Wharf in the year 2050. At high tide, the ocean would ebb just five feet below the wharf wall. A storm blowing into the Harbor from the northwest would push massive waves towards shore, creating a storm surge five feet high. The rows of white boats lining the wharf would knock together in the wind, and then start to list as the water rises above their moorings. Before a drop of rain had even fallen, the ocean would flow over the lip of Long Wharf, swamping the white mast that stands at its edge.

When the storm hits, rain will pour into the sea, raising the ocean another 15 feet and sending it crashing through the city. The first building to be inundated would be the trendy apartments whose sea-facing balconies have long been the envy of Harbor-going tourists. Then seawater will flow over the Harbor and across State Street, where it will hit the entrance to the Aquarium T stop and cascade down the escalator into the underground. At its worst, the surge could push all the way to the concrete and glass gate of Quincy Market. For as long as the storm lasts, the market would sit at the water’s edge, just as it did when it was first constructed in 1826, before the city was built out into the ocean.

This is the sort of flood that could hit Boston not once a century, but once every five years or more as sea levels rise. The new flood of the century would likely be worse than anything ever experienced in Boston’s history. You can see the extent of the new 100-year flood in this picture.

The light blue shows the extent of Boston's new flood of the century, a flood so severe it used to occur only every 500 years. Credit: Union of Concerned Scientists

While increased flooding from global warming may be unavoidable, there are steps Boston can take to reduce the predicted $20 billion of flood damage over the next century. The simplest method is building seawalls along the coast that will block surges heading landward. Seawalls have the benefit of allowing construction to continue uninhibited along the coasts, but they are expensive and cause coastal erosion. A cheaper and more effective solution, according to Kirshen, would be to limit development and flood-proof existing buildings in the new floodplains.

Neither solution is cheap or completely effective at preventing flood damage, but they are far better than the alternative. If nothing is done, for a few days every decade, the land the city has wrested from the Harbor may be reclaimed by the sea. Payment, perhaps, for reaching our fingers too far into the ocean.